Hot composting: building a batch that heats
A hot pile is built at once, read with a thermometer and turned when it cools. This chapter helps you decide whether that fits your week, then walks through one batch.
- Start here: the essentials
- Is a hot pile for you?
- Build a batch
- Read the temperature
- Turn when the center cools
- How long, and when it is done
- Working safely around a hot pile
- The sources side by side
- See it: Ingrid builds, Tomás decides no
- Practice: decide, then log a batch
- Your system
- Check your work
- Knowledge check and scenario
Start here Essentials
For a first pass, this chapter asks for one decision and, if the answer is yes, one batch. The decision is a written yes or no on your system plan. A written no finishes the chapter.
- Answer three questions and write yes or no on the system plan. The questions come from Washington State's master composters (is a hot pile for you, system plan).
- If yes, collect browns and greens until you have about a cubic yard, then build the batch in one go. Wet it to a wrung-out sponge (build a batch, pile calculator).
- Read the center temperature each day for the first week and write it in the log. Use a long-stem compost thermometer (read the temperature, bin log).
- Turn the pile when the center starts to cool, and log the turn. Check the moisture as you restack (turn when the center cools).
Minimum time: the decision is one sitting. A yes adds one build day and a first week of daily readings. Done when: the plan says yes or no to hot composting, and for a yes, the log shows a build date and a week of temperatures.
Is a hot pile for you? Understand
Hot composting is composting that you build and tend so that the pile warms well above air temperature. Clemson's extension calls it labor intensive but the method with the fastest results (Clemson's HGIC 1600), and it asks the most: material collected before you start, a pile built all at once, a thermometer, and turning. Choose your system explained why heat happens (microbes working fast in a big enough pile) and what the common figures claim. Here the question is whether you will do the work.
Washington State's master composters in Whatcom County give three questions for choosing between a hot (active) pile and a cold (passive) one (WSU Whatcom's backyard composting brochure). Their guidance is that yes answers to most of them point to a hot pile, and otherwise to cold composting and other passive methods. Convention
| Question | What a yes asks of you | If the answer is no |
|---|---|---|
| Do you need compost within about two months? | Hot composting gives the fastest results by Clemson's account, but only with the other two answers | A slow pile (The slow pile) gets there without the work, over months to a year or more |
| Do you have at least one cubic yard of material and room for it? | Collect it first, then build it at once. A cubic yard is a cube three feet on each side, 27 cubic feet | A smaller pile may stay cool. A bin that size can still run as add-and-cover |
| Will you turn the pile at least once a week? | Fork the pile apart and restack it, repeatedly, over the weeks the batch is hot | A pile that is not turned can run, but slowly. NC State says an unmixed pile takes three to four times longer (NC State's AG-791) |
Oregon State's compost specialist adds a seasonal test: aim for 0.5 to 1 cubic yard of fresh material during the peak garden season, and if you do not have that volume, cool-compost the rest of the year or consider a worm bin (Oregon State Extension's hot-compost troubleshooting article). A hot pile is often a seasonal project, with batches when leaves, trimmings and clippings are plentiful. Ingrid, the fictional example below, builds hot batches from May to September.
Notice what the questions leave out. They do not ask whether hot compost is better; they ask what you can do. Attention in this manual is tasks and how often: collect, build once, read the thermometer, turn. We give no minutes, because no source does.
The choice is a real one in both directions. A household with a small bin, scraps added a little at a time and a week with no room for turning has a complete method in The slow pile. A household with a yard full of leaves in October, a pile of clippings and a free weekend may reasonably want a batch. Write the answer on the plan either way; the review in Your compost system can change it.
Meat, fish, dairy, grease, pet waste and the other materials in What goes in's stays-out list stay out of a hot pile. A warm pile is not a reason to add them. If rats or bears are in your area, read rodents and bears in Site and bin before building anything outdoors. An open pile with food scraps in it is what those sections warn about.
Build a batch
A batch is one pile built at one time, from material you collected first, and then left to run its course without new additions. This is the main difference from add-as-you-go methods. Fresh scraps added to a heating pile cool it and restart the clock, so a hot system keeps collecting material in a separate place while the batch cooks. Texas A&M's composting guide says to collect enough material to build a one-cubic-yard pile, ideally near a 30:1 carbon-to-nitrogen ratio (Texas A&M's chapter on building a pile). What goes in gave the ratio idea; in practice you work by volume with browns and greens.
Size
Texas A&M gives about one cubic yard (3 feet by 3 feet by 3 feet, which is 27 cubic feet) as the usual minimum for a pile that heats (Texas A&M's Composting Fundamentals chapter). WSU Whatcom says a pile of at least one cubic yard keeps the pile from drying out too quickly. Convention Use the pile calculator to check any bin or stack against 27 cubic feet. The calculator page also explains why two extension pages give a smaller figure; we do not repeat their sentences. Measure the pile as you build it, because a loose heap that looks like a cubic yard often settles to less.
Mix
The default is EPA's volume rule from What goes in: at least two to three times as much browns as greens by volume (EPA's Composting at Home page). Convention Greens are the wet, nitrogen-rich materials (food scraps, fresh grass, fresh trimmings); browns are the dry, carbon-rich ones (dry leaves, straw, shredded paper, wood chips). Mix them as you stack rather than laying down thick separate layers; Arizona's extension publication gives that as its guidance (University of Arizona's AZ1632). If you add food scraps, bury them under at least 3 inches of browns, as New York State's guide advises (NYS DEC's Home Composting Guide).
Cut it small
Smaller pieces break down faster, because microbes work on surfaces. Texas A&M recommends particles less than 2 inches. Cornell's particle size page, citing earlier work, says grinding can double the decomposition rate and recommends pieces from about half an inch to two inches (Cornell's particle size page). The term particle size means the size of the pieces you put in. Too fine and packed, and air cannot get in, so leave some coarse, bulky pieces such as small twigs or straw. Chop kitchen scraps, shred leaves with a mower or by hand, and break up cardboard.
Wet it
EPA gives the consistency as a wrung-out sponge. NC State turns that into a check: a drop or two of water squeezed from a handful. NC State also says compost piles are most often too dry (NC State's AG-791). Wet dry browns as you stack, in a bucket or with a hose on a light spray, and stop when a handful squeezed hard gives a few drops. Soaked material pushes air out. The slow pile's squeeze test is the same test.
Where the batch sits
A batch can go in a bin, a pallet bay or a freestanding stack, as long as it holds its shape and can be reached from the side you will turn from. Site and bin covers sites and bins. A multi-bin system lets you keep collecting the next batch while one cooks. Keep rain from soaking the batch: Oregon State's article suggests a tarp or a simple roof.
| Decision | Our default | Source | Your exception |
|---|---|---|---|
| Size | About 27 cubic feet (a cubic yard) or more | Texas A&M | Oregon State says 0.5 to 1 cubic yard can heat if well built; check against your log |
| Browns to greens | Two to three parts browns to one part greens, by volume | EPA | Check your own mix against smell and moisture; Go deeper lists other proportions |
| Piece size | Under about 2 inches, with some coarse pieces left in | Texas A&M, Cornell | Wood chips and twigs break down slowly whatever the size |
| Moisture | A few drops from a squeezed handful | EPA, NC State | Hot, dry weather dries a pile fast: check at every visit |
Read the temperature
A compost thermometer is a probe thermometer with a stem long enough to reach into the middle of a pile. Clemson's extension suggests a long-stem thermometer checked at least 12 inches from the surface. Cornell's monitoring page says to leave the probe in place until the reading stabilizes, then move it to a new spot, and to take readings in several places at different depths from the top and sides (Cornell's Monitoring Compost Temperature page). New York State's guide says to take the temperature at the center of the pile, where it is hottest. For a pile with a bin around it, you can read through the top or push the probe in through the side.
Cornell says a well-constructed compost system will warm to 40 to 50°C (104 to 122°F) within two to three days. Oregon State's article says a well-built pile of 0.5 to 1 cubic yard should heat within a week. If a batch has not warmed after a week, the likely causes are too little volume, too dry, too wet or too many browns for the greens. The troubleshooting table has the first change for each.
What the readings mean
Decomposers work in temperature stages. Texas A&M calls the organisms that work from 50 to 113°F mesophilic (warm-loving) and those from 113 to 158°F thermophilic (heat-loving). A hot pile is one where the thermophiles take over. NC State gives the two numbers that most home guides repeat: the pile has to exceed 131°F to kill most pathogens harmful to humans and pets, and 145°F to destroy most weed seeds (NC State's AG-791). A pathogen is an organism that can cause disease. NC State also says temperatures near 160°F can kill the decomposers themselves. That is our default for this manual, because it is one named source with figures that other sources roughly match. Convention Other agencies give lower or higher figures and different times at temperature. Those comparisons are under the sources side by side, not here.
| Center reading | What it tells you, by NC State's figures | What you might do |
|---|---|---|
| Near air temperature after a week | The pile has not heated | Check size, moisture and the browns-to-greens mix; log what you change |
| Above air temperature, below 131°F | The pile is warming and has not reached NC State's pathogen figure | Log it. Leave diseased plants and seeded weeds out |
| 131°F to 145°F | Above NC State's pathogen figure and below its weed-seed figure | Log it. Seeded weeds stay out |
| Above 145°F | Above NC State's weed-seed figure at the probe | Log it. Watch for a reading near 160°F |
| Near 160°F or more | NC State says decomposers can die near here | Turning adds air and lets heat out; we turn at this point, as our convention, and log it |
A reading is one number at one spot. The center of a pile is hotter than its edges, so a pile that reached 131°F at the center did not reach it everywhere, and turning is part of how material from the edges gets to the middle. Home guidance on temperatures and times differs, and none of it is a tested standard for a backyard pile. The 131°F figure is the same number (55°C) as a federal rule for sewage sludge treated in controlled equipment (Go deeper). This manual does not say that your compost is free of pathogens or weed seeds, and a log cannot show it. Keep diseased plants and seeded weeds out unless your log shows a pile that held its heat through several turns, and even then treat the result as less than proof.
Log the temperature at the center on each visit in the bin and batch log, with the date, whether you turned it, the moisture and the smell. Daily readings in the first week show you the curve; after that, a reading each time you visit is enough. Daily is our suggestion, not a source's. Preference A pile that heated and then cooled is following the stages in Choose your system. A pile that rose and fell in a day, or did not rise, is telling you about size, moisture or mix.
Turn when the center cools
Turning means forking the pile apart and restacking it. It adds air to the pile, and it moves material from the cool edges to the hot center and back. Oregon State's extension guide gives a starting interval: restack when the pile starts to cool, which it puts at 5 to 10 days (Oregon State's EM 9475). Convention We use that as the default and let the thermometer set the exact day. Start checking at day 5, and turn when the center reading has dropped well below its peak.
- Wet any dry material before you start. This keeps dust down (see working safely).
- Move the outer material toward the middle. Pull the outer foot or so of the pile off, then restack it with what was at the edges now in the center. This is our method, not a figure from a source.
- Check moisture while the pile is open. Squeeze a handful. Add water with a light spray if the handful gives no drops.
- Read the new center temperature after a day and log the turn. A restacked pile may warm again.
Repeat: when the center cools again, turn again. Ingrid's June batch cooled by day 8, was turned that day, and was turned again on day 16. NC State's turning guidance is once a week; Penn State's is frequent turning in the first 3 to 4 weeks and once or twice after that (Penn State's home composting guide). These differences sit under Go deeper. The reader's exception is your thermometer: a pile that stays hot at day 10 does not need turning because the calendar says so, and a pile that cooled at day 4 can be turned at day 4.
A pile that was not turned can still finish, more slowly. If you skip a turn, log it. The log is this manual's way of letting your own pile settle the argument.
How long, and when it is done
Oregon State's guide describes a hot pile as taking about a month, followed by four to six weeks of curing, a rest period when the material is no longer heating and finishes stabilizing. Other sources give shorter and longer times: Missouri's extension puts the fast method at as little as six weeks (Missouri's G6956), and EPA puts a well-kept pile at about three to five months, then at least four weeks of curing. Your pile may take longer. Convention
Finished does not mean a date. EPA's signs for ready compost are a pile that no longer heats after mixing and food scraps that are no longer visible. Finished compost gives the signs, the germination test and curing in full, and it comes back to your log: a batch's last few readings, near air temperature, are one of the signs. See signs of finished compost and curing.
When a batch has finished heating, you can leave it to cure in place or, if you have a second bay, move it there. If you move it, start the next batch in the empty bay with material you collected in the meantime.
Working safely around a hot pile
- Do not reach into the center of an active pile. The USDA Natural Resources Conservation Service's Alaska dog-waste composting guide, a source we use only for this point, says not to plunge a hand into the center of an active pile, and notes that in very rare cases extremely high temperatures can cause a pile to ignite (NRCS Alaska's Composting Dog Waste fact sheet, hosted by EPA). Use the thermometer probe and a fork.
- Wear gloves and wash your hands afterward. The same guide notes that compost contains mold and fungus spores that may cause an allergic response in sensitive people.
- Wet dry material before you turn it. CDC's aspergillosis page says to try to avoid dusty areas, and to wear an N95 respirator if you cannot avoid them; it lists people with weakened immune systems or lung diseases as at higher risk (CDC's Preventing Aspergillosis page). The page does not mention compost, so we pass it on as a general dust precaution, and a doctor settles what applies to you. Ask a doctor
- Keep the pile away from buildings and fences. NC State advises more than six feet from your home or wooden structures.
The sources side by side Go deeper
The chapter gave one default for temperatures and for turning. This section sets out what the other sources say, so that you can see how much they differ and what is behind the figures. The Limits box above applies to everything here.
Temperatures, durations and turning
| Source | Temperatures | Duration at temperature | Turning |
|---|---|---|---|
| NC State (AG-791) | Above 131°F for most pathogens; above 145°F for most weed seeds; near 160°F can kill decomposers | Not given | Once a week; an unmixed pile takes three to four times longer |
| New York State DEC | 131°F for pathogens; 140°F for weed seeds; a hot pile reaches 130 to 160°F; above 170°F for more than a few hours is not recommended | When maintained for 72 hours | Twice a month |
| University of Arizona (AZ1632) | 130°F kills most pathogens and weed seeds | Seven consecutive days | Not given |
| Texas A&M (Don't Bag It) | Optimum range 135 to 175°F, with 150°F usually the most satisfactory | Not given | Not given |
| Clemson (HGIC 1600) | Most harmful pathogens and weed seeds destroyed over 140°F, checked at least 12 inches from the surface | Not given | Once a week |
| Penn State Extension | Not given | Not given | Frequently in the first 3 to 4 weeks, then once or twice |
| Utah State | 110 to 150°F destroys more pathogens, weed seeds and fly larvae; a rapid rise to 120 to 140°F | Maintained for several weeks | Weekly turning can give compost in one to two months; monthly, four to six months |
| University of Missouri (G6956) | 110 to 140°F in four to five days; optimum 135 to 140°F; occasionally 170°F | Not given (its week to a month is the time to reach 110°F) | Not given |
| WSU Whatcom brochure | 130 to 150°F reduce pathogens and weed seeds | Not given | At least once a week (its own question) |
| Oregon State (EM 9475; Extension article) | 135°F may kill pathogens and weed seeds; the article targets 130 to 135°F | Not given | Restack at 5 to 10 days when the pile cools (EM 9475); turn after the first three to five weeks (the article) |
Three things stand out. Utah's range starts at 110°F and Missouri's expected center range starts at 110°F, below the 130 or 131°F that others give for pathogens. Only three of the sources (New York, Arizona and Utah) name a time at temperature, and the times run from 72 hours to several weeks. And the sources do not agree on turning, from weekly to twice a month, though several tie it to the pile cooling. We use NC State's temperatures and Oregon State's turning interval as defaults, with the log as the check. Where a number comes from a Master Gardener or extension article, the word "may" or "should" in the source is carried here.
Two things to avoid: do not average these into a new rule, and do not read one agency's duration as a promise about your pile. Different agencies wrote for different purposes, and none tested a backyard pile.
Mixing proportion
The chapter used EPA's two to three parts browns to one part greens by volume. Oregon State's extension guide for hot composting says equal amounts of greens and browns. Both are volume rules of thumb for a mix that is not weighed, and the mix that works depends on how wet your greens are. If your batch smells sharp, like ammonia, it may have too many greens or too much water; Troubleshooting and the troubleshooting table cover it.
Federal standards, and where 131°F comes from
The federal Part 503 rule requires, for sewage sludge composted in a within-vessel or static aerated pile, a temperature of 55°C (131°F) or higher for three days, and for a windrow, 55°C or higher for 15 days or longer with at least five turnings during that period (40 CFR Part 503, Appendix B). The same appendix describes a lesser process: 40°C or higher for five days, with more than 55°C for four hours during those five days. It applies to sewage sludge, not home compost.
The USDA National Organic Program has a related rule for certified organic producers: compost must start at a carbon-to-nitrogen ratio of 25:1 to 40:1 and keep a temperature between 131 and 170°F for three days in an in-vessel or static aerated system, or for 15 days in a windrow turned at least five times (7 CFR 205.203). It governs those producers' own practice. Neither rule describes a household pile or tells a household how long its pile needs to stay hot, but both pair a duration with turning when the pile is a windrow.
Winter hot piles
New York State's guide suggests building a larger pile for winter and covering the top with about a foot of leaves or other lightweight browns, or setting straw bales or bagged leaves around it. Utah State says to increase the pile size or insulate it with an extra layer such as straw (Utah State's backyard composting guide). Neither source gives a result for how hot an insulated winter pile gets. If you try it, your log is the result.
What a Danish study measured
A Danish study followed six single-family home composting units for a year. The units mixed most often, weekly, emitted the most methane and nitrous oxide (Andersen and colleagues, Technical University of Denmark). It was six units in one country, and it does not say that turning a hot pile is a mistake or that it is climate-neutral. It is one reason this manual makes no climate claim for any home method.
See it: Ingrid builds, Tomás decides no See it
Ingrid Sorensen is a fictional person, and every fact about her here is invented for this manual. She is 61 and lives on 5 acres in a northern state with black bears. She has a large vegetable garden, and a three-bay pallet bin with each bay about 4 by 4 by 4 feet. She builds hot batches from May to September. Kitchen scraps go into a sealed tumbler in bear season and into the batches otherwise. No meat, fish or dairy goes outdoors. Her materials are grass from her own untreated meadow, garden trimmings and leaves.
What her June log shows. The center read 138°F on days 3 to 6. It was cooling by day 8, and she turned the batch on day 8 and again on day 16.
Known: the logged readings; the turn dates; the bay size. A bay of 4 by 4 by 4 feet is 64 cubic feet, about 2.4 cubic yards, so a bay filled to the top is above the 27 cubic feet figure. Her assumptions: the readings are from the center of the pile, taken with a long-stem thermometer, so the 138°F describes one spot. Open: how full the batch was (her measurement settles that); whether the edges and the top of the pile reached the same temperatures (more readings at other depths would show it, and nothing in her log does); whether the batch is finished (Finished compost, the signs and the germination test, settle that, not the log).
What the log does and does not say. By NC State's figures, 138°F at the center is above the 131°F pathogen figure and below the 145°F weed-seed figure. Under these assumptions the readings of 138°F on days 3 to 6 are above the first figure and below the second, and the log does not show that any other part of the pile got there. The first turn fell within Oregon State's 5 to 10 days. She keeps seeded weeds out of the batches regardless of temperature, which is the choice this manual makes by default, and her log does not show a pile hot enough at every spot to rely on for them. Whether bears come to a batch that takes kitchen scraps is Site and bin's question; this manual does not say what will keep a bear away.
What she writes in the log's notes. "Batch 3, June. Center 138 on days 3 to 6. Turned day 8 and 16. One probe spot. Seeded weeds not added."
Tomás Reyes is a fictional person, and every fact about him here is invented for this manual. He is 34 and lives alone in the downstairs of a rented duplex in a northern city where the ground freezes from December to March. He has a fenced backyard of about 18 by 24 feet and runs one enclosed plastic bin of about 11 cubic feet, add-and-cover.
Known: his bin is about 11 cubic feet, roughly 40 percent of 27; he chose low attention. His assumptions: he does not need compost within two months; he will not turn a pile weekly. Open: whether that changes (his review in Your compost system settles it).
The written no. Tomás reads the three questions and answers no to the second and third. On the plan he writes: "No hot composting. Reason: low attention, the bin is about 11 cubic feet, I do not want to turn weekly. Review in Your compost system." That is the whole chapter for him. It is a complete result, not a skipped step.
Free demonstrations of hot composting, from sources we fetched on 2026-10-26:
- Oregon State Extension's hot-compost troubleshooting article: what to check when a pile will not heat, smells or draws animals.
- Cornell's Monitoring Compost Temperature page: how to take readings in a pile.
Practice: decide, then log a batch Practice
Minimum: answer the three questions and write yes or no on the system plan with the reason. If yes, build one batch and log a week of center temperatures. Time: the decision is one sitting, once; a batch has a build day, daily readings for a week and a turn when the center cools. Materials: the system plan, the pile calculator, a long-stem compost thermometer, a fork, gloves, a hose or watering can, enough browns and greens for about a cubic yard.
- Answer the three questions on the plan. Do you need compost within about two months, have a cubic yard of material and room, and will you turn weekly? Write yes or no, with the reason in your own words.
- If yes, measure your bin or stack. Enter it in the pile calculator and compare it to 27 cubic feet.
- Collect browns and greens, then build the batch at once. Cut small, mix, wet to a wrung-out sponge, and put any food scraps in the middle under browns.
- Read the center each day for a week. Write the date and temperature in the bin log, with moisture and smell.
- Turn when the center cools, and log the turn. Check the moisture as you restack.
Take readings at three depths. On one day, read the temperature near the top, in the middle and near a side, and write all three in the notes column. The difference between them is the reason a reading at the center is not a reading of the pile.
Your system Your system
Check your work Check your work
Criteria
- Does the plan say yes or no to hot composting, with a reason in terms of your time, your material and your space?
- For a yes: does the log show a build date, a size you checked against 27 cubic feet, and a week of center temperatures?
- Did you log the turn and the reading afterward?
- Do the notes keep to what you saw ("center 138°F") and leave out claims the reading cannot support ("killed everything")?
Worked example
Ingrid's log has a batch name, a build in June, readings on days 3 to 6, turns on day 8 and day 16, and a note that it is one probe spot and that seeded weeds were not added. It does not say the batch is safe or finished. Tomás's plan has a no, three short reasons and a review date. Both pass.
Common mistakes
- Building the pile a bucket at a time. Fresh additions cool a heating pile. Collect first, then build.
- Reading the temperature at the edge. The center is where a pile is hottest. Push the probe in.
- Treating one number as a result. A single reading at one spot is not a result for the whole pile.
- Adding seeded weeds or diseased plants because the pile got warm. A pile that reached a temperature at its center did not reach it everywhere.
- Turning by the calendar when the pile has not cooled. The thermometer sets the day; the calendar is the starting guess.
- Adding meat, dairy or grease because the pile is hot. The stays-out list in What goes in is the same for hot and cold piles.
What is still unresolved
Whether your batch reached a temperature at every spot is not settled by a center reading. Whether it is finished is settled by the signs and the germination test in Finished compost. Whether a hot pile stays in your plan is settled by the review in Your compost system.