Calculators: browns, bin volume and worm bin size

Three calculators: how many browns to store for your scraps, how big a bin or pile is against one cubic yard, and how large a worm bin your scraps call for. All arithmetic; every assumption is listed.

1. Browns to store

Answers: how many gallons and bags of browns (dry leaves, shredded paper or cardboard) to keep for a year of scraps. Used in The slow pile and Leaves, grass and yard. Enter the gallons of food scraps you make in a week from your scrap log.

Assumptions behind the numbers

• The ratio is EPA's rule of thumb of two to three times as much browns as greens by volume (EPA's Composting at Home). Other sources give 1 to 1 up to 3 to 1; the choice here is yours.
• Loose leaves by volume, not weight. Leaves settle in a bag and shrink when shredded, so the count of bags is an estimate.
• Gallons convert to cubic feet at 7.48 gallons per cubic foot.
• The 30-gallon bag and the 52 weeks are this manual's defaults, not a source's. Change them to your bags and your season.

2. Bin and pile volume

Answers: how many cubic feet your bin or pile holds, how that compares with one cubic yard (27 cubic feet), the wire length for a round wire bin, and how long it takes to fill. Used in Site and bin and The hot pile. Measure in feet.

Assumptions behind the numbers

• One cubic yard, a cube three feet on each side, is 27 cubic feet. Texas A&M and Iowa State give about that as the minimum size for a pile that heats (Texas A&M, Iowa State). A smaller bin can still run as add-and-cover.
• The wire length for a round bin is the diameter times 3.14, the rule the University of Minnesota gives (UMN Extension), plus 6 inches of overlap, which is this manual's convention.
• Weeks to fill is an upper bound. Material shrinks as it breaks down, to about a third of its original size by EPA's account, so a bin fills more slowly than the arithmetic says.
• Two extension pages (Clemson and Penn State) write "3 cubic feet (3 feet x 3 feet x 3 feet)". A cube three feet on each side is 27 cubic feet, so that is a units error on those pages; this manual does not use the sentence.

3. Worm bin size

Answers: how much bin surface your scraps call for, a starting amount of worms and a starting feed. Used in Worm bins. Weigh one container of scraps once to get pounds; no source gives a conversion from gallons.

Assumptions behind the numbers

• Surface area: one square foot of bin surface for each pound of food scraps a week (NC State's worm composting bulletin). Depth: NC State gives 8 to 20 inches, while Oregon State and WSU Thurston County give about 12 inches at most, so 10 is the default.
• Starting worms: half a pound to one pound of worms for each pound of food a week, from the figures NC State, WSU Thurston County and Oregon State give.
• Starting feed: a quarter to a half pound of food per pound of worms a week, from Oregon State's guide (Oregon State's EM 9034), then raise it as the bin keeps up.
• The feeding figures across sources differ by about three and a half times, from one pound of food per pound of worms a week to half the worms' weight a day. Start low.