Composting Bin Size Calculator

Enter household size and food scraps per person per week. The best number is your own: keep a caddy for a week, weigh it, and divide by the people in the house. Three pounds each for four people is 12 lb a week.

Add weekly yard waste that will go in the bin as green material: fresh grass clippings, green trimmings, spent plants. The example adds 5 lb, making 17 lb of greens.

Set the browns-to-greens ratio by volume. At 2:1, the 2.55 gallons of greens need 5.1 gallons of dry leaves, straw or shredded cardboard alongside them, for 7.65 gallons of total weekly input.

Set turnover weeks: how long material accumulates before you empty the bin, move it to a second bay or stop adding. Eight weeks gives 61.2 gallons of raw input, which the 0.7 factor reduces to a recommended 42.8 gallons (5.7 cu ft).

The chart places that result against a countertop caddy (0.3 cu ft), a small tumbler (4 cu ft), a medium stationary bin (10 cu ft) and a 3 × 3 × 3 ft hot pile (27 cu ft). This household sits between the small tumbler and the medium bin; rounding up to the medium bin leaves room for bulky browns.

Weekly greens (lb) = household size × food scraps a person a week + weekly yard waste.

Weekly greens (gal) = weekly greens (lb) × 0.15. The calculator assumes each pound of greens fills 0.15 gallons, the same factor used by the compost impact calculator.

Total weekly input (gal) = weekly greens (gal) × (1 + browns ratio). A 2:1 ratio triples the greens volume; 3:1 quadruples it.

Raw volume (gal) = total weekly input × turnover weeks, with turnover floored at one week.

Recommended bin (gal) = raw volume × 0.7. Recommended bin (cu ft) = gallons ÷ 7.48.

Checking the 0.15 factor. It implies greens weighing about 6.7 lb a gallon, or roughly 50 lb per cubic foot. EPA's *Volume-to-Weight Conversion Factors* (April 2016) list food waste at 463 lb per cubic yard (about 0.44 gallons per pound) and cite other measurements of 22 to 45 lb per cubic foot. Every one of those measurements is lighter than the calculator's assumption, so real greens volume can be well above its figure. If your bin fills faster than predicted, this is why.

Two people, kitchen scraps only. 2.5 lb each a week, no yard waste, a richer 3:1 browns ratio and a 6-week turnover. Weekly greens are 5 lb, or 0.75 gal; adding browns at 3:1 gives 3.0 gal a week; six weeks of that is 18 gallons raw, and the recommended bin is 12.6 gallons, about 1.7 cubic feet. A small tumbler is more than enough, and far below the size a pile needs to heat itself.

Six people with a lawn. 3 lb each a week plus 15 lb of grass clippings, 2:1 browns and an 8-week turnover. Weekly greens are 33 lb (4.95 gal), total weekly input 14.85 gal, and the recommended bin is 83 gallons, about 11.1 cubic feet, a medium stationary bin or a double bay.

Household size and yard waste scale the answer linearly, but the browns ratio and turnover matter just as much. The two-person household used a higher browns ratio; the six-person household adds almost as much again in grass clippings as the family's own scraps. Halving the turnover from eight weeks to four halves any of these results.

The browns ratio is really about carbon and nitrogen. Cornell Composting's *Compost Chemistry* guidance puts the ideal starting carbon-to-nitrogen ratio at about 30:1 by weight. Below that, surplus nitrogen escapes as ammonia and the bin smells; above it, microbes lack nitrogen and the pile stays cool and slow.

Cornell's table shows why browns and greens have to be mixed. Greens are nitrogen-rich: vegetable scraps run about 15-20:1, grass clippings 15-25:1 and coffee grounds about 20:1. Browns are carbon-rich: autumn leaves 30-80:1, straw 40-100:1, wood chips or sawdust 100-500:1, and newspaper or corrugated cardboard around 560:1.

The ratio is by weight, but the calculator's browns input is by volume. Dry leaves and straw are light and bulky, so a bucket of them weighs far less than a bucket of wet scraps. Measuring browns in buckets rather than pounds is simply the practical way to work toward Cornell's weight-based target, and the EPA's two-to-three-times-by-volume rule is that kitchen version.

A practical sign you have it right: a bin that smells of ammonia or goes slimy needs more browns; one that stays dry and cold needs more greens or water.

Compost loses a lot of volume as it works. Air pockets collapse as material settles, and microbes convert carbon to CO2. Cornell notes the C:N ratio falls from about 30:1 to 10-15:1 in finished compost because each round of decomposition releases carbon. The EPA says a pile will shrink to about one-third of its original size after curing, and that a well-maintained pile is finished in about three to five months, while an untended one may take a year.

The calculator's 0.7 factor is gentler than that, because a bin that is still being filled holds a mix of fresh and part-decomposed material. For the four-person example, 61.2 gallons of raw input needs a 42.8-gallon bin while filling, but fully cured it would come down to roughly a third of 61.2 gallons, about 20 gallons of finished compost.

That gap is why two-bay systems work well: fill one side for the turnover period, then let it cure while the second side fills. Each bay only needs the calculator's size.

This is a volume-planning estimate. The 0.15 gal/lb factor sits at or beyond the dense end of EPA's food waste measurements, so real volumes are often larger. Size up rather than down. Bulky browns like whole leaves and straw also take more space per pound than the model assumes.

The 0.7 factor is a fixed allowance for settling during the fill period. Actual shrinkage depends on climate, moisture, particle size and turning, so a warm, frequently turned bin shrinks faster than a cold, untouched one.

The calculator sizes for volume, not heat. UNH Extension recommends a pile of at least one cubic yard, 3 ft wide by 3 ft tall by 3 ft long, noting that a smaller pile will not generate enough heat to break down the materials. If you want hot composting and your result is well under 27 cubic feet, build to that minimum instead.

Once your bin is sized, the Compost Impact Calculator estimates the CO2e avoided, finished compost volume, and nutrient value your composting produces from the same weekly waste inputs. The Garden Fertilizer NPK Calculator helps balance finished compost against other amendments to hit a target nutrient ratio, the Food Waste Impact Calculator quantifies what food waste costs before it ever reaches the bin, and the Recycling Impact Calculator covers the landfill-diversion side of your household waste stream.