A compost bin that is too small overflows every summer; one that is too large never heats up. This calculator sizes a bin from your household’s kitchen scraps and garden waste.
How much waste?
Typical figures for planning:
- kitchen scraps: about half a gallon per person per week (fruit and vegetable peels, coffee grounds, eggshells);
- garden waste: grass clippings, leaves and trimmings vary hugely with the season and the size of the yard.
For 4 people plus 2.5 gallons of garden waste a week, that is 4 × 0.5 + 2.5 = 4.5 gallons a week.
The bin volume
Compost shrinks as it decomposes, often to half its starting volume or less. The bin must hold the waste added during the time it takes to mature:
bin volume = waste per week × weeks to mature × (1 − shrinkage)
Over 6 months (about 26 weeks) with 50% shrinkage: 4.5 × 26 × 0.5 = 58 gallons, about 7.8 cu ft. A common 80-gallon plastic bin is enough. For a hot pile that heats up well, aim for at least 3 × 3 × 3 ft, about 27 cu ft.
One bin or two?
Many gardeners use two or three bins: one being filled, one maturing, and sometimes one of finished compost. This lets you stop adding to a batch so it can finish. Tumblers mix easily but are small; open wooden or wire bays are cheap and roomy.
Balance browns and greens
Good compost needs a mix of nitrogen-rich greens (kitchen scraps, fresh grass) and carbon-rich browns (dry leaves, cardboard, straw), often about 2 to 3 parts browns to 1 part greens by volume. Keep it as moist as a wrung-out sponge and turn it every few weeks for faster results. See the compost C:N calculator for a precise mix.
How much compost will I get?
Finished compost is roughly a third of the volume of the raw waste, about 70 gallons a year in this example: enough to top-dress a vegetable garden or several flower beds.
What not to compost at home
Meat, fish, dairy, oily food, pet waste, diseased plants and weeds that have gone to seed. Some cities offer curbside collection of food scraps or free bins; check your local program.