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Compost Calculator
Use this free compost calculator to get your pile's carbon-to-nitrogen (C:N) ratio right. Enter your greens and browns by weight or by volume, and it tells you the current ratio, how much of either material to add to reach the ideal 30:1, and how much finished compost you can expect.
How it works
Compost microbes need carbon for energy and nitrogen to build their bodies, and the balance between the two decides how fast a pile heats up and breaks down. The ideal mix is about 30 parts carbon to 1 part nitrogen. Too much nitrogen and the pile turns slimy and smells of ammonia; too much carbon and it sits cold and barely rots. This calculator estimates the ratio from the amounts you enter, using the standard working figures of 15:1 for greens and 60:1 for browns.
Greens are the wet, nitrogen-rich materials: grass clippings, kitchen scraps, coffee grounds, fresh manure, and green garden trimmings. Browns are the dry, carbon-rich materials: fallen leaves, straw, shredded newspaper and cardboard, sawdust, and small twigs. Enter each amount in pounds or cubic feet — the math works the same either way, as long as you use the same unit for both. The bin or pile size tells you how full your container is in volume mode, and gives a rough volume estimate in weight mode.
When the ratio is below 30:1, the pile has too much nitrogen, so the calculator works out how many pounds or cubic feet of browns bring it up to 30:1. When it is above 30:1, it works out the greens needed to bring it down. A finished pile shrinks a lot as microbes eat it and water evaporates, so the calculator estimates your finished compost at about half the starting amount — a common planning figure for home piles.
A few practical points make the numbers work better in real life. Shred or chop browns like leaves and cardboard so they break down faster, and mix layers rather than dumping one material in a thick slab. Keep the pile as damp as a wrung-out sponge, and turn it every week or two to add air. A pile of at least 3 feet on each side — about 27 cubic feet — holds heat well enough to compost quickly, which is why 27 cubic feet is the default bin size here.
Worked example
A gardener in Ohio starts a pile with 30 lb of grass clippings and kitchen scraps (greens) and 10 lb of dry leaves (browns), in a 27-cubic-foot bin.
Step 1: Current ratio — (15 × 30 + 60 × 10) ÷ (30 + 10) = (450 + 600) ÷ 40 = 1,050 ÷ 40 = 26.3:1. The pile is a little nitrogen-heavy.
Step 2: Browns needed — total browns for 30:1 = greens ÷ 2 = 30 ÷ 2 = 15 lb. The pile already has 10 lb, so add 15 − 10 = 5 lb of browns.
Step 3: Finished compost — total input is 40 lb, so the finished pile is about 40 × 0.5 = 20 lb.
The pile's C:N ratio is 26.3:1, so adding 5 lb of dry leaves brings it to the ideal 30:1, and the gardener can expect about 20 lb of finished compost — roughly enough to top-dress a small vegetable bed.
FAQ
What is the ideal C:N ratio for compost?
About 30 parts carbon to 1 part nitrogen. At 30:1 the microbes have enough carbon for energy and enough nitrogen to grow, so the pile heats up and breaks down quickly. Ratios from 25:1 to 35:1 still work well for a home pile.
What counts as greens and what counts as browns?
Greens are wet, nitrogen-rich materials such as grass clippings, fruit and vegetable scraps, coffee grounds, and fresh manure. Browns are dry, carbon-rich materials such as fallen leaves, straw, shredded paper and cardboard, sawdust, and twigs. The calculator treats greens as 15:1 and browns as 60:1.
How much finished compost will I get?
Expect roughly half the starting amount by volume — a pile shrinks as microbes consume material and moisture evaporates. The calculator uses a 50 percent estimate, so 40 lb of mixed material gives about 20 lb of finished compost.
How big should a compost pile be?
About 3 feet wide, 3 feet deep, and 3 feet tall, or 27 cubic feet, is the usual minimum for a pile that heats up well. Smaller piles still work but compost more slowly, while a larger pile holds heat longer and finishes faster.
Can I measure by volume instead of weight?
Yes. Switch the calculator to volume mode and enter cubic feet for both greens and browns. The ratio math is the same, and the calculator also shows what percentage of your bin the pile fills.