Compost Ratio Calculator
Balance your compost pile: split a bin into browns and greens, estimate the real carbon-to-nitrogen (C:N) ratio and moisture of a mix of up to four materials, or find how much browns to add to the greens you have to reach 30:1.
How it works
Split: browns = volume × browns ÷ (browns + greens), greens = volume × greens ÷ (browns + greens) · Weight = volume × bulk density · Dry matter = weight × (1 − moisture) · N = dry matter × %N, C = N × (C:N) · Mix C:N = Σ C ÷ Σ N (Cornell) · Mix moisture = Σ water ÷ Σ weight · Browns per volume of greens for target R = (C_g − R·N_g) ÷ (R·N_b − C_b)
Good compost needs roughly a 25–30:1 carbon-to-nitrogen ratio by weight and about 50–60% moisture. For typical materials that works out to about 2–3 volumes of "browns" (dry, carbon-rich) per volume of "greens" (moist, nitrogen-rich). Too many greens and the pile turns slimy and smells of ammonia; too many browns and it barely heats up. Split a bin divides any volume by the parts ratio you choose. It also estimates the real C:N and moisture for the materials you pick, because a part of sawdust holds far more carbon than a part of leaves, and it shows the volume ratio that would hit your target with that pair. Check a mix does the same for up to four materials. Balance a pile works out how much browns to add to the greens you have (or the reverse) to reach the target. The C:N and moisture estimates use Cornell’s mixture formula: each material’s volume is converted to weight, then to dry matter, nitrogen and carbon, and the totals are divided. The material figures are typical published values, so expect real piles to differ.
Worked example
A 27 ft³ (1 cubic yard) bin at a 3:1 browns-to-greens ratio: 4 total parts, so browns = 27 × (3 ÷ 4) = 20.25 ft³ and greens = 27 × (1 ÷ 4) = 6.75 ft³. With dry leaves (about 200 lb/yd³, 38% moisture, 60:1) and kitchen scraps (about 800 lb/yd³, 85% moisture, 15:1) that is about 150 lb of leaves and 200 lb of scraps. Their dry matter carries about 57 lb of carbon and 1.6 lb of nitrogen, so the C:N is about 36:1 and the moisture about 65%: slightly carbon-rich, so the pile will be slow but won’t smell. With these two materials, 30:1 needs about 1.7 parts leaves to 1 part scraps. Cornell’s grass-and-leaves example checks out the same way: 10 kg of grass (77% moisture, 2.4% N, 45% C) with 3.5 kg of leaves (35% moisture, 0.75% N, 50% C) gives about 30:1.
Sources: Cornell Waste Management Institute — Getting the right mix (moisture and C:N mixture equations) · Cornell Composting — Compost chemistry (C:N ratios of common materials) · University of Idaho Extension — Composting feedstocks and their characteristics · Penn State Extension — Home composting: a guide for home gardeners
Frequently asked questions
What counts as browns and greens?
Browns are dry, carbon-rich materials: fallen leaves, straw, shredded cardboard, wood chips, sawdust. Greens are moist, nitrogen-rich materials: vegetable scraps, fresh grass clippings, coffee grounds, manure.
Why does my compost smell bad?
A sour or ammonia smell almost always means too many greens (excess nitrogen) or too little air. Mix in more browns and turn the pile to add oxygen, and the odor should clear within a few days.
Is the ratio by weight or volume?
The 25–30:1 C:N target is by weight, but gardeners work by volume. The calculator converts your volumes to weight, then to dry matter, using typical densities and moisture. That is why 3 parts leaves to 1 part food scraps comes out near 36:1, while the same 3:1 with sawdust is far higher.
How much browns do I add to my kitchen scraps or grass clippings?
Choose Balance a pile, pick your green and brown materials and enter the amount you have. With dry leaves, kitchen scraps need about 1.7 volumes of leaves per volume of scraps to reach 30:1. Very carbon-rich browns such as cardboard or sawdust need much less.
How wet should compost be?
About 50–60% water by weight: damp like a wrung-out sponge. Below about 40% the microbes slow down; above about 65% the pile goes airless and smelly. Wet greens such as grass and food scraps raise moisture, while dry browns soak it up.
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This calculator is for educational purposes. Double-check important results before acting on them.