Composting Carbon to Nitrogen Ratio Calculator

Composting is the biological decomposition of organic materials into humus, a rich soil amendment. The speed and quality of composting is largely determined by the Carbon-to-Nitrogen ratio (C:N ratio) of your compost pile.

Why C:N ratio matters:

• Carbon (found in “browns”) provides energy for decomposing microorganisms
• Nitrogen (found in “greens”) provides protein for building microbial cells
• Ideal C:N ratio: 25:1 to 30:1

Too much carbon (high C:N, like 50:1 or higher) = slow decomposition, pile stays cool, takes months to years.
Too much nitrogen (low C:N, like 10:1) = pile heats up too fast, releases ammonia, smells bad.

C:N ratios of common materials:

The calculation, and the water nobody counts

A ratio cannot be averaged. What you can add up is the carbon and the nitrogen themselves, then divide one total by the other. That needs one more fact the C:N figure leaves out: how much of each material is water. The published ratios are measured on dry matter, and fresh grass is about 82% water against roughly 38% for fallen leaves, so a kilo of grass brings 180 g of solids to the pile and a kilo of leaves brings 620 g.

Dry weight = weight × (1 − moisture)
C:N of the mix = Σ dry weight ÷ Σ(dry weight ÷ its C:N)

Carbon makes up somewhere between 40 and 55% of the dry matter in every material on the list, so the dry weight stands in for it.

Mix a kilo of leaves at 60:1 with a kilo of grass at 17:1 and that gives 38:1. Carbon-heavy, and it wants more grass: about 2.25 kg of it for every kilo of leaves to reach 30:1. Two shortcuts get this wrong. Averaging the two ratios gives 38.5:1, close here only by luck. Running the sum on wet weight, as if the grass were all solids, gives 26.5:1 and calls the pile perfect, which is how a bucket-for-bucket heap of leaves and fresh clippings ends up sitting cold.

The water figures this calculator uses are typical values from the On-Farm Composting Handbook (NRAES-54, Cornell) as compiled in the Compost Foundation’s 2021 feedstock table. Coffee grounds and chicken manure come from Washington State University analyses in the same table.

Your own pile will differ. Clippings left to wilt for a day lose much of that water, and rain-soaked leaves gain it, so read the answer as a guide to which way to go rather than a lab result. If you mix by the bucket, the compost ratio calculator does the same sum by volume.

Target range: 25:1 to 30:1 for hot, active composting. Anything from 20:1 to 35:1 will compost well.

Practical tips:

• A simple rule of thumb: 3 parts browns to 1 part greens by volume
• Chop or shred materials into pieces under 2 inches (5 cm) for faster decomposition
• Keep moisture at 40–60%, the pile should feel like a wrung-out sponge
• Turn the pile every 1–2 weeks to add oxygen

Metric and imperial note: This calculator works on weight, so any consistent unit works: kilograms, pounds or ounces. Only the proportions matter. A cup or a bucket is a volume, and a bucket of leaves weighs a fraction of a bucket of kitchen scraps.