Last updated: 2026-06-21

Compost C:N Ratio

A compost C:N ratio calculator estimates the carbon-to-nitrogen balance of a compost mix. Enter the mass of your carbon-rich browns and nitrogen-rich greens along with each material's own C:N ratio, and it returns the blended ratio and whether the mix sits in the band that composts quickly and without smell. It is for anyone building a compost pile, balancing a bin, or troubleshooting one that has gone wet and sour.

Why the C:N ratio matters

Composting microbes use carbon for energy and nitrogen to build proteins, and they work best when the two are supplied in roughly the right proportion. The widely used target is about 25 to 35 parts carbon to one part nitrogen. With too much nitrogen the surplus escapes as ammonia and the pile smells; with too much carbon the microbes run short of nitrogen and the pile breaks down slowly.

How the blend is calculated

Each material has its own C:N ratio. When materials with a similar carbon content are mixed, the carbon term cancels and the blended ratio is the total mass divided by the sum of each material's mass divided by its ratio:

where $m_b$ and $m_g$ are the masses of browns and greens and $r_b$ and $r_g$ are their C:N ratios. This is a mass-weighted harmonic mean, which is the correct way to average ratios that share a common numerator — here, carbon.

Worked example

Mixing 5 kg of dry leaves at 60:1 with 3 kg of food scraps at 15:1:

The blend lands near 28:1, comfortably inside the balanced band, so this mix should heat up and compost cleanly. If it instead came out below 20:1 you would add more browns; the ratio falls naturally over time as carbon leaves the pile as carbon dioxide, ending near 10 to 15:1 in finished compost. To put that finished compost to work, see Soil Amendment Calculator and Raised Bed Soil Calculator.