What C:N ratio makes good compost?

A starting blend of roughly 25 to 30 parts carbon to 1 part nitrogen by mass is ideal. Microbes use carbon for energy and nitrogen to build protein in about that proportion. Too much carbon and the pile composts slowly; too much nitrogen and it loses nitrogen as ammonia and smells.

Why does dry matter matter in the calculation?

Published C:N ratios refer to dry mass, but feedstocks contain very different amounts of water. Fresh food waste might be 30 percent dry matter while straw is 90 percent. The tool multiplies each material's fresh weight by its dry-matter fraction so the blend reflects actual dry carbon and nitrogen, not just wet weight.

How is the blended ratio computed?

For each material the tool finds dry mass, then splits it into carbon and nitrogen using its C:N ratio. It sums total carbon and total nitrogen across all materials and divides total carbon by total nitrogen. This mass-balance approach is more accurate than averaging the ratios directly.

What are browns and greens?

Browns are high-carbon, dry materials like straw, dead leaves, woodchips, and cardboard. Greens are high-nitrogen, moist materials like fresh grass, manure, and food scraps. A workable pile is mostly browns by volume with enough greens to hit the target ratio.

My ratio is too high. What do I add?

Add nitrogen-rich greens such as fresh manure, grass clippings, or food waste, which have low C:N ratios. If it is too low, add carbon-rich browns like straw, shredded cardboard, or woodchips. The tool recalculates instantly so you can adjust amounts until you are in range.

Compost Recipe C:N Ratio Calculator

A compost pile heats and breaks down fastest when its carbon-to-nitrogen ratio starts near 25 to 30 to 1. This calculator blends your feedstocks using their dry carbon and nitrogen content — not just wet weight — to compute the true mix C:N ratio and tell you whether to add browns or greens.

How it works

Averaging published C:N ratios is wrong because materials differ in water and mass. The tool does a proper mass balance:

dry mass        = fresh weight x dry-matter fraction
nitrogen (dry)  = dry mass / (C:N ratio + 1)
carbon (dry)    = dry mass - nitrogen
blended C:N     = total carbon / total nitrogen   (summed over all materials)

Splitting each material’s dry mass into its carbon and nitrogen shares, summing, and dividing gives the real blended ratio. Embedded values cover common feedstocks; greens carry low ratios (manure ~15, food waste ~17, grass ~17) and browns carry high ones (straw ~80, woodchips ~400, dry leaves ~55).

Example

Mix 40 kg fresh straw (C:N 80, 90% dry) with 60 kg fresh manure (C:N 15, 25% dry). Straw dry mass is 36 kg, manure 15 kg. The blend lands around 30 to 1 — squarely in range and ready to heat up.

Tips

Aim to finish a fresh pile between 25 and 35 to 1; it will fall as composting proceeds.
If the ratio is too high (too much carbon) the pile stalls — add greens.
If it is too low (too much nitrogen) it smells of ammonia and loses fertility — add browns.