Composting Carbon to Nitrogen Ratio Calculator
Calculate the C:N ratio for your compost pile from browns and greens by weight.
Returns your ratio vs the ideal 25-30:1 and a mix adjustment guide.
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:
| Material | C:N Ratio | Type |
|---|---|---|
| Wood chips / sawdust | ~400:1 | Brown |
| Cardboard (shredded) | ~350:1 | Brown |
| Paper (newspaper) | ~175:1 | Brown |
| Dry straw | ~80:1 | Brown |
| Dry leaves | ~60:1 | Brown |
| Coffee grounds | ~20:1 | Green |
| Manure (cow / horse) | ~20:1 | Green |
| Fresh grass clippings | ~17:1 | Green |
| Food scraps (veg/fruit) | ~15:1 | Green |
| Manure (chicken) | ~10:1 | Green |
The calculation, and why the obvious formula is wrong
The tempting move is to average the C:N ratios by weight. It does not work, because a ratio is not something you can average. Mix a kilo of leaves at 60:1 with a kilo of grass at 17:1 and averaging gives 38.5:1, which would tell you the pile is carbon-heavy and needs more greens.
Follow the actual carbon and nitrogen instead. Both kilos carry roughly the same carbon, but the grass carries three and a half times the nitrogen. Add the carbon up, add the nitrogen up, divide:
C:N of the mix = Total weight ÷ Σ(weight of each material ÷ its C:N)
That gives 26.5:1 for the same two kilos, which is dead centre of the ideal band. This calculator uses that formula. The averaging shortcut always reads high, so it will tell you to add greens to a pile that is already perfect, and a pile pushed green-heavy is the one that goes slimy and smells of ammonia.
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 uses weight ratios — any consistent unit (kg, lbs, or even cups) works since we’re computing a ratio.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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