Compost Ratio Calculator
Balance carbon and nitrogen in a compost mix to hit the ideal C:N ratio.
Please note: An estimate based on the figures you entered. Real-world results vary with conditions this calculation cannot see, so treat it as a starting point rather than a final answer.
What the Compost Ratio Calculator does
Composting microbes need carbon for energy and nitrogen to build cells, in a ratio of roughly 30 to 1. Getting the mix right is what separates a heap that heats up and breaks down in weeks from one that either stinks of ammonia or simply sits there.
Formula
Nitrogen in a material = Weight × Nitrogen %Carbon = Nitrogen × its C:N ratioMix ratio = Total carbon ÷ Total nitrogenIdeal range: 25:1 to 35:1
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Material names | text | Yes | Use the keys from the reference table: grass, foodScraps, coffee, manureCow, manurePoultry, leaves, straw, cardboard, sawdust, woodChips. |
| Weight of each | text | Yes | In kilograms, one per material. |
| Target C:N ratio | number | Yes | Composting works best between 25:1 and 35:1. Accepts 1 or more. |
How to use it
- Enter Material names, Weight of each and Target C:N ratio.
- Select Calculate.
Worked example
100 kg of grass clippings mixed with 50 kg of dry leaves.
- Materials
- grass, leaves
- Weights
- 100, 50
Grass gives 2.4 kg N and 48 kg C; leaves give 0.45 kg N and 27 kg C. Total 75 kg C to 2.85 kg N — a ratio of 26.3:1, right in the ideal band.
Frequently asked questions
What happens if the ratio is wrong?
Too much nitrogen and the heap goes anaerobic and smells of ammonia as nitrogen escapes. Too much carbon and the microbes run out of nitrogen, so decomposition slows to a crawl.
Why is carbon calculated from nitrogen?
Because materials are published with a nitrogen percentage and a C:N ratio. Multiplying the two gives carbon directly, which is more reliable than separate carbon figures.