Compost ratio calculator (carbon to nitrogen)
This compost ratio calculator works out the carbon-to-nitrogen (C/N) ratio and the moisture of a mix of up to three materials – and how much of one material you need for a target such as 30 : 1. 10 kg of grass clippings with 5 kg of leaves give a C/N ratio of 28.6 : 1 at 67 % moisture; 5.9 kg of leaves would bring it to 30 : 1. The formulas come from the Cornell Waste Management Institute.
Source: Cornell Waste Management Institute – Cornell Composting: C/N Ratio and Moisture Content (T. Richard, N. Trautmann, ©1996; formulas and grass/leaves example; read 09.10.2026). Updated: .
How it is calculated
C/N of the mix = total carbon ÷ total nitrogen, both counted on the dry matter. Cornell’s formula weights each material by its wet weight Q, its carbon C, its nitrogen N and its moisture M: R = Σ Q·C·(100 − M) ÷ Σ Q·N·(100 − M). Carbon follows from %C = %N × C/N.
Typical values of compost materials
| Material | N (% dry) | C/N | Moisture |
|---|---|---|---|
| Grass clippings | 3.4 | 17 : 1 | 82 % |
| Food waste | 1.9–2.9 | 14–16 : 1 | 69 % |
| Vegetable scraps | 2.7 | 19 : 1 | 87 % |
| Horse manure | 1.6 | 30 : 1 | 72 % |
| Leaves | 0.9 | 54 : 1 | 38 % |
| Straw | 0.7 | 80 : 1 | 12 % |
| Sawdust | 0.24 | 442 : 1 | 39 % |
| Cardboard | 0.10 | 563 : 1 | 8 % |
The values are averages from Appendix A of the On-Farm Composting Handbook; for food waste the calculator uses the middle of the range (2.4 % N, 15 : 1). Cornell recommends a C/N ratio of about 30 : 1 at the start; finished compost is usually close to 10 : 1. Moisture should be 50–60 %. Too little nitrogen and the pile does not heat up; too much and it can get too hot or go anaerobic.
Examples
- 10 kg grass + 5 kg leaves: C/N 28.6 : 1 and 67.3 % moisture; 5.9 kg of leaves give 30 : 1.
- 10 kg food waste + 10 kg leaves: C/N 31.7 : 1 and 53.5 % moisture. For 30 : 1 with 10 kg food waste: 8.3 kg leaves, 3.6 kg straw or 2.3 kg cardboard.
- Cornell’s example: 10 kg grass (2.4 % N, 45 % C, 77 % moisture) needs 3.5 kg of leaves (0.75 % N, 50 % C, 35 % moisture) for 30 : 1 – but then the moisture is above 66 %. With 6.8 kg of leaves (the amount for 60 % moisture) the ratio is just under 37 : 1.
For raised beds and garden soil see the raised bed soil calculator; fertilizer amounts are in the fertilizer calculator.
Limits
The table values are typical, not measured for your pile: grass can contain 2–6 % nitrogen, leaves 0.5–1.3 %. Cornell notes that moisture is often the more critical value – when in doubt err on the high C/N side, which composts more slowly but with fewer problems. For exact numbers a lab can test nitrogen and carbon.
Frequently asked questions
What is the best C/N ratio for compost?
Cornell recommends about 30 : 1 at the start. During composting carbon escapes as CO₂, and finished compost is usually close to 10 : 1.
How many browns per greens?
It depends on the materials. With the handbook values 10 kg of grass clippings need about 5.9 kg of leaves for 30 : 1, but only 2.6 kg of straw.
How do I calculate the C/N ratio of a mix?
Add up the carbon and the nitrogen of all materials on a dry-matter basis and divide: R = Σ Q·C·(100 − M) ÷ Σ Q·N·(100 − M).
Why does moisture matter for the C/N ratio?
Carbon and nitrogen sit in the dry matter. Wet grass at 82 % moisture brings much less dry matter per kilogram than dry straw at 12 %.
How wet should compost be?
Cornell gives 50–60 % moisture. Above 60 % the pile may go anaerobic and smell.
What C/N ratio does cardboard have?
About 563 : 1 at 0.10 % nitrogen – a strong carbon source. 2.3 kg of cardboard balance 10 kg of food waste to 30 : 1.
Sources and legal basis
- Cornell Waste Management Institute – Cornell Composting: C/N Ratio and Moisture Content (T. Richard, N. Trautmann, ©1996; formulas and grass/leaves example; read 09.10.2026)
- On-Farm Composting Handbook, Appendix A, Table A.1 – typical characteristics of selected raw materials (Cornell Waste Management Institute; read 09.10.2026)
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