Compost Calculator
How much brown material to mix with green for a target carbon-to-nitrogen ratio, or the C:N and moisture of a mix, by Cornell’s two-material method.
Result
Enter first material, c:n, first material, nitrogen, first material, moisture, second material, c:n, second material, nitrogen and second material, moisture to see the result.
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Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the Compost Calculator
- For each of the two materials, enter its carbon-to-nitrogen ratio, its nitrogen content and its moisture, from a test or a table you trust.
- Enter how much of the first material you have and the C:N you are aiming for.
- Read how much of the second material to add, or switch mode to find the C:N and moisture of a mix you already have.
Cornell’s two-material method
dry nitrogen of a material = weight × (1 − moisture) × nitrogen sharemix C:N = Σ dry nitrogen × C:N ÷ Σ dry nitrogensecond = first dry nitrogen × (target − C:N₁) ÷ (N₂ × (C:N₂ − target) × (1 − moisture₂))mix moisture = Σ weight × moisture ÷ Σ weightThe formula is Richard and Trautmann’s, from Cornell’s composting guide, rearranged with each material’s carbon written as its nitrogen times its C:N. Weights are as they come, wet, so the moisture turns them into dry matter first. The target must lie between the two materials’ ratios: no amount of a material at 67 to 1 brings a mix above 67.
Reading the result
The page also shows the moisture of the resulting mix. If it comes out very wet or very dry, the pile may need a third, drier or wetter material, or water, whatever its C:N.
With your numbers
- Mix C:N = Σ weight × (1 − moisture) × nitrogen × C:N ÷ Σ weight × (1 − moisture) × nitrogen
Quick reference
Material values vary widely; a test of your own materials beats a table.
| Job | Result |
|---|---|
| Cornell grass and leaves, 30 to 1 | Second material to add: 3.47 kg |
| Same, 25 to 1, 20 lb of grass | Second material to add: 3.4 lb |
| Dry materials, 30 to 1 | Second material to add: 9.82 kg |
| C:N of 10 kg grass with 6.8 kg leaves | C:N of the mix, to 1: 36.7 |
| Equal weights of grass and leaves | C:N of the mix, to 1: 41.2 |
| Cornell grass and leaves, 100 lb of grass | Second material to add: 34.74 lb |
Common questions about the Compost Calculator
- What is the right carbon-to-nitrogen ratio for compost?
- Cornell’s composting guide gives about 30 to 1 as the usual starting point for a pile, which is where the target starts here. It is a starting point rather than a rule, and you can enter any target between your two materials’ ratios.
- Why does the page need nitrogen and moisture as well as C:N?
- Because the ratio of a mix is not the average of the two ratios. It is the total carbon divided by the total nitrogen, and a material with more nitrogen and less water pulls the mix further towards its own ratio. Cornell’s method weighs each material by its dry matter and its nitrogen for exactly this reason.
- How much leaves do I need for 10 kilograms of grass clippings?
- With Cornell’s figures, grass at 2.4% nitrogen, C:N 18.75 and 77% moisture and leaves at 0.75% nitrogen, C:N 66.67 and 35% moisture, about 3.5 kilograms of leaves bring the mix to 30 to 1. The same guide finds that 6.8 kilograms of leaves are needed to bring the mix down to 60% moisture, a reminder that water can matter more than the ratio.
- Where do I find C:N values for my materials?
- Extension services and composting guides publish typical values, and a laboratory can test your own. Real materials vary widely with their source, age and moisture, which is why the page holds no table and asks you for the values.