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Compost Calculator

Combine up to four materials using their wet mass, moisture, dry carbon percentage and C:N ratio. Optionally estimate water to add or moisture to remove for your own target.

Material 1
Material 2 (optional)
Material 3 (optional)
Material 4 (optional)

Results depend on the source and basis of your material data. Enter the C:N ratio and carbon percentage for dry matter; the calculator does not infer them. It does not model decomposition, temperature, aeration, pathogens, maturity, local rules or growing suitability, and does not prescribe a universal target. Water estimates are simple mass-balance estimates, not a composting recommendation.

How do I calculate the C:N ratio of a compost mix?

For each material, estimate dry mass from its wet mass and moisture. Multiply dry mass by the dry carbon percentage to estimate carbon mass, then divide by that material’s C:N ratio to estimate nitrogen mass. Add the carbon and nitrogen estimates separately; the mix ratio is total carbon divided by total nitrogen.

Frequently asked questions

How do I calculate the C:N ratio of a compost mix?

For each material, estimate dry mass from its wet mass and moisture. Multiply dry mass by the dry carbon percentage to estimate carbon mass, then divide by that material’s C:N ratio to estimate nitrogen mass. Add the carbon and nitrogen estimates separately; the mix ratio is total carbon divided by total nitrogen.

Are my compost material details uploaded?

No. This calculation runs locally in your browser; material names and measurements are not sent to a server or saved by this tool.

Can I calculate the C:N ratio using only each material’s ratio?

Not reliably. The calculation also needs each material’s dry mass and estimated carbon fraction, because C:N values alone do not tell how much carbon or nitrogen is in the amount added.

Does the target moisture tell me how to make good compost?

No. It only estimates water mass needed to reach the percentage you enter. Compost quality also depends on materials, particle size, oxygen, temperature, time and other conditions.