Compost Mix Calculator

Direct answer

Calculate the carbon mass and nitrogen mass represented by each entered material, add each total, then divide total carbon by total nitrogen. Cornell Composting describes this mass-based C:N approach and notes that moisture and material availability can affect practical results; this tool only reports the ratio from your inputs.

Variables and formula

• Material mass: mass of each of two compost ingredients.
• Carbon content: carbon percentage by mass for that ingredient.
• Nitrogen content: nitrogen percentage by mass for that ingredient.

C:N ratio = Total carbon mass / Total nitrogen mass

Reproducible example

For 10 kg of a 50% carbon, 0.75% nitrogen material and 10 kg of a 45% carbon, 2.4% nitrogen material, carbon mass is 9.5 kg and nitrogen mass is 0.315 kg. 9.5 / 0.315 = 30.16:1.

Interpretation

The ratio is a traceable property of the entered composition data. It is not a prescription for what materials are safe, suitable or permitted to compost.

Assumptions and limitations

• Both materials use the same mass basis.
• Composition values are measured or suitable estimates for the actual materials.
• Moisture, oxygen, temperature, contamination and local requirements are outside the model.

Common mistakes

• Entering a C:N ratio where the calculator asks for carbon and nitrogen percentages.
• Combining dry-basis and wet-basis data.
• Ignoring moisture and aeration because the ratio looks plausible.

Assumptions

• Masses and composition percentages describe the same basis for both materials.
• The entered composition values are suitable estimates or measurements for the actual materials.
• Moisture, oxygen, temperature, particle size, contaminants and local composting rules are not calculated.

Sources and methodology

CalcMotive publishes the formula and assumptions so you can decide whether the estimate fits your use case. See our methodology standards.

• C/N Ratio; Cornell Composting; accessed Aug 9, 2026