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Composting: C/N ratio and mix adjustment

How much carbon and nitrogen your mix has, and what to add to balance it.

Pre-sizing and estimating tool: the results do not replace a design, calculation, or report signed by a qualified professional, nor verification against the applicable standards.

Preliminary estimateRevision 12/11/2026
Material A (rich in carbon, e.g., straw)
Material B (rich in nitrogen, e.g., manure)
Target

The mix C/N ratio is the mass of carbon divided by the mass of nitrogen. A common reference range is 25 to 30, but the ideal point depends on the material and the method.

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—Mix C/N ratio
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C/N ratio by the proportion of material A

Mix of 100 kg of dry material, varying the fraction of A (the rest is B).

Tap the chart to see the values.

C/N ratio of each material

Ratio of each material on its own, for comparison.

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What if the data changes?

Drag the controls to simulate data different from what you entered (from 30% less to 30% more) and watch the result change instantly.

Tap the chart to see the values.

Approximate C/N ratio of common materials

Approximate literature ranges, only to guide the choice of materials. Your own material analysis is worth more than the table value.

Step by step with your numbers

The calculation done in steps, with the values you entered.

    The calculation memo gathers this page's inputs, results, equations, assumptions, and references, with fields for project, responsible person, and review. The full report also includes the charts and interpretations, in color. In the print window, choose Save as PDF.

    How to use

    Enter the dry mass and the carbon and nitrogen contents of two materials: one rich in carbon (such as straw) and one rich in nitrogen (such as manure). Enter the target C/N ratio. The calculator shows the ratio of the mixture and how much of the material that corrects the ratio to add.

    Hypothetical example

    400 kg of dry straw (40% C and 0.5% N) and 300 kg of dry manure (30% C and 2.0% N). Target of 25.

    Carbon = 400 × 0.40 + 300 × 0.30 = 250 kg. Nitrogen = 400 × 0.005 + 300 × 0.02 = 8 kg.

    C/N ratio = 250 ÷ 8 = 31.3. Since it is above the target, adding manure corrects the ratio: x = (25 × 8 − 250) ÷ (0.30 − 25 × 0.02) = 250 kg of manure.

    Precautions

    • Use dry mass and contents measured in the lab; table values are only estimates.
    • Microorganism respiration loses carbon during the process, and the ratio falls over the course of composting.
    • Monitor the temperature and moisture of the windrow: the ratio alone does not guarantee composting.

    Method, assumptions, and references

    Equations used

    C = Σ (mass × %C) ÷ 100; N = Σ (mass × %N) ÷ 100
    C/N ratio = C ÷ N
    Mass to add x = (t × N − C) ÷ (c − t × n), with c and n the fractions of C and N in the chosen material and t the target ratio

    Assumptions and limits

    • Dry mass and contents measured in the lab; table contents are estimates.
    • Does not consider the carbon lost through respiration during composting, which lowers the ratio over the process.
    • The target ratio is a common reference; the ideal point depends on the material and the method.

    Standards and technical documents

    • Kiehl, E. J. Organic Fertilizers. Piracicaba: Agronômica Ceres, 1985.
    • Inácio, C. T.; Miller, P. R. M. Composting: science and practice for organic waste management. Rio de Janeiro: Embrapa Solos, 2009.

    Frequently asked questions

    What happens if the ratio is high?

    Decomposition slows down because the microorganisms lack nitrogen. Adjust the mix with a nitrogen-rich material.

    What if the ratio is low?

    There is excess nitrogen, with a risk of ammonia loss and bad odor. Add carbon-rich material, such as straw or sawdust.

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