C/N Ratio Calculator
Enter carbon and nitrogen contents (same unit) to compute the carbon-to-nitrogen ratio C/N = carbon ÷ nitrogen, used for compost mix, organic-matter decomposition rate, and soil nitrogen immobilization or release.
Input Data
Results
At a glance:Enter carbon and nitrogen contents (same unit) to compute the carbon-to-nitrogen ratio C/N = carbon ÷ nitrogen, used for compost mix, organic-matter decomposition rate, and soil nitrogen immobilization or release.
Formula
C/N = carbon ÷ nitrogen.
$$C/N = \frac{C}{N}$$How to Use
- Enter the carbon and nitrogen contents in the same unit.
- The calculator returns the C/N ratio.
Approximate C/N of common materials and compost role
| Material | Approx. C/N | Compost role / note |
|---|---|---|
| Fresh grass / kitchen scraps (wet) | ≈ 15–20 : 1 | High-N 'green', fast decomposition, pair with carbon material |
| Mature compost / humus | ≈ 10–15 : 1 | Stable, directly supplies N |
| Ideal initial compost mix | ≈ 25–30 : 1 | Balanced C/N, smooth fermentation, good N retention |
| Dry straw / leaves | ≈ 60–80 : 1 | High-C 'brown', add N material to speed up |
| Sawdust / woody | > 200 : 1 | Extremely high C, alone severely immobilizes soil N |
Values vary widely with moisture and type; composting practice mixes 'green (high-N) + brown (high-C)' to bring overall C/N to about 25–30:1.
FAQ
Why is 24:1 often called 'ideal' for C/N?
Because decomposer microbes have a body C/N around 8–12:1, and while building themselves they respire away about 2 parts of carbon for every 1 part they assimilate into their bodies (as CO₂). In total, building 1 part of nitrogen-related tissue takes about 24 parts of carbon. So when the decomposed material's C/N is about 24:1, carbon and nitrogen supply best balance and decomposition is smoothest. Above it carbon is surplus and microbes lack nitrogen, grabbing it from soil; below it nitrogen is surplus and released.
What units for carbon and nitrogen content?
Any units as long as they are the same, because C/N is a ratio. Common inputs are carbon and nitrogen mass percentages (%) from elemental analysis, or grams per kg of sample (g/kg). Make sure both numbers share the same basis (e.g. both on a dry-weight basis); do not mix fresh weight and dry weight or the ratio distorts.
What if C/N is too high, and how to fix it?
Too high C/N (carbon-rich, nitrogen-poor) slows decomposition and microbes pull soil inorganic nitrogen, causing short-term crop N deficit (yellowing, stunted growth). Fix by adding high-N material (manure, urea, fresh greens) to bring overall C/N to about 25–30:1, or fully compost the high-C material before application — avoid burying fresh sawdust or straw directly.
Is a low C/N always good?
Not entirely. Low C/N means nitrogen is relatively ample, decomposition is fast, and crops get N, but too low (e.g. large amounts of fresh manure) causes nitrogen excess that easily volatilizes as ammonia (odor and N loss) or leaches as nitrate, polluting groundwater. Composting and fertilization balance 'enough N supply' against 'avoiding N loss'; the 25–30:1 initial mix is that compromise.
Does soil itself have a C/N, and what does it mean?
Yes. Soil organic matter C/N is usually about 10–14:1, reflecting highly humified, relatively stable organic matter. Higher soil C/N means slower organic-matter decomposition and conservative N supply; lower means faster mineralization and more N supply. Monitoring soil C/N helps judge organic-matter quality and N supply potential — part of soil-fertility assessment.
Related Tools
References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.