Compost C:N Ratio Calculator
A fast, clean compost pile comes down to balancing carbon-rich browns against nitrogen-rich greens. The target carbon-to-nitrogen ratio is around 25:1 to 30:1 by mass. This calculator blends two materials, each with its own mass and known C:N ratio, using the standard mass-balance method, then reports the combined ratio and whether it falls inside your chosen target band. Enter each material's C:N ratio from a reliable composting reference; all values are editable inputs.
Compost C:N ratio formula
Carbon fraction = C:N / (C:N + 1)
Nitrogen fraction = 1 / (C:N + 1)
Total carbon = sum of (mass * carbon fraction)
Total nitrogen = sum of (mass * nitrogen fraction)
Blended C:N = total carbon / total nitrogen
This mass-balance method treats each material's C:N ratio as the split of its mass into carbon and nitrogen, ignoring inert moisture and ash for simplicity. The result is the effective carbon-to-nitrogen ratio the microbes experience.
Tuning your compost mix
- If the blended ratio is above your target, add more greens (low C:N) such as grass or food scraps.
- If it is below target, add more browns (high C:N) such as dry leaves, straw or shredded cardboard.
- Keep the pile moist (like a wrung-out sponge) and aerated for the microbes to work.
- C:N ratios of materials vary; use a reliable reference and adjust the inputs to match what you have.
Compost C:N ratio: frequently asked questions
What is the ideal compost C:N ratio?
Composting microbes work best with a carbon-to-nitrogen ratio in the range of about 25:1 to 30:1 by mass. Too much carbon (high ratio) slows decomposition; too much nitrogen (low ratio) causes odour and ammonia loss. This calculator works out the blended ratio of your mix and flags whether it sits in the target band, which you can adjust as an input.
How is the blended C:N ratio calculated?
For each material, multiply its mass by its carbon fraction and by its nitrogen fraction. Carbon fraction is C:N ratio divided by (C:N + 1); nitrogen fraction is 1 divided by (C:N + 1). Sum the carbon contributions and the nitrogen contributions across all materials, then divide total carbon by total nitrogen. This is the standard mass-balance approach used in composting guidance.
What are typical C:N ratios of materials?
Carbon-rich browns such as dry leaves, straw and cardboard have high C:N ratios (often 50:1 to several hundred to 1). Nitrogen-rich greens such as fresh grass clippings, food scraps and manure have low ratios (often 10:1 to 25:1). Enter the figure for each material from a reliable composting reference; the values here are editable inputs.
Why does the C:N ratio matter?
Microbes use carbon for energy and nitrogen to build proteins. A balanced ratio keeps the pile decomposing efficiently, heating properly and avoiding both stalling and odour. Getting the mix right is the single biggest factor in fast, clean composting.
Official sources
- U.S. Environmental Protection Agency: Composting.
- U.S. Department of Agriculture, Natural Resources Conservation Service: NRCS conservation resources.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.