Compost Carbon to Nitrogen Ratio Calculator
A balanced compost pile decomposes quickly and produces finished compost without odours. The key factor is the carbon to nitrogen (C:N) ratio of your ingredients blend. Cornell University and other land-grant extension services recommend a blended C:N ratio of 25:1 to 30:1 for rapid aerobic decomposition. Enter up to three ingredients with their weights and C:N ratios to calculate your blended ratio.
Blended C:N ratio formula
Blended C:N = (W1 x CN1 + W2 x CN2 + W3 x CN3) / (W1 + W2 + W3)
Where W = weight of ingredient, CN = C:N ratio of that ingredient
This weighted average formula is from Cornell Composting Science and Engineering. Weight units must be consistent (all pounds, all kilograms, or all parts by weight). The formula computes the mass-weighted average C:N ratio of the blend.
Common C:N ratios by material
- Sawdust: 500:1 (very high carbon).
- Wood chips: 400:1.
- Cardboard (shredded): 350:1.
- Straw (dry): 80:1.
- Dry leaves: 60:1.
- Fresh grass clippings: 20:1.
- Vegetable scraps: 15:1.
- Chicken manure (fresh): 10:1.
- Blood meal: 4:1 (high nitrogen amendment).
Compost C:N calculator: frequently asked questions
What is the ideal carbon to nitrogen ratio for composting?
The ideal C:N ratio for active composting is approximately 25:1 to 30:1. At this ratio, microbial decomposition is fastest. A ratio above 40:1 slows composting because microbes become nitrogen-limited. A ratio below 20:1 can cause ammonia loss and odour problems because excess nitrogen is released as ammonia gas.
What are high-carbon (brown) compost materials?
High-carbon brown materials include dry straw (C:N 80:1), wood chips (C:N 400:1), sawdust (C:N 500:1), shredded cardboard (C:N 350:1), and dry leaves (C:N 60:1). These provide the structural carbon skeleton for the compost pile.
What are high-nitrogen (green) compost materials?
High-nitrogen green materials include fresh grass clippings (C:N 20:1), vegetable scraps (C:N 15:1), fresh manure (C:N 10:1 to 15:1), and coffee grounds (C:N 20:1). These supply the nitrogen that fuels microbial growth.
How do I calculate the blended C:N ratio for a mixture?
Blended C:N ratio = sum of (weight x C:N ratio for each ingredient) / sum of (weight for each ingredient). This weighted average gives the overall C:N ratio of the mixture. This calculator supports up to three ingredients.
Can I compost materials with a very high C:N ratio like wood chips?
Yes, but you need to balance them with high-nitrogen materials. Wood chips (C:N approximately 400:1) require a large proportion of nitrogen-rich material to bring the blend to the 25:1 to 30:1 target. Wood chips are also useful as a bulking agent to maintain pile aeration even if they decompose slowly.
Official sources
- Cornell Waste Management Institute: Compost Science and Engineering.
- USDA Natural Resources Conservation Service: Composting and Soil Health.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.