Compost C:N Ratio Calculator
The carbon-to-nitrogen ratio governs how quickly a compost pile breaks down. Too much carbon and decomposition stalls; too much nitrogen and the pile becomes anaerobic and smelly. The USDA recommends a blended C:N ratio of 25:1 to 30:1 for active composting. This calculator takes the volume of browns (dry leaves, straw, cardboard) and greens (grass clippings, kitchen scraps, manure) and estimates the blended C:N ratio using the typical average values: 80:1 for browns and 20:1 for greens. It also tells you how many additional parts of browns to add if the ratio is below target, or how much green material to add if the ratio is too high.
Compost C:N ratio formula
Blended C:N = (V_browns x 80 + V_greens x 20) / (V_browns + V_greens)
Target: 25 to 30
Average C:N values used: browns = 80 (representing a mix of dry leaves and straw), greens = 20 (representing grass clippings and kitchen scraps). These are conservative midpoints from USDA Cooperative Extension composting guides.
Interpreting your result
- C:N below 20:1: Too much nitrogen. Add more browns. The pile may smell of ammonia.
- C:N 20:1 to 25:1: Slightly nitrogen-rich. Acceptable but add a little more browns for faster composting.
- C:N 25:1 to 35:1: Ideal zone. Pile should heat up quickly and compost in 4 to 8 weeks with turning.
- C:N 35:1 to 50:1: Slightly carbon-rich. Composting will proceed but may take longer.
- C:N above 50:1: Too much carbon. Add more greens or a nitrogen amendment such as blood meal or fresh manure.
Compost ratio calculator: frequently asked questions
What is the ideal C:N ratio for compost?
The USDA and most composting guides recommend a carbon-to-nitrogen ratio of 25:1 to 30:1 by weight. Ratios much higher than 30:1 slow decomposition; ratios below 20:1 produce ammonia odors.
What are browns and greens in composting?
Browns are high-carbon materials: dry leaves (60:1 C:N), straw (75:1), cardboard (350:1), and wood chips (400:1). Greens are high-nitrogen materials: grass clippings (20:1), food scraps (15:1), and coffee grounds (20:1). Mixing roughly 3 parts browns to 1 part greens by volume usually produces a 25:1 ratio.
How do I use this calculator?
Enter the volume of browns and greens you have. The calculator uses average C:N values (browns 80:1, greens 20:1) to estimate the blended ratio of your pile. Adjust your inputs until the result lands in the 25 to 35 range.
Why is my compost pile not heating up?
The pile may have too many browns (high C:N, low nitrogen) so decomposition is slow, or it may be too dry or too small. Aim for a pile at least 3 x 3 x 3 feet, keep it moist like a wrung-out sponge, and add nitrogen-rich greens if the ratio is above 40:1.
Can I use this ratio with weight instead of volume?
Volume is easier to measure in practice. If you have weights, use them directly, as C:N ratios are based on dry weight. The calculator uses volume as a proxy with the assumption of typical bulk densities for browns and greens.
Official sources
- USDA Natural Resources Conservation Service, composting guide: nrcs.usda.gov.
- USDA National Institute of Food and Agriculture: nifa.usda.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.