C:N Ratio Results
All C:N values are on a dry-weight basis per NRAES-54 and CREF Appendix B. Actual ratios vary by material freshness, fertilization, and species. See full methodology.
Carbon to Nitrogen Ratio Calculator
Blend your compost ingredients to hit the ideal 25–35:1 C:N ratio. Uses 45+ feedstocks from NRAES-54 and CREF Appendix B.
Understanding the Carbon to Nitrogen Ratio in Composting
The carbon to nitrogen (C:N) ratio is the single most important parameter in active composting. It determines whether the microorganisms decomposing your pile can thrive — or whether they're starved of nitrogen or choked by ammonia. The ideal starting C:N ratio for rapid hot composting is 25:1 to 35:1,[4] with 30:1 most commonly recommended.[3]
Why C:N Ratio Matters
Microbes that break down organic matter require roughly 30 parts carbon for every 1 part nitrogen — the C:N ratio of their own cell biomass. When your pile's C:N is in range, decomposition is rapid and temperatures rise into the thermophilic zone (130–160°F). If C:N is too low (below 20:1), excess nitrogen volatilizes as ammonia gas — causing the characteristic sharp odor of a pile with too many grass clippings or fresh manure. If C:N is too high (above 40:1), decomposition slows dramatically because nitrogen is limiting.
Browns vs. Greens: A Simplified Mental Model
The popular "3 parts browns to 1 part greens" rule of thumb approximates the 30:1 C:N target, but is not accurate enough for calculating actual ratios. "Browns" include dry leaves (40–80:1), straw (50–150:1), and cardboard (150–500:1). "Greens" include fresh grass clippings (9–25:1), food scraps (15–25:1), and manures (5–25:1). The C:N calculator is more precise because it accounts for the actual ratio of each specific material, not just its color.
How This Calculator Works
The calculator uses the dry-mass weighted harmonic mean formula from the Cornell Waste Management Institute. It first converts each ingredient's volume to dry mass using bulk density and moisture content from NRAES-54 and CREF data, then computes the blended C:N ratio. Simple arithmetic averaging of C:N ratios — a common error — produces incorrect results because it ignores the relative mass contribution of each material.
Once you have your C:N ratio, use the Moisture Calculator to verify your pile moisture is in the optimal 40–65% range, and the Pile Temperature Tracker to monitor decomposition progress.
Frequently Asked Questions
What is the ideal C:N ratio for composting?
The ideal starting C:N ratio is 25:1 to 35:1, with 30:1 most commonly cited as optimal. Piles in this range decompose actively. Below 20:1, excess nitrogen may volatilize as ammonia (causing odor). Above 40:1, decomposition slows because microbes run short of nitrogen.
Why does the calculator use gallons instead of pounds?
Most home composters measure materials by volume (buckets, wheelbarrows) rather than by weight. The calculator converts gallons to dry mass using bulk density and moisture content from the CREF feedstock database, then applies the C:N formula. You can also enter liters when using metric mode.
What is the Dry/Typical/Wet condition toggle?
How do I calculate C:N ratio by hand?
The correct formula uses dry-mass weighted harmonic mean: first calculate the dry mass of each material (volume × bulk density × (1 − moisture fraction)), then compute R_mix = (sum of dry masses) / (sum of dry_mass_i / R_i). Simple averaging of C:N ratios is a common error that produces incorrect results.
My C:N is too high — what should I add?
A high C:N (above 35:1) means you have too many "browns." Add nitrogen-rich "greens": fresh grass clippings, vegetable scraps, coffee grounds, chicken manure, or blood meal. Aim to bring the ratio down to 25–35:1.
My C:N is too low — what should I add?
A low C:N (below 20:1) means you have too many "greens." Add carbon-rich "browns": dry leaves, straw, cardboard, newspaper, or wood chips. Excess nitrogen can cause ammonia odor and attract pests.
Are C:N ratios the same as "browns to greens" ratios?
Not exactly. Browns-to-greens is a simplified rule of thumb (often cited as 3:1 by volume), while C:N ratio is a precise chemical measurement. C:N calculation is more accurate because it accounts for the actual carbon and nitrogen content of each specific material.
What does the moisture percentage in the results mean?
The blended moisture percentage is the weighted average moisture of your pile. A healthy active pile should be 40–65%. If below 40%, the calculator shows how much water to add. If above 70%, add dry carbon materials to reduce moisture.