Compost C:N Ratio Calculator
Mix compost ingredients and get the blended carbon-to-nitrogen (C:N) ratio, with guidance toward the ideal 25โ30:1. Runs in your browser.
Inputs by weight (any consistent unit โ kg, lb, or buckets)
Ideal โ should heat up and break down well
Target is roughly 25โ30:1. Blend C:N = total mass รท ฮฃ(mass รท material C:N), using dry-basis C:N values from Cornell Waste Management Institute / extension references. Real ratios vary with moisture and material condition โ adjust by feel (damp, not soggy; warms within days). Not a precise lab value.
About this tool
A compost pile breaks down fastest and cleanest when the balance of carbon to nitrogen โ the C:N ratio โ is right, around 25 to 30 parts carbon for every part nitrogen. Microbes use carbon for energy and nitrogen to build protein; too much nitrogen (excess 'greens') goes anaerobic and smells of ammonia, while too much carbon (excess 'browns') just sits there and decomposes slowly. This calculator lets you enter the weights of common ingredients โ food scraps, grass, and manure on the green side; leaves, straw, cardboard, and sawdust on the brown side โ each tagged with its typical C:N ratio from Cornell Waste Management Institute and extension references, and computes the blended ratio with the standard mass-weighted formula. It then tells you whether to add more greens or browns to hit the target. The material C:N values are dry-basis averages and real piles vary with moisture and how fresh the inputs are, so use the result as a strong starting guide and adjust by feel: a working pile is damp like a wrung sponge and warms within a few days. Everything runs in your browser.
How to use it
- Enter the weight of each ingredient you're adding (use any consistent unit).
- Greens are nitrogen-rich; browns are carbon-rich โ both columns are labeled with their C:N.
- Read the blended C:N ratio and the guidance.
- Add greens or browns until you land near 25โ30:1.
Frequently asked questions
- What C:N ratio should compost be?
- About 25โ30:1 (carbon to nitrogen) is the sweet spot for fast, hot, odor-free composting. Higher means it breaks down slowly; much lower can turn smelly and lose nitrogen as ammonia.
- How is the blended ratio calculated?
- With the mass-weighted formula: blend C:N = total mass รท the sum of (each material's mass รท its C:N ratio). It assumes a comparable carbon fraction across materials, which is the standard simplification used by home-compost calculators.
- What are "greens" and "browns"?
- Greens are nitrogen-rich and usually moist โ food scraps, fresh grass, coffee grounds, manure (low C:N). Browns are carbon-rich and dry โ leaves, straw, cardboard, sawdust (high C:N). A good pile layers both; by volume people often use roughly 2โ3 parts browns to 1 part greens.
- My pile smells bad โ what does that mean?
- Usually too much nitrogen (greens) or too wet/compacted, going anaerobic. Add and mix in dry browns to raise the C:N and aerate. A pile that just sits cold and intact usually needs more greens and moisture.
- How accurate are the C:N values?
- They are dry-basis averages from composting references; real values vary with material freshness, species, and moisture. Treat the calculated ratio as a well-grounded estimate to guide your mix, not a lab measurement.
- Is anything uploaded?
- No. The calculation runs entirely in your browser with no network request.