Compost C:N Ratio Calculator
Divides grams of carbon by grams of nitrogen in a compost mix and says whether the pile needs more browns or more greens to sit near the ideal 25-30:1.
C:N + status
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C:N ratio: the 25–30:1 target
The microbes that break down organic matter burn carbon for energy and use nitrogen to build their proteins. Get the balance right and things move fast: the ideal carbon-to-nitrogen ratio is 25–30:1 by mass for hot composting, with the pile hitting 55–65 °C within days. Drop below 20:1 and you smell ammonia from the extra nitrogen; go past 40:1 and decomposition grinds to a halt. Browns (carbon): dry leaves 60:1, paper/cardboard 170:1, straw 80:1, sawdust 500:1. Greens (nitrogen): fresh grass clippings 20:1, animal manure 15:1, kitchen scraps 15–30:1, coffee grounds 20:1. Example: mix 1 kg dry leaves (60:1) with 1 kg grass clippings (20:1) and you average around (60+20)/2 ≈ 40:1, so toss in a little more grass or scraps to land near 30:1. If you'd rather eyeball it by volume, the usual guideline is 3 parts browns to 1 part greens.
Applications
This shows up everywhere from backyard bins to Bokashi fermentation (which cares less about C:N), organic farming, regenerative gardening, and vermicomposting, where worms do best around 20–25:1. At the other end of the scale, municipal operations track windrow C:N day by day.
FAQ
My pile smells bad. What's wrong? An ammonia smell means too much nitrogen, so mix in browns. A rotten or sulfur smell means it's gone anaerobic, so turn the pile to let oxygen back in.
Can I compost meat or dairy? Keep them out of open piles, where they draw pests and stink. Bokashi or a closed digester can deal with them instead.
How long to finished compost? With the C:N dialed in, hot composting takes 4–8 weeks. Leave it to cold composting and you're looking at 6–12 months.
Moisture matters too? Definitely. Shoot for 50–60% moisture, the point where a squeezed handful releases a couple of drops. Too dry and the microbes stall; too wet and it turns anaerobic.
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.