Compost C:N Ratio Calculator
30:1 or it either stinks or sits there.
Work out Compost C:N Ratio. 30:1 or it either stinks or sits there. Counts the thing everyone forgets to count.
Carbon to nitrogen ratio
33.3:1
Well balanced — this should heat up and break down quickly
Compost microbes need roughly thirty parts carbon to one part nitrogen to build their own bodies while burning the rest for energy. Get it right and the heap heats to 55-65 °C within days; get it wrong in either direction and it stalls. Too much nitrogen — all grass clippings, say — goes anaerobic and smells of ammonia, which is the nitrogen you wanted escaping into the air. Too much carbon and nothing happens at all, because there is no nitrogen for the microbes to multiply with. This weights by volume, which is an approximation; the exact calculation uses dry mass and moisture content. It is the right shape and quite good enough to keep a heap out of trouble, but do not treat the second decimal place as meaningful.
How the Compost C:N Ratio Calculator works
The carbon-to-nitrogen ratio of a compost mix, against the 25:1 to 35:1 target that makes a heap heat up. Too much nitrogen and it goes anaerobic; too much carbon and nothing happens at all.
Also known as: green to brown compost ratio · why does my compost smell · carbon nitrogen ratio for composting · compost not breaking down
Frequently asked questions
What is the ideal compost ratio?
Between 25:1 and 35:1 carbon to nitrogen by weight, with 30:1 the usual target. At that ratio microbes have enough nitrogen to build their bodies and enough carbon to burn for energy, and the heap reaches 55-65 °C within days.
What counts as green and what counts as brown?
Greens are nitrogen-rich and usually wet — grass clippings, vegetable scraps, manure, coffee grounds. Browns are carbon-rich and dry — leaves, straw, cardboard, wood chips. The colour is a memory aid rather than a rule.
Why does my compost smell of ammonia?
Too much nitrogen. That smell is the nitrogen you wanted escaping into the air, and it means the heap has gone anaerobic. Add browns and turn it to get air in — the smell usually clears within a day or two.
Why is nothing happening in my compost heap?
Usually too much carbon, or too little moisture. A pile of dry leaves at 60:1 will sit for a year. Add greens, check it feels like a wrung-out sponge, and turn it.
How accurate is a volume-based ratio?
Good enough to keep a heap out of trouble and no better. The exact calculation weights by dry mass, moisture and actual nitrogen content, all of which vary with how fresh the material is. Do not treat the second decimal place as meaningful.
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