Compost ratio calculator
"Two parts brown to one part green" is a decent rule of thumb until you are composting chicken manure and cardboard. Put in what you actually have and see the real number.
Enter how many parts (by volume — buckets, wheelbarrows, whatever you measure in) of each material you are adding.
Browns (carbon)
Greens (nitrogen)
Too carbon-rich: the pile will break down slowly and stay cool. Add greens (grass clippings, kitchen scraps, coffee grounds).
What the C:N ratio actually measures
The microbes doing the work need carbon for energy and nitrogen to build proteins. They consume roughly 25–30 parts carbon for every part nitrogen. Hit that blend and the pile heats to 130–150°F within a few days and finishes in weeks. Miss it in either direction and you get one of two familiar failures.
The two ways piles fail
Too much nitrogen (below 25:1) is the smelly failure. Excess nitrogen has nowhere to go and off-gasses as ammonia, the pile goes anaerobic, and you end up with a slimy mat that your neighbours notice. The fix is always more browns — dry leaves, straw, shredded cardboard.
Too much carbon (above 30:1) is the boring failure. Nothing smells, nothing happens, and the pile sits there for a year. Wood chips at 400:1 and sawdust at 325:1 are the usual culprits — a single wheelbarrow of chips can swamp a season of kitchen scraps.
Why chicken manure changes the maths
At roughly 7:1, chicken manure is the most nitrogen-dense thing most homesteaders have on hand — several times richer than grass clippings. If you keep a flock and clean the coop into the compost, you need substantially more browns than the standard advice assumes. This is exactly the case where a rule of thumb fails and a calculator earns its keep. Deep-litter coop bedding is a partial exception, since the shavings come with their own carbon.
The two things the ratio doesn't cover
- Moisture. Aim for a wrung-out sponge. A perfect C:N ratio in a bone-dry pile still does nothing.
- Particle size and air. Chopped material composts far faster, and turning every week or two keeps oxygen in. A well-balanced pile that never gets turned will still go anaerobic in the middle.
A note on the numbers
The C:N values here are typical figures from university extension composting guides. Real materials vary — grass clippings from a fertilised lawn run richer than the 20:1 listed, and leaves differ by species. Treat the output as a starting point, then read the pile: heat, smell and how fast it shrinks tell you more than any calculator.