Buckets of browns per bucket of greens
To reach about 30 : 1, this is how much of each brown to add per bucket of each green, by volume, calculated from the same data the calculator uses:
Three things stand out. Horse manure with its bedding is already close to 30 : 1 and composts well on its own. Grass clippings balance with about their own volume of dry leaves. And fresh chicken manure is so rich in nitrogen that it needs a very large amount of carbon, which is why it smells of ammonia when piled up neat and burns plants when used fresh. Mix it thinly through plenty of browns, or let it age with its bedding first.
Why the calculator works by weight
Most "greens and browns" advice counts buckets, but carbon and nitrogen are chemistry, and chemistry is by weight. A 10 L bucket of fruit and vegetable scraps weighs about 9.4 kg, and 87 % of that is water. A 10 L bucket of loose dry leaves weighs about 1.2 kg. The calculator converts each bucket to wet weight, then to dry matter, then to carbon and nitrogen, and adds them up. This is the standard method from Cornell University's composting program:
where %C = C:N ratio × %N (both on a dry-weight basis)
Averaging the C:N ratios of the ingredients, as some quick calculators do, gives the wrong answer. A bucket of chicken manure and a bucket of leaves do not average to 30 : 1, because the manure carries far more nitrogen per bucket.
The numbers behind it
These are the typical or average values from the On-Farm Composting Handbook (Table A.1), converted to metric. Loose straw density is from agricultural-engineering measurements. Coffee grounds (C:N about 20) behave as a green; newspaper (C:N roughly 400–850) behaves like cardboard.
Moisture, air and heat
- Moisture: aim for 50–60 %, which feels like a wrung-out sponge. Squeeze a handful: a few drops is right; a trickle is too wet; crumbly and dusty is too dry. Fresh greens are mostly water, so a heap heavy in scraps or clippings drifts wet. Dry browns fix it.
- Air: the calculator shows the mix's density. Below about 600 kg/m³ there is enough open space for air. Above that, and certainly above 700 kg/m³, air struggles to move, and the heap goes sour. Bulky browns such as straw, prunings and leaves keep it open.
- Heat: a heap needs roughly 1 m³ (about 1 × 1 × 1 m) to hold its own heat. Commercial compost made to Australian Standard AS 4454 must reach at least 55 °C for three consecutive days after each turn, which is what kills most weed seeds and pathogens. Small home heaps rarely manage that all the way through, so keep seeding weeds and diseased plants out.
- Shrinkage: expect finished compost to be well under half the starting volume. The calculator assumes about 40 %.
When the compost is ready, the compost calculator works out how much you need to dig in or spread.
Compost questions
What is the best carbon to nitrogen ratio for compost?
About 25–30 : 1 by weight. Lower than about 20 and nitrogen escapes as ammonia; much higher than 40 and the heap slows right down for lack of nitrogen.
How many browns to greens should I use?
It depends entirely on which ones. By volume: about 1 bucket of dry leaves per bucket of grass clippings, 2–3 per bucket of kitchen scraps, or half a bucket of shredded cardboard per bucket of scraps. The table above covers the common combinations.
Why does my compost smell?
An ammonia smell means too much nitrogen: add browns. A rotten-egg or sour smell means it is too wet and short of air: turn it and mix in dry, bulky browns. A healthy heap smells earthy.
Are coffee grounds green or brown?
Green. Spent coffee grounds have a C:N ratio of about 20, similar to kitchen scraps, despite their brown colour.
Sources and method
- Material data: Rynk et al., On-Farm Composting Handbook (NRAES-54), Appendix A, Table A.1, as published by the Cornell Waste Management Institute. Bulk densities converted from lb/yd³ (× 0.593 = kg/m³).
- Mixture formula and target ratio: Cornell Composting – C/N ratio.
- Loose straw density (about 24–111 kg/m³ depending on moisture): published measurements of wet and dry wheat straw bulk density; 60 kg/m³ used.
- Pasteurisation at 55 °C or more for three consecutive days (AS 4454): EPA South Australia – compost guideline.
Written by DueRiva · Last reviewed 23 September 2026 · Report an error