Donaldson R F Compost C:N Ratio Calculator
Work out the blended carbon:nitrogen ratio of your compost mix, and see whether it needs more "greens" or more "browns".
How it is worked out
Blended C:N ratio = the sum of (amount × that material's C:N ratio) ÷ total amount, a simple weighted average by weight. The commonly recommended starting ratio is around 25–30:1 — too high (too much carbon, "browns") and composting slows right down; too low (too much nitrogen, "greens") and the pile can turn smelly and anaerobic.
Source: C:N ratios for each material — grass clippings 19:1, food waste 15:1, cow manure 20:1, horse manure 25:1, leaves 40–80:1 (60:1 used here), paper 170:1, wood/sawdust 300–700:1 (500:1 used here) — and the ~30:1 target, from Cornell Waste Management Institute: Composting Ingredients. Checked 7 October 2026.