Compost Ratio Calculator

In short A compost heap works when its carbon-to-nitrogen ratio lands somewhere between about 25:1 and 35:1. Above roughly 40:1 the microbes run short of nitrogen and the heap sits cold for a year; below about 20:1 the surplus nitrogen leaves as ammonia, which is what the smell is and is fertiliser walking away. So the practical question is how much brown material to add to a given amount of green — and almost every calculator online answers it wrongly. THE MISTAKE IS AVERAGING THE RATIOS. Mix equal weights of dry leaves at 55:1 and fresh grass at 17:1 and the average of the two ratios is 36. The true answer is 38:1, because leaves are drier and carry more dry matter per kilo. Do the same with sawdust at 400:1 and grass at 17:1 and the average says 208:1 while the truth is 83:1 — out by a factor of two and a half. The reason is simple once seen: a heap ratio is TOTAL CARBON over TOTAL NITROGEN, and carbon and nitrogen add up while ratios do not. Sawdust is 0.125% nitrogen, so a kilo of it contributes almost none, and the mixture ends up weighted towards whatever is actually supplying the nitrogen. That is why sawdust is a far weaker corrective than its headline number suggests, and why leaves at 55:1 do more work per kilo than they look like they should. This page does the additive arithmetic, in both directions: give it two materials and weights and it returns the mixture, or give it a target and it solves for the parts of brown per part of green exactly. It also shows what the naive average would have told you, so the size of the error is visible rather than asserted. Two honest caveats. The carbon fractions and C:N ratios are published typical values and every one is a range — grass clippings run from about 12:1 to 25:1 depending on how recently the lawn was fed. And moisture matters as much as the ratio: a heap wants 50 to 60% water, wrung-out-sponge damp, and a perfectly balanced heap of dry material does nothing at all. That is exactly why an autumn pile of leaves sits untouched until spring rain finds it.