Compost C:N Ratio Calculator

The carbon-to-nitrogen ratio, written C:N, compares the amount of carbon to the amount of nitrogen in a compost pile. Carbon-rich browns such as dry leaves and cardboard feed the microbes' energy needs, while nitrogen-rich greens such as grass clippings and food scraps build their bodies. The balance between the two sets how fast and how well a pile breaks down.

A C:N ratio of about 25 to 30 parts carbon to 1 part nitrogen is widely cited as ideal for efficient composting. Too much carbon and the pile breaks down slowly; too much nitrogen and it can become wet and smelly as excess nitrogen is lost as ammonia. Aiming for roughly 30:1 keeps microbes working quickly without odour.

This calculator divides the carbon parts you enter by the nitrogen parts to give the C:N ratio as a single number to one part nitrogen. For example, 300 parts carbon and 10 parts nitrogen gives 300 divided by 10, which is 30, written 30:1. Use consistent units for both, such as the carbon and nitrogen content by weight.

Browns are carbon-rich, dry and woody: dead leaves, straw, sawdust, paper and cardboard. Greens are nitrogen-rich, moist and fresh: grass clippings, vegetable scraps, coffee grounds and manure. A practical rule of thumb is to layer roughly two to three parts browns to one part greens by volume, then adjust if the pile is too slow or too smelly.

Composting diverts organic waste from landfill, where it would otherwise generate methane, a potent greenhouse gas. A well-balanced pile composts efficiently and produces a useful soil amendment. Government environmental agencies encourage home composting as a way to cut waste and improve soils, and a sensible C:N balance makes the process work.