Compost C:N Ratio Calculator
Works out the carbon to nitrogen ratio of a compost mix.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
C:N = Σ(weight × (1 − moisture) × N% × C:N) ÷ Σ(weight × (1 − moisture) × N%)
The nitrogen in each material comes from its dry matter, and the carbon follows from its C:N. A simple average of the C:N figures gets the wrong answer, because wet and nitrogen-poor materials count for less nitrogen than their weight suggests.
What to watch out for
Weigh a bucket of each material to turn buckets into weights. A figure for material 2 is shown only when adding or changing that material can reach the target.
Frequently Asked Questions
What is the best C:N ratio for compost?
About 25–30 : 1 is the usual aim. Below about 20 : 1 nitrogen is lost as ammonia, and well above 40 : 1 the heap works slowly.
Why not just average the two ratios?
Because the materials hold different amounts of nitrogen. By weight, 20 kg of leaves and 50 kg of grass at the starting figures come to 29.6 : 1, not the plain average of the two ratios.