Compost Calculator
Balance greens and browns to the ideal compost C:N ratio.
About this tool
Add each material you're composting and how much of it, in parts by weight. The tool estimates the mix's C:N ratio from typical values and tells you whether to add more greens or browns.
The ideal compost mix
Composting microbes use carbon for energy and nitrogen for growth, and they work fastest when the heap has roughly 25โ30 parts carbon to 1 part nitrogen. 'Greens' are nitrogen-rich: grass clippings (around 17:1), vegetable and fruit scraps, coffee grounds and manure (chicken manure about 10:1). 'Browns' are carbon-rich: dry leaves (about 60:1), straw (80:1), cardboard (350:1), newspaper and sawdust (400โ500:1). Because nitrogen content differs as well as the ratio, you can't just average the ratios. The tool works out the carbon and nitrogen each material contributes from typical nitrogen percentages and combines them. Two parts grass, one part kitchen scraps and two parts dry leaves comes to about 24:1, close to ideal. Below about 20:1 the heap tends to smell of ammonia and turn slimy; above about 40:1 it breaks down slowly. Honest limits: real materials vary a lot with moisture, freshness and source, so treat the result as a guide and watch the heap: it should heat up, smell earthy and stay as damp as a wrung-out sponge. Turning it adds the air microbes need. The fertilizer calculator and the plant spacing calculator are related.
Frequently asked questions
What is the best C:N ratio for compost?
About 25 to 30 parts carbon to 1 part nitrogen.
What are greens and browns in compost?
Greens are nitrogen-rich materials like grass and food scraps; browns are carbon-rich like dry leaves, straw and cardboard.
Why does my compost smell?
Usually too much nitrogen or too little air. Add browns and turn the heap.
Why isn't my compost breaking down?
Often too much carbon, too dry or not enough air. Add greens and water, and turn it.
Is my data sent anywhere?
No. Everything is calculated in your browser, and nothing is stored or sent.