Compost Ratio Calculator

What the C:N ratio actually means

A compost pile is a microbial farm. The bacteria and fungi doing the work need carbon as their energy source and nitrogen to build proteins and reproduce. If there is too much nitrogen relative to carbon, the excess is lost as ammonia — that is the sharp smell of a slimy, soggy pile. If there is too much carbon, the microbes run short of nitrogen, reproduce slowly, and the pile sits cold and unchanged for months. The balance point most extension services quote is 25 to 30 parts carbon to 1 part nitrogen, and that range is a practical target rather than a precise figure: published C:N values for the same material vary between sources, moisture content changes them, and the microbes themselves adjust as the pile matures. Use the ratio to steer the pile in the right direction, not to chase an exact number.

Greens and browns, in practice

Greens are fresh, moist, nitrogen-rich materials: grass clippings, kitchen scraps, coffee grounds, fresh manure and green weeds. Browns are dry, carbon-rich materials: autumn leaves, straw, shredded cardboard, sawdust and wood chips. The classic rule of thumb — two to three parts brown to one part green by volume — gets most piles close to the target, because greens are dense and wet while browns are light and bulky. Layer them rather than dumping one material in a thick slab: a thick layer of grass clippings mats into an airless, smelly sheet, and a thick layer of whole leaves does the same. Shredding or tearing browns makes a real difference — shredded leaves can break down in months, while whole leaves can sit for a year. Keep the pile about as damp as a wrung-out sponge, and turn it when it cools down; turning adds the oxygen the microbes need.

Fixing a pile that is not working

A pile that smells like ammonia has too much nitrogen or too little air: fork in dry browns and turn it to introduce oxygen. A pile that is cold and unchanged usually needs greens, water, or both — add fresh grass clippings or food scraps, wet it thoroughly, and turn it. A pile that is dry and dusty needs water more than anything else; dry leaves will not rot no matter how long you wait. A pile full of flies around the top is usually exposed food scraps: bury scraps in the middle of the pile and cover with browns. Sawdust and wood chips deserve special caution because their C:N ratios are around 400:1 — they soak up nitrogen from whatever is around them, so use them sparingly in a hot pile and never dig fresh sawdust into garden beds, where it will starve your plants of nitrogen while it breaks down.

What not to compost

Meat, dairy, fats and cooked food attract rats and smell bad in a home pile. Diseased plants and weeds that have gone to seed belong in the green bin unless your pile reliably gets hot — above about 55°C (130°F) for several days — which kills most seeds and pathogens; most home piles do not stay that hot for that long. Treated timber, glossy or coated paper, and pet waste from meat-eating animals should stay out. Finished compost should smell earthy and look dark and crumbly. Use it to feed your beds — the raised-bed soil calculator shows how much compost a new bed needs, the fertiliser calculator helps with feeding, and the mulch calculator covers using compost as a mulch. For what to grow in compost-fed soil, see the planting calendar.