What is a compost C:N ratio?
The carbon-to-nitrogen (C:N) ratio describes how much carbon is in your pile relative to nitrogen. Compost microbes need carbon for energy and nitrogen to build proteins. Give them roughly 30 parts carbon for every 1 part nitrogen and they multiply fast, heating the pile to the 131°F–160°F zone where weed seeds and pathogens die.
Too much nitrogen (below ~20:1) smells like ammonia, goes slimy, and loses nitrogen to the air. Too much carbon (above ~40:1) sits there barely warm for months because the microbes are starved of nitrogen.
Greens and browns, translated into numbers
"Greens" are nitrogen-rich (grass clippings, food scraps, manure — typically 10:1 to 25:1). "Browns" are carbon-rich (leaves, straw, cardboard — typically 40:1 to 500:1). The calculator above does the weighted math for you: it converts each material's wet weight to dry weight, sums total carbon and total nitrogen, and divides.
Approximate feedstock values used
These are widely published typical values (dry-weight basis), not lab analyses of your specific materials. Treat them as planning estimates.
| Material | C:N | %C | %N | Moisture |
|---|
Sources: Cornell Waste Management Institute feedstock tables (compost.css.cornell.edu); Mississippi State Extension "Composting for the Mississippi Gardener" feedstock table; UC Agriculture feedstock carbon/nitrogen content table. Figures vary between sources; values above use commonly cited midpoints and are labeled approximate.
Frequently asked questions
What is the ideal C:N ratio for a compost pile?
About 25:1 to 30:1. This range gives microbes enough nitrogen to build cells while burning carbon for energy, producing the fastest heating and decomposition. Finished compost ends up closer to 10:1–15:1 because carbon leaves the pile as CO2.
My pile smells like ammonia. What is wrong?
That is the classic sign of excess nitrogen (ratio too low, often under 15:1). Turn the pile to add oxygen and mix in carbon-rich browns like straw, shredded cardboard, or dry leaves until the solver reads around 30:1.
My pile will not heat up at all. What do I add?
A pile that stays cold is usually too carbon-heavy (above 40:1), too dry, or too small. Add nitrogen-rich greens — grass clippings, food scraps, coffee grounds, or manure — moisten to a wrung-out sponge feel, and make sure the pile is at least 3×3×3 feet.
How do I measure pounds or gallons of leaves and straw?
Weigh a full bucket or bag on a bathroom scale once, then count by container. For volume, the calculator converts gallons to weight using each material's approximate bulk density — handy for loose leaves and wood chips that are hard to weigh.
Does moisture matter as much as the C:N ratio?
Yes. Microbes need 40%–60% moisture by weight. Below 15% decomposition stalls; above 65% the pile goes anaerobic and stinks. The calculator shows a weighted moisture estimate and flags piles that are too dry or too wet.