Compost Carbon-to-Nitrogen Ratio Calculator
Add your compost ingredients by weight to get the blended carbon-to-nitrogen ratio. See whether to add more browns or greens to reach the ideal 25-35:1 ratio. Nothing uploaded.
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Pile C:N Ratio
Add materials to see your C:N ratio.
Learn more: blending your compost carbon-to-nitrogen ratio
How the blended ratio is worked out
Each material's moisture is stripped out first, because only dry matter counts as carbon or nitrogen. The calculator treats that dry matter as 45% carbon, then divides the carbon figure by the material's own C:N ratio to get its nitrogen.
Carbon and nitrogen are then totalled separately across everything in the pile, and the blend is the carbon total divided by the nitrogen total. That is why the answer is almost never the average of the individual ratios shown in the dropdown.
Why equal weights don't land on the midpoint
Put 5 kg of fresh grass clippings (17:1) next to 5 kg of plain cardboard (350:1) and the calculator returns 76.7:1. The midpoint of those two numbers is 183.5:1, so the blend sits nowhere near it.
Two things cause the gap. Cardboard is listed at 10% moisture and grass at 80%, so a kilo of cardboard puts 0.405 kg of carbon into the pile while a kilo of grass puts in 0.09 kg.
The nitrogen total does the rest. In that same mix the grass supplies about 82% of the nitrogen off 18% of the carbon, and a large nitrogen total is what holds the ratio down.
Reading the verdict
Under 20:1 the calculator flags the pile as nitrogen-heavy and warns it may smell. From 20:1 to 35:1 it reads as ideal, with 25:1 to 35:1 named as the sweet spot inside that band.
Between 35:1 and 50:1 the pile still composts, just slowly. Past 50:1 it is carbon-heavy enough that the tool asks for greens or water.
The edges are closer together than they look. 3 kg of grass with 3 kg of dry autumn leaves comes out at 36.5:1, a whisker into the slow band, while 4 kg of food scraps with 2 kg of the same leaves lands at 28.6:1.
FAQ
Why does moisture matter for a carbon-to-nitrogen calculation?
Wet material carries a lot of weight that is neither carbon nor nitrogen. Stripping moisture out first puts every ingredient on a dry-weight basis, so a soggy kilo of vegetable peel is not credited with the same organic content as a dry kilo of straw.
My blend reads below 20:1. What should I add?
Browns, and the higher their own ratio the less of them you need. Wood chips at 500:1, sawdust at 400:1 and cardboard at 350:1 move the number fastest, while straw at 80:1 and dry leaves at 50:1 nudge it up more gently.
My blend reads above 50:1. Does adding water fix it?
Water helps a dry pile break down, but it will not change the number here. Every material carries a fixed moisture figure in the calculator, so the only way to bring a carbon-heavy blend down is to add greens such as food scraps at 15:1, vegetable peel at 12:1 or fresh chicken manure at 8:1.