Compost C:N Ratio Calculator

How it works

A compost pile's carbon-to-nitrogen ratio is the total carbon in the pile divided by the total nitrogen. Each ingredient contributes carbon and nitrogen in proportion to its dry matter, so the calculator first removes the water from each weight, multiplies the dry matter by its carbon and nitrogen percentages, adds the carbon and the nitrogen separately, and only then divides.

That is why the result is not the average of the ingredients' ratios, the quick estimate some guides give. Wet grass clippings are mostly water and carry little dry matter per pound; a pound of dry leaves carries several times as much. And a nitrogen-rich ingredient contributes more nitrogen per pound of dry matter than its ratio alone suggests. Averaging the ratios treats every ingredient as equal.

If you know an ingredient's C:N ratio rather than its carbon percentage, choose C:N ratio and the calculator works out carbon as ratio × nitrogen. The nitrogen percentage is still needed: it is what sets how much each ingredient weighs in the mix.

To reach a target, pick one ingredient by its position in the lists. Adding more of it moves the ratio toward that ingredient's own C:N, so a high-carbon (brown) ingredient raises a low ratio and a high-nitrogen (green) ingredient lowers a high one. The calculator solves for the extra weight that lands exactly on the target, or says when the chosen ingredient pulls the wrong way. Cornell's composting guide gives about 30:1 as the usual starting recommendation; Penn State Extension's home composting guide gives 25–30:1.

Formula

dry matterᵢ   = weightᵢ × (1 − moistureᵢ ÷ 100)
carbonᵢ       = dry matterᵢ × %Cᵢ ÷ 100      (%Cᵢ = C:Nᵢ × %Nᵢ in C:N mode)
nitrogenᵢ     = dry matterᵢ × %Nᵢ ÷ 100
mix C:N       = Σ carbonᵢ ÷ Σ nitrogenᵢ
mix moisture  = 1 − Σ dry matterᵢ ÷ Σ weightᵢ
extra weight of ingredient k to reach target T:
  x = (T × Σ nitrogen − Σ carbon) ÷ [dₖ × (%Cₖ − T × %Nₖ) ÷ 100]
  where dₖ = 1 − moistureₖ ÷ 100; x must come out positive

Example

Cornell's composting guide mixes 10 kg of grass clippings (77% moisture, 2.4% N, 45% C) with 6.8 kg of leaves (35% moisture, 0.75% N, 50% C). The grass holds 10 × 0.23 = 2.3 kg of dry matter and the leaves 6.8 × 0.65 = 4.42 kg. Carbon is 2.3 × 0.45 + 4.42 × 0.50 = 3.245 kg and nitrogen 2.3 × 0.024 + 4.42 × 0.0075 = 0.08835 kg, so the mix is 3.245 ÷ 0.08835 = 36.7:1 (Cornell: "a little less than 37:1") at 60.0% moisture.

To bring it down to 30:1 with more grass: (30 × 0.08835 − 3.245) ÷ [0.23 × (0.45 − 30 × 0.024)] = −0.5945 ÷ −0.0621 = 9.57 kg more grass clippings. Starting from the grass alone (leaves 0 kg), 30:1 needs (30 × 0.0552 − 1.035) ÷ [0.65 × (0.50 − 30 × 0.0075)] = 3.47 kg of leaves, which Cornell rounds to 3.5 kg.

Assumptions and limitations

• Carbon and nitrogen percentages are on a dry-matter basis and moisture is percent water in the material as weighed. Values in published tables, including the typical figures in the field help, vary widely between batches; a lab analysis of your own material is more reliable.
• All carbon and nitrogen are counted as equally available. Woody materials such as sawdust and wood chips hold much of their carbon in lignin, which breaks down slowly, so a mix rich in them behaves as if its C:N were lower than calculated.
• The target is an input. About 30:1 is the usual starting recommendation in Cornell's composting guide, which notes that a mix too low in nitrogen will not heat up and one too high in nitrogen may overheat or go anaerobic.
• The ratio describes the starting mix only. Carbon is lost as carbon dioxide during composting, so the ratio of finished compost is lower.
• Moisture of the mix is reported for reference; the calculator does not adjust for it. Cornell's worked example aims for 60% moisture at the start.

Frequently asked questions