Compost C:N Ratio Calculator
The carbon to nitrogen ratio of a compost mix from the weights and analyses of two materials.
Results
What this tool does
A compost heap that never heats up and one that reeks of ammonia have the same cause: the wrong balance between carbon and nitrogen. Gardeners call them browns and greens, and the useful target is around thirty parts carbon to one of nitrogen by weight. Since the ratio depends on the mass of each element and not on the mass of the material, mixing is a weighted calculation rather than a simple average.
Formula
C:N = (m1 C1 + m2 C2) / (m1 N1 + m2 N2)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
m1 | Weight of material 1 | kg |
c1 | Carbon in material 1 | % |
n1 | Nitrogen in material 1 | % |
m2 | Weight of material 2 | kg |
c2 | Carbon in material 2 | % |
n2 | Nitrogen in material 2 | % |
CN | Carbon to nitrogen ratio | — |
CC | Carbon in the mix | kg |
NN | Nitrogen in the mix | kg |
TM | Total weight | kg |
Worked example
- Weight of material 110 kg
- Carbon in material 145 %
- Nitrogen in material 10.5 %
- Weight of material 25 kg
- Carbon in material 245 %
- Nitrogen in material 23 %
- Carbon to nitrogen ratio33.75
- Carbon in the mix6.750 kg
- Nitrogen in the mix0.2000 kg
- Total weight15.00 kg
Limitations
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
Frequently asked questions
What ratio should I aim for?
Between about 25 and 35 parts carbon to one of nitrogen. Too much carbon and the heap simply sits there cold, because the microbes have no nitrogen to build bodies with; too much nitrogen and the surplus escapes as ammonia, which is both a smell and a waste. You need the carbon and nitrogen percentages of your actual materials — they are widely tabulated and vary enormously, from about 500:1 for sawdust to 15:1 for fresh grass.