Compost C/N Ratio Calculator (Carbon to Nitrogen)
Is your compost pile too brown or too green? Pick a material for each row, or choose Custom and type your own carbon and nitrogen percentages. Preset percentages are typical dry-weight values; real materials vary. Use the same weight unit in every row.
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Formula
- Carbon in a material = weight x carbon % / 100; nitrogen likewise
- Blend C/N = total carbon / total nitrogen
- Weight x of material 1 for a target T:
x = (T x N_other - C_other) / (C1 - T x N1), where C and N are in weight units (percent / 100 x weight per unit)
Typical values used by the presets (dry basis)
| Material | Carbon % | Nitrogen % | About C/N |
|---|---|---|---|
| Dry leaves | 48 | 0.8 | 60 |
| Grass clippings | 44 | 2.6 | 17 |
| Kitchen scraps | 38 | 2.5 | 15 |
| Straw | 46 | 0.6 | 77 |
| Sawdust | 50 | 0.1 | 500 |
| Coffee grounds | 40 | 2 | 20 |
| Chicken manure | 35 | 3.5 | 10 |
Worked example
10 units of leaves and 10 of grass clippings: carbon 4.8 + 4.4 = 9.2, nitrogen 0.08 + 0.26 = 0.34, so C/N = 9.2 / 0.34 = 27.06, inside 25 to 30. For a target of 30, material 1 would be (30 x 0.26 - 4.4) / (0.48 - 30 x 0.008) = 3.4 / 0.24 = 14.17 units.
When to use it
Use it when building a pile, or when one is slow or smelly. A typical starting target is 25 to 30 parts carbon to 1 part nitrogen.
Common mistakes
- Mixing weight units between rows.
- Using fresh, wet weights when the percentages are on a dry basis.
- Expecting a precise answer from preset percentages; use Custom if you have a lab figure.
How it differs from nearby calculators
This page balances the ingredients of a pile before it is made. The compost calculator sizes how many cubic yards of finished compost to spread on a lawn or bed, and the fertilizer NPK calculator works with bagged fertilizer analysis.
Frequently asked questions
What C:N ratio should compost have?
A starting ratio of roughly 25 to 30 parts carbon to 1 part nitrogen is the commonly cited target for fast, hot composting. Piles far above 30 break down slowly; far below 25 can lose nitrogen as ammonia and smell.
How do you calculate the C:N ratio of a mix?
Add up the carbon and nitrogen in each material (weight x percentage), then divide total carbon by total nitrogen. Equal weights of leaves (48 % C, 0.8 % N) and grass (44 % C, 2.6 % N): C = 0.48 + 0.44 = 0.92, N = 0.008 + 0.026 = 0.034 per unit pair, ratio 27.06.
Are the preset percentages exact?
No. They are typical dry-weight values from composting references and vary widely with age, moisture and source. If you have a lab figure or a label, choose Custom and enter it.
How much of material 1 do I need to reach the target?
The tool solves (C1 x + C_other) / (N1 x + N_other) = target for the weight x of material 1. In the default example it is 14.17, about 4.2 more than the 10 entered. A result of 0 means the target cannot be reached by changing material 1 alone.
What is the C/N ratio of leaves mixed with chicken manure?
Using the typical presets, 10 units of dry leaves and 10 of chicken manure give carbon 4.8 + 3.5 = 8.3 and nitrogen 0.08 + 0.35 = 0.43, so the ratio is 19.3. That is below 25, so the blend is nitrogen-rich and needs more browns.
What is a good brown to green ratio for compost?
There is no single volume rule because wet greens weigh more than dry browns. By weight on a dry basis, equal parts dry leaves and grass clippings give 27.06, which is inside the typical 25 to 30 range. Use the calculator with your own materials.
What happens if the C/N ratio is too high?
A ratio above about 30 means little nitrogen for the microbes, so the pile breaks down slowly. For example 10 units each of leaves and coffee grounds gives 31.43; adding some grass or scraps lowers it.