Compost Ratio Calculator
Calculate the carbon-to-nitrogen ratio for your compost pile and get recommendations for perfect compost.
Did this tool work for you?
How to use this calculator
Weighted average C:N = (sum of each material's kg × its C:N ratio) / total kg. Ideal C:N ratio is 25–30:1 for active decomposition.
- 1
Select your brown (carbon-rich) material and enter its weight in kg.
- 2
Select your green (nitrogen-rich) material and enter its weight in kg.
- 3
Read the estimated C:N ratio and check if it falls in the ideal 25–30:1 range.
- 4
Follow the recommendation to adjust your mix for faster, better compost.
Frequently asked questions
How does this tool work out the C:N ratio?
It takes a mass-weighted average of the two C:N figures built into the tool: (brown kg × brown ratio + green kg × green ratio) ÷ total kg. It is a simple estimate for planning a pile, not a laboratory analysis, and it ignores moisture and the real nitrogen content of your particular materials.
Which carbon-to-nitrogen figures does it use?
Browns: dry leaves 60, straw 80, newspaper or paper 170, cardboard 400, wood chips 400. Greens: food scraps 15, fresh manure 15, grass clippings 20, coffee grounds 20, fresh plant cuttings 30. Published values for these materials vary widely, so treat the figures as the ones this tool uses, not universal constants.
What counts as the ideal range?
The tool flags 25 to 30 parts carbon to 1 part nitrogen as ideal. Below 15 it reports too much nitrogen, 15 to 25 slightly nitrogen-rich, above 30 up to 50 slightly carbon-rich, and above 50 too much carbon.
Where does the weeks-to-compost figure come from?
It is a lookup by band, not a prediction: 16 weeks for the below-15 band, 10 for 15 to 25, 6 for the ideal band, 12 for 30 to 50 and 20 above 50. Temperature, particle size, moisture and turning frequency all change real timing.
Can I mix more than one brown or green?
No. You choose one brown material and one green material with an amount for each. For a blend, run the tool for each pairing, or combine materials by hand into a single weight using the closest listed type.
Why does a pile of mostly wood chips show hundreds to one?
Because the tool uses 400 for wood chips and cardboard, and the average is weighted by mass. Ten kilograms of wood chips with five of grass clippings comes out at 273.3:1 here, so the result moves strongly toward whichever material you add more of.
What happens if I enter zero for one side?
The result becomes that single material’s own figure, for example 60.0:1 for dry leaves alone. If both amounts are zero the tool asks you to enter amounts instead of showing a ratio.
Does it measure by weight or volume?
Weight, in kilograms. Materials such as leaves and straw are light for their volume, so a bucket count will not translate directly. Weigh a sample bucket if you only know volumes.
Carbon-to-nitrogen ratio for a compost pile
What the ratio expresses
A C:N ratio compares how much carbon a material holds against its nitrogen. Dry, woody materials carry a lot of carbon per unit of nitrogen and so have high ratios; fresh kitchen waste and manure sit at the low end. A pile is a blend, so its overall figure lies somewhere between the ratios of what goes in. This calculator treats that blend as an average weighted by kilograms, then compares the result with a target band of 25 to 30 to 1.
Figures built into this tool
The table lists the carbon-to-nitrogen figure the calculator assigns to each selectable material. Reference values for compost feedstocks differ between sources and between batches of the same material, so these are working numbers rather than measurements.
| Material | Group | C:N figure used |
|---|---|---|
| Wood chips | Brown | 400 |
| Cardboard | Brown | 400 |
| Newspaper / paper | Brown | 170 |
| Straw | Brown | 80 |
| Dry leaves | Brown | 60 |
| Fresh plant cuttings | Green | 30 |
| Grass clippings | Green | 20 |
| Coffee grounds | Green | 20 |
| Food scraps | Green | 15 |
| Fresh animal manure | Green | 15 |
Worked examples with the calculator’s output
The table below shows six pairings run with fixed amounts, and the ratio, verdict and timing the tool returned for each. Only the last one lands inside the 25 to 30 band.
| Brown | Green | Result | Verdict | Time band |
|---|---|---|---|---|
| 10 kg dry leaves | 5 kg grass clippings | 46.7:1 | Slightly carbon-rich | 12 weeks |
| 10 kg dry leaves | 10 kg food scraps | 37.5:1 | Slightly carbon-rich | 12 weeks |
| 10 kg wood chips | 5 kg grass clippings | 273.3:1 | Too much carbon | 20 weeks |
| 6 kg cardboard | 20 kg plant cuttings | 115.4:1 | Too much carbon | 20 weeks |
| 5 kg dry leaves | 20 kg food scraps | 24.0:1 | Slightly nitrogen-rich | 10 weeks |
| 2 kg straw | 9 kg fresh manure | 26.8:1 | Ideal | 6 weeks |
Reading the numbers
Compare the first two rows: doubling the green material in the leaf pile moved the ratio from 46.7 to 37.5, which shows how little ground a modest change in greens covers when browns have a high figure. The leaves and food scraps in the fifth row slipped just under 25 and earned a different message and a longer time band, because the thresholds are hard cut-offs rather than a gradual scale. When a result sits near a boundary, treat the verdict as a nudge and judge the pile by how it behaves.
Limits of a ratio-only estimate
The calculator knows nothing about moisture, particle size, bin size or temperature, all of which affect how a pile behaves. It also does not model materials that are not on the list, such as sawdust, hay or seaweed. Use it to choose proportions for a first batch, then adjust by observation. If your own materials come with a tested C:N figure, weigh that against the built-in value. When a result is far outside the band, change one amount at a time and rerun to see how much of the other material it takes to move the ratio into range.
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →