Composting Carbon to Nitrogen Ratio Calculator
Calculate the C:N ratio for your compost pile from browns and greens by weight.
Returns your ratio vs the ideal 25-30:1 and a mix adjustment guide.
Composting is the biological decomposition of organic materials into humus — a rich soil amendment. The speed and quality of composting is largely determined by the Carbon-to-Nitrogen ratio (C:N ratio) of your compost pile.
Why C:N ratio matters:
- Carbon (found in “browns”) provides energy for decomposing microorganisms
- Nitrogen (found in “greens”) provides protein for building microbial cells
- Ideal C:N ratio: 25:1 to 30:1
Too much carbon (high C:N, like 50:1 or higher) = slow decomposition, pile stays cool, takes months to years. Too much nitrogen (low C:N, like 10:1) = pile heats up too fast, releases ammonia, smells bad.
C:N ratios of common materials:
| Material | C:N Ratio | Type |
|---|---|---|
| Wood chips / sawdust | 400–500:1 | Brown |
| Cardboard (shredded) | 350:1 | Brown |
| Dry straw | 75–100:1 | Brown |
| Dry leaves | 60–80:1 | Brown |
| Paper (newspaper) | 175:1 | Brown |
| Food scraps (veg/fruit) | 15–20:1 | Green |
| Fresh grass clippings | 17–20:1 | Green |
| Coffee grounds | 20:1 | Green |
| Manure (chicken) | 10–15:1 | Green |
| Manure (cow) | 20–25:1 | Green/Neutral |
| Fresh vegetable trimmings | 12–15:1 | Green |
The calculation: Weighted Average C:N = Σ(Weight × C:N) ÷ Σ(Weight)
Where each material’s weight and C:N ratio are combined proportionally.
Target range: 25:1 to 30:1 for hot, active composting.
Practical tips:
- A simple rule of thumb: 3 parts browns to 1 part greens by volume
- Chop or shred materials into pieces under 2 inches (5 cm) for faster decomposition
- Keep moisture at 40–60%, the pile should feel like a wrung-out sponge
- Turn the pile every 1–2 weeks to add oxygen
Metric and imperial note: This calculator uses weight ratios — any consistent unit (kg, lbs, or even cups) works since we’re computing a ratio.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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