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.