Compost Carbon-to-Nitrogen (C:N) Ratio Calculator
Calculate the C:N ratio of a compost mix from greens and browns.
Get target 25:1 to 30:1 ratio, weight balance, and material lookup for active hot composting.
Compost Carbon-to-Nitrogen (C:N) Ratio
The optimal ratio for hot composting is 25:1 to 30:1 by weight (carbon to nitrogen). Outside this range, the pile heats slowly or releases ammonia.
Common organic materials (typical C:N):
| Material | C:N Ratio |
|---|---|
| HIGH NITROGEN (“greens”): | |
| Grass clippings (fresh) | 12-25:1 |
| Vegetable scraps | 12-20:1 |
| Coffee grounds | 20:1 |
| Manure (cow, pig) | 12-18:1 |
| Manure (chicken) | 7:1 |
| Manure (horse, with bedding) | 25-30:1 |
| Garden weeds (green) | 20:1 |
| Fish waste | 4:1 |
| Blood meal | 3:1 |
| Alfalfa hay | 12:1 |
| Seaweed | 19:1 |
| HIGH CARBON (“browns”): | |
| Leaves (autumn) | 50-80:1 |
| Straw | 50-150:1 |
| Hay (dry) | 25-40:1 |
| Sawdust (untreated) | 200-500:1 |
| Wood chips | 400-700:1 |
| Cardboard / paper | 350-500:1 |
| Pine needles | 60-110:1 |
| Corn stalks | 60-75:1 |
| Newsprint (shredded) | 175:1 |
| Peat moss | 58:1 |
The mix formula: C:N = (Σ Mass_i × C%_i) / (Σ Mass_i × N%_i)
For practical mixing, use C:N ratio directly: Final C:N = (M₁ × CN₁ + M₂ × CN₂) / (M₁ + M₂)
Where M is mass and CN is the C:N ratio of each material.
Why ratio matters:
- C:N below 20:1: Excess nitrogen → ammonia odor, fly attraction
- C:N 25-30:1: Sweet spot, heats fast (130-160°F), kills weed seeds
- C:N 30-40:1: Slower decomposition, no smell, safer for novice
- C:N over 50:1: Nitrogen-starved, decomposes very slowly, sometimes years
Hot pile management:
- Pile size: minimum 3’×3’×3’ (27 ft³) to retain heat
- Moisture: 50-60% (squeeze handful, drips just slightly)
- Turn: every 4-7 days for hot composting
- Internal temperature: target 130-160°F (54-71°C) for 3+ days to kill pathogens
- Finished compost ratio: drops to ~15:1 (humus stable)
Quick rule of thumb: For a 30:1 mix, layer 2-3 parts browns to 1 part greens by VOLUME. Volume isn’t perfectly accurate (browns are usually less dense) but works well for backyard piles.
Common mistakes:
- Adding too much sawdust without high-nitrogen offset (pile sits cold for months)
- All grass clippings (slimy, anaerobic, smells like ammonia)
- Wrong moisture (dry = no decomposition; wet = anaerobic)
- Pile too small (won’t reach hot temperatures)
- Adding meat / dairy (rodents, smell: backyard piles only do plant-based)
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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