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)