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Compost C:N Calculator.
Compost C:N Calculator
Hot compost needs a 25:1 to 35:1 carbon-to-nitrogen ratio. Pick your feedstocks, set quantities, and the calculator finds the blend.
Why C:N matters
The microbes that break down organic matter into compost need both carbon (energy) and nitrogen (protein for building bodies). Their preferred diet is roughly 25-30 parts carbon to 1 part nitrogen by weight.
| C:N ratio | What happens |
|---|---|
| 15:1 or lower | Too much N; pile reeks of ammonia; nitrogen lost as gas; slow decomposition. Add carbon-rich "browns". |
| 20:1 to 25:1 | Decomposes fast but burns through N; less stable end product. |
| 25:1 to 35:1 | Sweet spot. Heats to 130-160°F. Decomposes in 30-90 days. Rich, stable end product. |
| 40:1 or higher | Too much carbon; pile won't heat; takes years; finished compost may temporarily tie up soil N. Add "greens". |
Typical feedstock C:N values
| Greens (high N) | C:N | Browns (high C) | C:N |
|---|---|---|---|
| Fresh grass clippings | 15-20:1 | Wood chips (fresh) | 300-500:1 |
| Vegetable / fruit scraps | 15-25:1 | Sawdust | 300-700:1 |
| Coffee grounds | 20:1 | Cardboard | 350:1 |
| Manure (cow) | 20:1 | Newspaper | 175:1 |
| Manure (chicken) | 7:1 | Straw | 75:1 |
| Manure (horse) | 25:1 | Dry leaves (oak) | 60:1 |
| Garden weeds (green) | 20-30:1 | Pine needles | 80:1 |
| Alfalfa hay | 12:1 | Hay (older / dry) | 25-40:1 |
| Comfrey leaves | 10-15:1 | Corn stalks (dry) | 60-75:1 |
| Seaweed | 19:1 | Peat moss | 60:1 |
Hot composting checklist
- C:N ratio 25-35:1. The calculator above gives you this.
- Pile size at least 1 cubic yard (3' × 3' × 3'). Smaller piles can't retain heat.
- Moisture 50-60%. Squeezed handful drips a few drops; wrings out otherwise.
- Aeration. Turn the pile every 7-14 days OR build with passive aeration channels (perforated PVC, pallets at base).
- Particle size variety. Mix coarse (structure for air) and fine (surface area). All-fine = compacted; all-coarse = dries out.
- Temperature monitoring. Pile thermometer. Target 130-160°F sustained for 3+ days to kill weed seeds and pathogens.
- Curing. After hot phase ends (~30 days for a turned pile), let mature for another 30-90 days at ambient temperature before using.
Method comparison
| Method | Time | Effort | Pathogen kill? |
|---|---|---|---|
| Cold pile (just dump it) | 1-3 years | Lowest | No (cool temperatures) |
| Standard hot pile (turn weekly) | 2-4 months | Moderate | Yes if thermophilic phase reached |
| Berkeley method (turn every other day) | 14-21 days | High | Yes |
| Vermicompost (worm bin) | 2-4 months | Moderate | No (mesophilic worms; not hot) |
| Bokashi (anaerobic ferment) | 2-3 weeks ferment + 4 weeks soil bury | Low | No (no heat) |
| Static aerated pile (forced air) | 1-3 months | Setup-heavy, run-easy | Yes |
Free under CC BY 4.0. Cite as "OAT Compost C:N Calculator (openagriculturetechnology.com)". Feedstock C:N ratios from Cornell Composting Resources and standard composting literature.