Forget the ratio.
Read the recipe.
A mobile-first compost calculator that turns C:N math into plain-language recipes. “Add 2.5 buckets of dried leaves.” Not “32:1.”
Pick a material and a quantity to see the ratio.
Compost material reference
Rotio sizes the carbon and nitrogen in a pile and returns a recipe in buckets. This reference holds the field data behind that math: what a 5-gallon bucket of each material weighs, the target ratio for each composting method, the squeeze test for moisture, and a symptom-to-fix table for piles that have gone wrong.
What a 5-gallon bucket of each material weighs
This is the conversion most calculators skip. Two buckets of dried leaves and one bucket of fresh grass look carbon-heavy by volume, but the grass weighs more than five times as much per bucket, so the blend leans nitrogen. Weight is what the microbes respond to, not volume. Bucket weight = bulk density (lb/ft³) × 0.668. Values are air-dry; after heavy rain, weigh a sample bucket and switch the tool to pounds.
| Material | C:N | Density (lb/ft³) | Per 5-gal bucket |
|---|---|---|---|
| Vegetable and fruit scraps | 15:1 | 28 | 18.7 lb |
| Coffee grounds, used | 20:1 | 25 | 16.7 lb |
| Grass clippings, fresh | 17:1 | 20 | 13.4 lb |
| Cow manure, fresh | 16:1 | 45 | 30.1 lb |
| Chicken manure, fresh | 10:1 | 35 | 23.4 lb |
| Horse manure, with bedding | 30:1 | 25 | 16.7 lb |
| Garden trimmings, mixed | 22:1 | 14 | 9.4 lb |
| Alfalfa hay | 12:1 | 10 | 6.7 lb |
| Grass clippings, dried | 25:1 | 8 | 5.3 lb |
| Material | C:N | Density (lb/ft³) | Per 5-gal bucket |
|---|---|---|---|
| Sawdust, fresh | 500:1 | 18 | 12.0 lb |
| Wood chips, hardwood | 400:1 | 15 | 10.0 lb |
| Hay, dry mixed | 32:1 | 10 | 6.7 lb |
| Newspaper, shredded | 170:1 | 8 | 5.3 lb |
| Office paper, shredded | 150:1 | 6 | 4.0 lb |
| Corrugated cardboard, shredded | 350:1 | 6 | 4.0 lb |
| Wheat straw | 80:1 | 5 | 3.3 lb |
| Dried autumn leaves, mixed | 60:1 | 4 | 2.7 lb |
| Pine needles | 75:1 | 3 | 2.0 lb |
Target C:N ratio by composting method
The 25:1–35:1 band is a hot-composting target. Slower methods tolerate a wider, more carbon-leaning range because they are not chasing pathogen-kill temperatures.
| Method | Target C:N | Working temp | Time to finished | Turning |
|---|---|---|---|---|
| Hot / thermophilic | 25:1–35:1 | 130–160 °F | 8–12 weeks | Every 5–10 days |
| Cold / passive | 35:1–50:1 | Ambient | 12–18 months | None |
| Vermicompost (worm bin) | 30:1–40:1 | 55–77 °F | 8–16 weeks | None; refresh bedding |
| Sheet / lasagna (in place) | 30:1–40:1 | Ambient | 4–12 months | None |
| Bokashi (anaerobic ferment) | Not C:N driven | Sealed, ambient | 2 wk, then bury | None |
Moisture targets and the squeeze test
Aim for the wrung-sponge feel. Activity stalls below roughly 40% moisture and goes anaerobic above 65%. Grab a handful from the core and squeeze.
| Squeeze result | Approx. moisture | Reading | Action |
|---|---|---|---|
| Crumbles, dusty | Under 35% | Too dry | Water during a turn, or add wet greens |
| Holds shape, no drip | 50–60% | Ideal | Maintain |
| A few drops appear | About 65% | Slightly wet | Add dry browns, turn |
| Water streams out | Over 70% | Too wet | Add browns, turn, cover from rain |
Troubleshooting by symptom
A misbehaving pile is almost always telling you which way its ratio or moisture has drifted.
| Symptom | Likely cause | Ratio move | Fix |
|---|---|---|---|
| Ammonia or rotten smell | Excess nitrogen, low C:N, wet | Raise carbon | Mix in shredded browns, turn, add bulk |
| Won't heat up | Too much carbon, too dry, or pile too small | Lower carbon | Add greens, check moisture, build to 3 ft³+ |
| Heats then cools fast | Nitrogen or moisture used up | Add greens | Re-wet, add greens, turn |
| Slimy, matted clumps | Compacted wet greens, no structure | Add bulky carbon | Layer in straw or wood chips for airflow |
| Dries out and stalls | Too dry, too exposed | Hold moisture | Water on turns, shade in summer |
| Rodents or flies | Exposed food scraps at the surface | Bury greens | Cover with 4 in of browns; no meat or dairy |
| Slow and cold, no odor | High C:N, normal for cold composting | Optional | Shred or add greens to speed it up |
Materials to keep out of the pile
These carry pathogens, attract pests, or introduce contaminants a backyard pile cannot break down safely.
| Material | Why | Better route |
|---|---|---|
| Meat, fish, bones | Pests, pathogens, odor | Municipal organics, or bokashi then bury |
| Dairy, grease, cooking oil | Pests, anaerobic souring | Bokashi or trash |
| Dog or cat waste | Persistent parasites and pathogens | Trash; never near food crops |
| Glossy or coated paper | Clay coatings and heavy inks | Curbside recycling |
| Diseased plants | Spreads pathogens in a cool pile | Municipal hot compost or trash |
| Weeds gone to seed | Seeds survive a cold pile | Hot pile only, or solarize first |
| Treated or painted wood | Arsenic, copper, biocides | Hazardous waste |
| Coal or charcoal ash | Sulfur and heavy metals | Trash; wood ash only, sparingly |
Particle size and how fast each class breaks down
Surface area drives decomposition. Anything over roughly 100:1 benefits from shredding before it goes in.
| Material class | Shred first? | Breakdown (hot pile) | Note |
|---|---|---|---|
| Soft greens (scraps, grass) | No | 2–4 weeks | Fast, but mats if piled thick |
| Tree leaves | Recommended | 4–8 wk whole, 2–4 shredded | Mow to shred |
| Straw and hay | Chopping helps | 8–16 weeks | Slow without added nitrogen |
| Cardboard and paper | Shred or tear | 8–12 weeks wet | Wet thoroughly before adding |
| Wood chips and bark | Optional | 6–24 months | Best as a bulking agent |
| Sawdust | Use sparingly | 6–18 months | Very high C:N; compacts easily |
Sources and standards
| Reference | What it provides |
|---|---|
| Cornell On-Farm Composting Handbook (NRAES-54, Rynk 1992) | C:N values, bulk densities, mass-weighted blend formula |
| WSU Compost Fundamentals | Material C:N, moisture and aeration targets |
| BioCycle / Compost Magazine | Field values for less-common materials |
| US EPA pathogen-reduction guidance (PFRP) | 131 °F sustained 3 days for pathogen kill |
| US Composting Council, TMECC | Lab test methods for C:N and compost maturity |
Related concepts
- PFRP and pathogen kill temperatures: the 131 °F-for-3-days rule that makes hot compost safe for food crops.
- Vermicomposting: worm-bin bedding and feedstock ratios, which sit just above the hot-pile band.
- Bokashi fermentation: an anaerobic pre-treatment that lets meat and dairy enter the soil after burial.
- Compost maturity testing: how labs confirm a finished pile using volatile solids and respiration.
- Bulk density and air-fill porosity: why a pile needs coarse carbon for structure, not just the right ratio.