Compost C:N Ratio Calculator
Balance your compost pile. Pick up to six materials such as grass clippings, kitchen scraps, manure, leaves, straw or cardboard, enter how much of each you have, and see the carbon-to-nitrogen ratio, estimated moisture, pile size and exactly how much brown or green material to add to reach 30:1.
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Volumes are converted with typical loose bulk densities.
How to use the Compost C:N Ratio Calculator
- Choose how you measure: buckets, cubic feet, wheelbarrows or weight.
- Pick each material in your pile and enter its amount.
- Set the target ratio (30:1 is the classic goal).
- Choose the brown and green you have on hand for balancing.
- Press Calculate C:N ratio to see the ratio, moisture and what to add.
The compost C:N formula
The ratio of a mix is total carbon ÷ total nitrogen, not the average of the materials' ratios. For each material:
- Dry weight = wet weight × (1 − moisture %)
- Nitrogen = dry weight × N %
- Carbon = nitrogen × C:N
To reach a target ratio T, the amount x of a balancing material (with carbon c and nitrogen n per pound) is x = (T × N − C) ÷ (c − T × n) when adding browns, or (C − T × N) ÷ (T × n − c) when adding greens.
Example: 3 buckets of grass clippings, 6 buckets of dry leaves and 2 buckets of vegetable scraps give about 14.6 lb of carbon and 0.49 lb of nitrogen, a ratio of about 30:1, right on target. That mix is around 70% moisture, so a few handfuls of dry shredded cardboard would help it stay airy.
Typical values used for common materials
| Material | Type | C:N | N (dry) | Moisture | Loose lb per cu yd |
|---|---|---|---|---|---|
| Grass clippings (fresh) | Green | 17:1 | 2.5% | 82% | 400 |
| Fruit and vegetable scraps | Green | 18:1 | 2.5% | 85% | 1,000 |
| Coffee grounds | Green | 20:1 | 2.3% | 60% | 700 |
| Green weeds and plant trimmings | Green | 20:1 | 2.2% | 80% | 400 |
| Cow manure | Green | 19:1 | 2.1% | 80% | 1,400 |
| Horse manure (some bedding) | Green | 30:1 | 1.5% | 70% | 1,000 |
| Chicken manure (fresh) | Green | 9:1 | 4.5% | 70% | 1,200 |
| Alfalfa hay or pellets | Green | 16:1 | 2.7% | 10% | 300 |
| Dry leaves | Brown | 54:1 | 0.9% | 38% | 200 |
| Grass hay (old) | Brown | 32:1 | 1.4% | 12% | 150 |
| Straw | Brown | 80:1 | 0.6% | 12% | 100 |
| Pine needles | Brown | 80:1 | 0.6% | 15% | 150 |
| Corn stalks | Brown | 60:1 | 0.7% | 12% | 100 |
| Shredded cardboard | Brown | 560:1 | 0.08% | 8% | 150 |
| Shredded newspaper | Brown | 600:1 | 0.07% | 8% | 150 |
| Wood chips | Brown | 400:1 | 0.12% | 40% | 450 |
| Sawdust (untreated) | Brown | 440:1 | 0.11% | 39% | 400 |
Values are rounded typical figures from published composting references; real materials vary with source, age and how tightly they are packed.
Tips for a balanced pile
- Stockpile dry leaves in fall so you have browns ready for summer grass clippings and kitchen scraps.
- Shred or chop browns: smaller pieces break down faster and mix better with greens.
- Layer or mix greens into browns rather than dumping a thick mat of fresh grass, which compacts and smells.
- Turn the pile when the center cools, checking moisture each time.
- Leave out meat, dairy, oily food, pet waste and diseased plants in a home pile.
Frequently asked questions
What is the best C:N ratio for compost?
About 25:1 to 35:1, with 30:1 the usual target. Below about 20:1 the extra nitrogen escapes as ammonia and the pile smells; above about 40:1 microbes run short of nitrogen and the pile composts slowly.
How do you calculate the C:N ratio of a compost mix?
Work out how much carbon and nitrogen each material contributes, add them up, and divide total carbon by total nitrogen. For each material, dry weight = wet weight × (1 − moisture), nitrogen = dry weight × N%, and carbon = nitrogen × its C:N ratio. Simply averaging the C:N numbers gives the wrong answer because materials carry very different amounts of nitrogen.
Is it 2 parts browns to 1 part greens?
A volume ratio of about 2 to 3 parts browns to 1 part greens is a handy rule of thumb, but it depends on the materials. Fluffy dry leaves are light and fairly low in carbon compared with sawdust, while wet food scraps are heavy. The calculator uses weight, moisture and nitrogen to give a real ratio.
What are greens and browns in compost?
Greens are moist, nitrogen-rich materials such as grass clippings, fruit and vegetable scraps, coffee grounds and fresh manure (C:N below about 30). Browns are dry, carbon-rich materials such as dry leaves, straw, cardboard, newspaper, wood chips and sawdust (C:N above 30).
How wet should compost be?
Aim for about 40 to 60 percent moisture, like a wrung-out sponge: damp to the touch but only a drop or two of water when squeezed. Too dry and decomposition stalls; too wet and the pile goes anaerobic and smells.
How big does a compost pile need to be to heat up?
About 1 cubic yard (3 × 3 × 3 ft) or larger holds enough heat for hot composting at 130 to 160 °F. Smaller piles and bins still compost, just more slowly and at cooler temperatures.
How accurate are the C:N values?
They are typical published averages. Real materials vary a lot: grass clippings can range from about 9:1 to 25:1 and straw from about 50:1 to over 100:1. Use the result as a starting point and adjust by smell, heat and texture.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.