Balance up to three compost materials for a 30:1 carbon-to-nitrogen ratio and 60% moisture with Cornell’s composting equations, including the two-goal solution
Updated October 2026
Compost microbes use carbon for energy and nitrogen to build protein. Cornell’s composting program puts the usual starting target at about 30 parts carbon to 1 part nitrogen. Too little nitrogen and the pile will not heat up. Too much, and it can overheat or go anaerobic and smell. As carbon leaves as carbon dioxide, the ratio falls, and finished compost is typically close to 10:1. Moisture is the other half. Cornell’s examples aim for 60%, and in its grass-and-leaves example moisture turns out to be the more critical of the two goals.
The C:N of a mix is the total carbon divided by the total nitrogen, both on a dry-weight basis. Each material contributes its wet weight × (100 − moisture%) × its carbon or nitrogen percentage. So R = Σ Q × C × (100 − M) ÷ Σ Q × N × (100 − M), and the mix moisture is Σ Q × M ÷ Σ Q.
Cornell’s example. Grass clippings are 77% water, 45% carbon and 2.4% nitrogen (C:N 18.75). Leaves are 35% water, 50% carbon and 0.75% nitrogen (C:N 66.67). Food scraps are 80% water, 42% carbon and 5.0% nitrogen.
The calculator’s default is that rounded mix, and it returns 29.9:1 at 60.2% moisture.
| Material | C:N (Cornell table) | Moisture | Role |
|---|---|---|---|
| Grass clippings | 10 to 25:1 | about 80% (77% in the worked example) | Green |
| Vegetable scraps | 10 to 20:1 | 80 to 90% | Green |
| Fruit wastes | 20 to 50:1 | 80 to 90% | Green |
| Coffee grounds | 20:1 | not listed | Green |
| Horse manure | 20 to 50:1 | not listed | Green to neutral |
| Autumn leaves | 40 to 80:1 | about 40% (35% in the worked example) | Brown |
| Straw | 50 to 150:1 | not listed | Brown |
| Sawdust | 200 to 750:1 | about 40% | Brown |
| Newspaper | 400 to 900:1 | not listed | Brown |
| Corrugated cardboard | 600:1 | not listed | Brown |
Cornell calls these representative ranges, not universal values. Leaves alone range from about 15:1 (alder) to 113:1 (larch) in its leaf table, and maple and oak run 52 and 47. To use a C:N figure in the calculator, pick a nitrogen percentage and set carbon = N × C:N, which is Cornell’s own conversion. A lab test of your actual material beats any table.
| Mix (kg, Cornell material values) | C:N | Moisture | Verdict |
|---|---|---|---|
| 10 grass + 3.5 leaves (C:N solution only) | 30.0:1 | 66.2% | Right ratio, too wet |
| 10 grass + 6.8 leaves (moisture solution only) | 36.7:1 | 60.0% | Right moisture, ratio a bit high |
| 10 grass + 13 leaves + 8 food scraps | 29.9:1 | 60.2% | Both goals, Cornell’s solution |
| 10 grass + 10 leaves | 41.2:1 | 56.0% | Dry side, carbon-heavy |
| 10 food scraps + 5 leaves | 19.8:1 | 65.0% | Wet and nitrogen-rich: needs more browns |
Cornell’s equations use weight, because C:N and moisture are percentages by mass. A bucket of wet grass weighs far more than a bucket of dry leaves, so a “two buckets of browns to one of greens” rule can land anywhere. Weigh a bucket of each material once, then convert. If the pile stays cold, it usually needs nitrogen or water. If it smells, it usually needs dry carbon and air. Cornell suggests treating water as one of the unknowns when your ingredients are dry. Water raises moisture without changing the C:N.
It balances carbon, nitrogen and water only. It does not cover particle size, aeration, pile size or temperature. It ignores how much of the carbon microbes can actually reach: woody materials such as sawdust release carbon slowly. It also does not cover which materials are safe to compost at home. Your local extension office or municipal compost program has rules for meat, dairy, pet waste and diseased plants.