Compost Browns to Greens Ratio Calculator
Getting the mix right is the difference between fast, sweet-smelling compost and a slimy, stinky pile. This free compost browns to greens ratio calculator tells you exactly how many pounds of browns (leaves, straw, cardboard) to add to your greens (food scraps, grass clippings, coffee grounds) to hit the ideal 25:1 carbon-to-nitrogen ratio — or any target ratio you set.
Enter your numbers
Your results
| Greens in the pile | — |
| Greens C:N ratio | — |
| Browns C:N ratio | — |
| Resulting mix ratio (estimated) | — |
Estimates: C:N ratios are typical published values and vary with moisture, freshness, and source. Actual pile performance also depends on particle size, air flow, and moisture.
How to use this calculator
Weigh your greens — food scraps, grass clippings, coffee grounds — and pick the matching material from the dropdown to get its typical carbon-to-nitrogen ratio. Then pick your browns material (dry leaves, straw, or cardboard). Results update instantly as you change anything.
The target ratio defaults to 25:1, the classic compost sweet spot. If your pile smells sour or slimy (too nitrogen-heavy), keep the target where it is and add the browns the calculator suggests. If your pile is barely heating (too carbon-heavy), try a lower target or add more greens.
How it works
Compost microbes need roughly 25 to 30 parts carbon for every 1 part nitrogen to work fast. The formula figures out how much browns (carbon-rich) to add to your greens (nitrogen-rich) so the blend lands on your target ratio:
Browns needed = greens weight × (target − greens ratio) ÷ (browns ratio − target)
For example, 10 lb of food scraps (15:1) with dry leaves (60:1) and a 25:1 target: 10 × (25 − 15) ÷ (60 − 25) = 100 ÷ 35 ≈ 2.86 lb of leaves. The resulting mix ratio shown is the target ratio itself — the formula is solved to hit it exactly — and the verdict judges the target against the ideal 20:1–35:1 band.
One caveat: the math treats each material's ratio as a single number, but real C:N values shift with moisture, freshness, and source — grass clippings lose nitrogen as they dry, and wet cardboard behaves differently from dry. That is why every ratio here is labeled as an estimate: use the result as your starting point, then let your nose and thermometer confirm.
Frequently asked questions
What is the ideal browns to greens ratio for compost?
Aim for a carbon-to-nitrogen (C:N) ratio of about 25:1 to 30:1 — roughly 25 parts carbon to 1 part nitrogen. In practice that usually means adding roughly 2 to 4 times as much browns as greens by weight, depending on the materials. A pile in this range heats up quickly, breaks down fast, and stays odor-free.
Why does my compost pile smell like ammonia?
That smell means your pile is too nitrogen-heavy — there are more greens than the microbes can balance with carbon. Add dry browns like leaves or shredded cardboard, turn the pile to add air, and use this calculator to find the right browns-to-greens weight for your materials.
Are the C:N ratios in this calculator exact?
No — they are typical published values and are labeled as estimates. Real C:N ratios vary with moisture, freshness, season, and source; for example, fresh grass clippings are more nitrogen-rich than dried ones. Treat the result as a starting point and adjust your pile by feel: smell and heat tell you a lot.
Can I measure browns and greens by volume instead of weight?
Yes, most gardeners do, and a rough 1:1 mix of browns to greens by volume gets you close for common materials. Weighing is more accurate because cardboard and dry leaves are fluffy while food scraps are dense. If you only have volumes, weigh a bucket of each material and use those weights in the calculator.
What happens if my compost is too carbon-heavy?
A carbon-heavy pile heats up slowly and can sit unchanged for months — microbes lack the nitrogen they need to work. Fix it by adding nitrogen-rich greens such as grass clippings or food scraps, and keep the pile moist like a wrung-out sponge.