About the compost C:N ratio calculator
A compost pile is really a colony of microbes, and those microbes need carbon for energy and nitrogen to build proteins in a fairly specific proportion. Get the carbon-to-nitrogen ratio right and the pile heats up, breaks down fast, and stays sweet-smelling. Get it wrong and it either turns into a cold, dry stack of leaves or a wet, ammonia-reeking mess. This calculator blends the "browns" and "greens" you plan to add and reports the resulting C:N ratio so you can adjust before you build the pile.
The target window is a C:N ratio of roughly 25 to 30 parts carbon to 1 part nitrogen. Carbon-rich browns sit well above that, dry leaves around 60:1, straw near 75:1, cardboard about 350:1, and wood chips up to 500:1. Nitrogen-rich greens sit below it: fresh grass clippings near 20:1, food scraps about 15:1, coffee grounds around 20:1, and manure near 15:1. Because browns carry so much more carbon per unit weight, a balanced pile usually mixes more browns than greens. Too much green starves the pile of carbon and it goes slimy and smelly; too much brown leaves the microbes short on nitrogen and decomposition crawls.
How it works: the formula
The blended ratio is the total carbon contributed by all materials divided by the total nitrogen, which works out to a weight-weighted average of each material's own C:N:
C:N_blend = (browns_kg x brown_CN + greens_kg x green_CN)
/ (browns_kg + greens_kg)
- browns_kg and greens_kg are the weights of each material group you are adding.
- brown_CN is the carbon-to-nitrogen ratio of your browns (leaves around 60, straw around 75).
- green_CN is the ratio of your greens (food scraps around 15, grass around 20).
- Aim for a blended result between 25 and 30; higher means add greens, lower means add browns.
Worked example
Mixing 10 kg of dry leaves (C:N 60) with 4 kg of food scraps (C:N 15):
- Carbon weighting from browns: 10 x 60 = 600.
- Carbon weighting from greens: 4 x 15 = 60.
- Sum: 600 + 60 = 660.
- Total weight: 10 + 4 = 14 kg.
- Blended C:N: 660 / 14 = 47:1.
Reference: C:N ratio of common materials
| Material | Type | Approx C:N |
|---|---|---|
| Food scraps | Green | 15:1 |
| Manure | Green | 15:1 |
| Coffee grounds | Green | 20:1 |
| Fresh grass clippings | Green | 20:1 |
| Dry leaves | Brown | 60:1 |
| Straw | Brown | 75:1 |
| Cardboard / paper | Brown | 350:1 |
| Wood chips / sawdust | Brown | 500:1 |
Common pitfalls
- Too much green. A pile heavy in grass and food scraps drops below 20:1, goes anaerobic, and smells of ammonia; add browns to fix it.
- Too much brown. Above 40:1 the microbes run short of nitrogen and the pile sits cold and dry for months; mix in greens or a nitrogen source.
- Confusing volume with weight. Wood chips and leaves are light and fluffy; the formula uses weight, so a big pile of leaves weighs less than it looks.
- Treating all browns as equal. Cardboard at 350:1 and sawdust at 500:1 carry far more carbon than leaves at 60:1, so a little goes a long way.
- Ignoring moisture and air. Even a perfect ratio fails if the pile is bone-dry or compacted; keep it damp like a wrung sponge and turn it for oxygen.
Frequently asked questions
What is the ideal C:N ratio for compost?
Roughly 25 to 30 parts carbon to 1 part nitrogen. In that window microbes have enough carbon for energy and enough nitrogen to build cells, so the pile heats up and breaks down quickly without smelling. Above about 40:1 it stalls cold and dry; below about 20:1 it turns wet and ammonia-scented.
What counts as a brown versus a green?
Browns are dry, carbon-rich materials such as fallen leaves, straw, cardboard, and wood chips. Greens are moist, nitrogen-rich materials such as grass clippings, food scraps, coffee grounds, and manure. The names refer to chemistry, not literal color, so brown coffee grounds are a green and pale straw is a brown.
Why is my compost slimy and smelly?
That is the classic sign of too much green: the C:N ratio has dropped too low, the pile has gone anaerobic, and excess nitrogen is escaping as ammonia. Mix in plenty of browns such as shredded cardboard or dry leaves to raise the carbon, and turn the pile to add oxygen.
Should I add more browns or more greens by weight?
Usually more browns by weight, because greens are far more nitrogen-dense per unit weight. A common rule of thumb is two to three parts browns to one part greens, but the exact mix depends on which materials you use; this calculator gives you the precise blended ratio so you can fine-tune it.
Does a perfect C:N ratio guarantee fast compost?
No. The ratio is necessary but not sufficient. The pile also needs moisture like a wrung-out sponge, oxygen from periodic turning, and enough mass to retain heat. A perfectly balanced pile that is bone-dry or tightly compacted will still break down slowly.