Compost C:N Ratio Calculator & Balance Greens & Browns
Balances grass & straw
Mix your greens and browns by weight and get the pile's carbon-to-nitrogen ratio — with a verdict against the ideal 25–35:1 window and advice on exactly what to add for fast, hot compost.
Your mix has a carbon-to-nitrogen ratio of 28.3:1 — ideal — composts fast. Right in the sweet spot. Keep it moist like a wrung sponge and turn regularly.
Next: aim for a 25–35:1 blend (roughly 2–3 parts browns to 1 part greens by volume), keep it as moist as a wrung-out sponge, and turn the pile every week or two to keep it hot and aerobic.
C:N values are typical averages and vary with how fresh, dry or woody each material is — treat the ratio as a guide.
Compost C:N — key facts
- Ideal C:N
- 25–35 : 1 (≈30:1)
- Too low (<25)
- smelly, too wet — add browns
- Too high (>35)
- slow, woody — add greens
- Browns:greens
- ≈ 2–3 : 1 by volume
- Grass clippings
- C:N ≈ 20:1
- Straw
- C:N ≈ 75:1
- Moisture
- like a wrung sponge
- Privacy
- Runs in your browser; nothing uploaded
Why the C:N ratio decides your compost
Composting is microbial work, and the microbes need carbon for energy and nitrogen to build proteins as they multiply. When the blend sits near 30 parts carbon to 1 part nitrogen, they thrive — the pile heats, weed seeds and pathogens die off, and material breaks down in weeks. Stray too far and it stalls: too much nitrogen (low C:N) goes wet and smelly as excess nitrogen is lost as ammonia, while too much carbon (high C:N) sits cold and woody because microbes run short of nitrogen.
This calculator works from the actual masses you add, not a vague rule, because greens and browns differ wildly in how much carbon and nitrogen they carry. It sums the carbon and nitrogen across every ingredient, reports the blend's C:N, and tells you which way to nudge it. Keep the pile as damp as a wrung-out sponge and turn it for air, and a balanced ratio will compost fast.
Balance the pile
Get the exact C:N of your mix and the nudge needed to hit the 25–35:1 sweet spot.
Fix a cold pile
See when a pile is too carbon-heavy and which greens to add to make it heat up.
Stop the smell
Spot a nitrogen-heavy, anaerobic mix and add browns to bring it back into balance.
Use any ingredients
Add as many greens and browns as you like; the ratio updates from their real weights.
Frequently Asked Questions
What is the ideal C:N ratio for compost?+
Around 25–35 parts carbon to 1 part nitrogen (often quoted as 30:1) is the sweet spot. In that range microbes have the carbon for energy and the nitrogen for protein to multiply fast, so the pile heats up and breaks down quickly. This tool tells you where your mix sits and how to correct it.
What are greens and browns in composting?+
'Greens' are nitrogen-rich, usually moist materials — fresh grass, kitchen and vegetable scraps, manure, coffee grounds. 'Browns' are carbon-rich, usually dry — dead leaves, straw, sawdust, cardboard. A good pile blends both; this calculator carries typical C:N values for each.
What ratio of browns to greens should I use?+
By volume, roughly 2–3 parts browns to 1 part greens usually lands near the ideal C:N, because browns are far more carbon-dense per unit nitrogen. By weight it differs, which is why this tool calculates the actual ratio from the masses you enter rather than relying on a rule of thumb.
Why won't my compost heat up?+
A pile that stays cold is usually too high in carbon (too many browns) or too dry. Microbes lack the nitrogen or moisture to multiply. Add greens (grass, manure, scraps) and water to a wrung-sponge dampness, and the C:N will drop toward the active range.
Why does my compost smell bad?+
A sour, ammonia or rotten-egg smell usually means too much nitrogen (too many greens) and not enough air — the pile has gone anaerobic. Mix in browns to raise the C:N, turn it to add oxygen, and the smell should clear.
How is the C:N ratio calculated here?+
For each material the tool multiplies its weight by its nitrogen content to get nitrogen mass, then by its C:N ratio to get carbon mass. It sums carbon and nitrogen across all ingredients and divides total carbon by total nitrogen for the blend's C:N.
Does moisture affect composting too?+
Yes — alongside C:N, a pile needs moisture (about as damp as a wrung-out sponge) and air. Too dry and microbes stall; too wet and it goes anaerobic and smelly. Turning the pile restores oxygen and evens out moisture.
Can I compost only kitchen scraps?+
You can, but scraps alone are nitrogen-heavy (low C:N) and tend to go wet and smelly. Mixing in browns such as dry leaves, shredded cardboard or straw balances the ratio, improves airflow and produces better compost faster.
How long does compost take?+
A well-balanced, turned hot pile can finish in a few weeks to a couple of months; a neglected or carbon-heavy pile can take a year or more. Getting the C:N into the 25–35 range, keeping it moist and turning it are the biggest levers on speed.
Are the C:N values exact for my materials?+
They're representative averages — actual C:N varies with how fresh, dry or woody a material is (for instance, old straw is higher in carbon than fresh). Use the result as a strong guide and adjust greens or browns to taste as the pile progresses.