Compost Ratio Explained: Greens vs Browns
“Greens” and “browns” are composting shorthand for nitrogen-rich and carbon-rich materials. Get the balance right and your pile heats up and breaks down fast. Get it wrong and you get sludge — or a pile that just sits there. Here is how the ratio works, with the actual numbers.
Aim for a carbon-to-nitrogen (C:N) ratio of 25–30:1 by weight. Greens like kitchen scraps and grass clippings supply nitrogen (around 15–20:1); browns like dry leaves and cardboard supply carbon (60–500:1). Weigh your materials and balance them with a calculator, then adjust whenever the pile smells or stalls.
What the C:N ratio actually measures
Every compostable material contains carbon and nitrogen. The microbes that do the composting burn carbon as fuel and use nitrogen to build proteins and multiply. The C:N ratio states how many parts of carbon exist for each part of nitrogen, by weight.
Kitchen scraps at 20:1 contain 20 parts carbon for each part nitrogen — relatively nitrogen-rich. Sawdust at 500:1 is almost all carbon, with barely any nitrogen at all. Neither is “better”; the microbes need both, in the right proportion.
Greens: the nitrogen suppliers
“Green” means nitrogen-rich, moist and usually fresh — not literally green. Coffee grounds are brown, but they are a green. These are the materials that heat a pile up:
| Material | C:N ratio | Notes |
|---|---|---|
| Kitchen scraps (fruit & veg) | 20:1 | The everyday green; chop large pieces to speed things up |
| Grass clippings | 20:1 | Add in thin layers so they do not mat into a slimy sheet |
| Coffee grounds | 20:1 | A green despite the colour; worms love them |
| Manure (herbivore) | 15:1 | The strongest common green; use herbivore manure only |
Browns: the carbon suppliers
“Brown” means carbon-rich and usually dry. These supply the energy and the structure that keeps air flowing through the pile:
| Material | C:N ratio | Notes |
|---|---|---|
| Dry leaves | 60:1 | Shred or mow over them to stop them matting |
| Straw | 80:1 | Hollow stems add air; a classic composting brown |
| Shredded paper | 175:1 | Plain paper only; avoid glossy inserts |
| Cardboard | 350:1 | Remove tape, tear up and soak before adding |
| Wood chips | 400:1 | Very slow; better as a base layer than a main ingredient |
| Sawdust | 500:1 | Use sparingly; it mats when wet and locks up nitrogen |
The 25–30:1 target
At 25–30 parts carbon per part nitrogen, microbes have the balanced diet they need to multiply fast. Below 25:1 there is more nitrogen than they can use, and the excess escapes as ammonia — the classic stinky pile. Above 30:1 they run short of nitrogen and the pile goes cold and stalls.
A handy rule of thumb: mix about 1 part greens to 2–3 parts browns by volume. Volume, not weight — browns are fluffy, so a bucket of leaves weighs far less than a bucket of scraps. When you want it exact, weigh everything in pounds and run the numbers.
Worked examples
The weighted ratio is (weight × ratio of each material, added up) ÷ total weight.
Balanced: 60 lb grass clippings (20:1) + 20 lb dry leaves (60:1) → (60×20 + 20×60) ÷ 80 = 2400 ÷ 80 = 30:1. In range.
A touch too green: 40 lb kitchen scraps (20:1) + 5 lb dry leaves (60:1) → (800 + 300) ÷ 45 ≈ 24.4:1. Fix: about 3.5 lb of dry leaves brings it to 27:1.
Far too brown: 15 lb kitchen scraps (20:1) + 45 lb dry leaves (60:1) → (300 + 2700) ÷ 60 = 50:1. Fixing it would take roughly 197 lb of grass clippings — which is why it is easier to build the mix correctly from the start than to rescue a very carbon-heavy pile.
Balance your own pile
Enter your materials in pounds and get your exact ratio — plus how much to add when it drifts off target.
Try the Compost Mix BalancerDiagnosing your pile
Your pile will tell you when the ratio is off. Match the symptom to the fix:
| Symptom | Likely problem | Fix |
|---|---|---|
| Strong ammonia smell | Too much nitrogen (below ~25:1) | Turn in browns: dry leaves or torn cardboard |
| Slimy, matted, flies | Too much nitrogen, plus too wet | Add browns, turn, and cover in heavy rain |
| Cold, nothing happening | Too much carbon (above ~30:1) | Add greens, water, and turn the pile |
| Dry and dusty | Needs water (or too carbon-heavy) | Water until like a wrung-out sponge; recheck the ratio |
| Steaming and shrinking | Just right (25–30:1) | Keep doing what you are doing |
Frequently asked questions
Do I really need to weigh everything?
No. The 1:2–3 volume rule gets you close enough for most home piles. Weighing pays off when a pile keeps failing — it turns guesswork into a number you can fix.
Is the ratio measured by weight or volume?
Weight. C:N ratios are determined by laboratory analysis of weight, and the maths only works with weights. Volume misleads because a bucket of leaves weighs far less than a bucket of kitchen scraps.
Why do some sources say 30:1 is ideal?
30:1 is the top of the ideal band and a fine single number to remember. The full working range is 25–30:1; anywhere inside it composts well.
Can I just layer greens and browns without any maths?
Yes — the classic method is alternating layers of roughly 1 part greens to 2–3 parts browns by volume, then adjusting by smell and heat. The maths is there for when layering is not working.
Does turning the pile change the ratio?
No. Turning adds oxygen, which helps microbes work and clears the symptoms of a bad ratio faster, but the carbon-to-nitrogen proportion stays the same until you add material.
What C:N ratio does finished compost have?
Around 10–15:1. Carbon is lost as carbon dioxide while the pile cooks, so the ratio falls as compost matures. Dark, crumbly, earthy-smelling compost is done, whatever the starting number was.
Related guides
How to Compost at Home for Beginners
Bins, layers, turning and troubleshooting — the full walkthrough.
How Much Soil Do I Need for a Raised Bed?
Work out the cubic feet before you fill your beds.
Best Soil Mix for Raised Beds
The 60/30/10 recipe, explained with numbers.
Material C:N values are typical figures from university extension composting guides; real batches vary with moisture, age and source.
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