The compost ratio is the balance of nitrogen-rich greens and carbon-rich browns in a compost heap, and it is the single most important factor in whether a heap works quickly and cleanly or sits there smelling and doing nothing. Get the ratio right and the organisms in the heap have the food they need to break material down fast and hot. Get it wrong and the heap either goes slimy and smelly or stalls and stays dry and unchanged for months. This guide explains the ratio in plain terms, gives the numbers for common materials, and shows how to fix a heap that has gone wrong.
The standard advice is roughly one part green to two or three parts brown by volume. That ratio is not a law — it is a starting point that works for most garden heaps — but it is a good one, because most gardeners add far too many greens and not enough browns. Grass clippings, kitchen scraps and fresh weeds are easy to add in bulk, while browns take more effort to collect, so the heap drifts green and the problems follow.
Greens and browns: what they are
Greens are nitrogen-rich, wet and quick to rot. They feed the organisms that do the breaking down, and they provide the moisture the heap needs. Common greens include grass clippings, vegetable and fruit peelings, fresh weeds, fresh manure, coffee grounds, and green leaves. Greens are usually soft, damp and bright green, and they rot quickly on their own — a pile of grass clippings alone goes slimy within days.
Browns are carbon-rich, dry and slow to rot. They provide the energy the organisms need, give the heap structure and air spaces, and soak up excess moisture from the greens. Common browns include autumn leaves, straw, shredded cardboard and paper, twigs and prunings, sawdust, wood chips, and dried stems. Browns are usually dry, brown and stiff, and they rot slowly on their own — a pile of dry leaves alone sits unchanged for months.
The ratio works because the organisms breaking down the heap need both nitrogen and carbon in roughly the right proportions. The scientific measure is the carbon-to-nitrogen ratio, or C:N ratio, and the ideal for composting is roughly 25–30 parts carbon to 1 part nitrogen. Greens have a low C:N ratio — grass clippings are around 20:1, vegetable scraps around 15–25:1, manure around 15–20:1 — while browns have a high one — straw around 80:1, leaves around 60:1, cardboard around 350:1, sawdust around 500:1. Mixing one part green to two or three parts brown by volume lands the heap in roughly the right C:N range. You do not need to calculate the exact ratio — the volume rule of thumb gets you close enough for a garden heap. Use the compost ratio calculator if you want to work out the amounts for your heap.
The ratio in practice
Build the heap in layers. Start with 15 cm of coarse browns at the bottom for drainage and air. Then add a layer of greens about 5–8 cm thick, then a layer of browns about 15–20 cm thick, then another layer of greens, and so on, watering each layer lightly as you go. Aim for roughly one bucket of greens to two or three buckets of browns. Add a few spadefuls of garden soil or finished compost between layers to introduce organisms. Keep the heap damp like a wrung-out sponge, and turn it every few weeks, mixing the layers together. A heap built this way should heat up within a few days in warm weather, reaching 55–65°C in the middle, and should smell like damp earth, not like rotting food.
The ratio matters most when you add a large amount of one material at once. A big load of grass clippings needs an equal bulk of browns mixed in immediately, or it will go slimy. A big load of autumn leaves needs greens or manure added, or it will sit unchanged. Small, regular additions of mixed material are more forgiving than large dumps of a single ingredient, so add a handful of browns every time you add kitchen scraps, and keep a bag of shredded cardboard or a pile of leaves beside the heap for that purpose.
Common materials and their ratios
| Material | Green or brown | Rough C:N ratio | Notes |
|---|---|---|---|
| Grass clippings | Green | 20:1 | Add in thin layers with browns, or it goes slimy |
| Vegetable peelings | Green | 15–25:1 | Chop small, mix with browns |
| Fresh weeds | Green | 20:1 | Avoid weeds with mature seeds unless heap runs hot |
| Coffee grounds | Green | 20:1 | Good nitrogen source, add in moderation |
| Fresh manure | Green | 15–20:1 | Use well-rotted where possible; see safety notes |
| Autumn leaves | Brown | 60:1 | Shred them first to speed rotting |
| Straw | Brown | 80:1 | Excellent structure, good for layering |
| Shredded cardboard | Brown | 350:1 | Soak or wet it first; remove tape and staples |
| Sawdust | Brown | 500:1 | Use sparingly; very high carbon, can rob nitrogen |
| Wood chips | Brown | 400:1 | Slow to rot; use as mulch or in small amounts |
| Twigs and prunings | Brown | 300:1 | Chop small or they take years |
Fixing a heap that has gone wrong
Smelly and slimy. Too many greens, too much water, or not enough air. Add dry browns — shredded cardboard, straw or dry leaves — at roughly twice the volume of the smelly material, turn the heap thoroughly to let air in, and check the drainage. The smell should fade within a few days.
Dry and doing nothing. Too many browns, or not enough water. Add greens — grass clippings, kitchen scraps or fresh manure — water the heap until it is damp like a wrung-out sponge, and turn it to mix. A dry heap in a hot summer may need watering every week.
Not heating up. The heap may be too small — it needs to be at least a cubic metre to hold heat — or the ratio may be off, or it may be too dry or too cold. Check moisture, add greens if it is mostly browns, turn it, and make sure it is at least a cubic metre. A heap in winter heats slowly, which is normal.
Full of flies. Usually caused by food scraps left exposed on the surface. Bury kitchen scraps under a layer of browns, keep the heap covered, and do not add meat, dairy or cooked food, which attract rats and flies.
Not breaking down after months. The material may be too coarse — chop or shred it smaller — or the heap may be too dry, too cold, or lacking nitrogen. Add greens, water it, turn it, and be patient: woody material takes longer than soft material, and a cool heap takes six to twelve months.
Compost ratio for special cases
For a worm bin, the ratio is different: worms need mostly browns as bedding, with small amounts of greens as food added gradually. See our vermicompost guide. For leaf mould, there is no ratio at all — it is just leaves left to rot for one to two years. For a hot heap that you want to finish in two to three months, the ratio matters most, and turning every week or two speeds it further. For a slow, cold heap, the ratio matters less — the material will rot eventually, just more slowly — but a heap with far too many greens will still smell, so add browns even in a slow heap. For compost tea, the ratio does not apply: tea is brewed from finished compost, not from raw materials.
Common mistakes
The most common mistake is adding greens without browns — a heap of grass clippings and kitchen scraps alone will smell and go slimy. The second is the opposite: a heap of dry leaves and cardboard alone that does nothing for months. The third is measuring the ratio by weight instead of by volume — browns are light and bulky, so a weight-based ratio gives far too few browns. Always measure by volume, by the bucket. The fourth is adding a large amount of sawdust or wood chips, which are so high in carbon that they rob nitrogen for months. The fifth is never turning the heap, so the middle goes airless regardless of the ratio. And the sixth is expecting a precise number: the 1:2–1:3 ratio is a guide, not a formula, and a heap that is roughly right and turned regularly will work well.
Safety notes
Wear gloves when handling compost and manure, and wash your hands before eating. Use only mature, well-rotted manure in the heap, and never add pet waste, meat, dairy or diseased plants. Fresh manure should be composted for several months before use on food beds, and never applied directly to edible crops near harvest. Wear a mask when turning a dry heap, because compost dust can irritate lungs. If you are unsure about safe composting practice, check your local extension service for advice.
Frequently asked questions
What is the ideal compost ratio?
Roughly one part green to two or three parts brown by volume. That lands the heap near the ideal carbon-to-nitrogen ratio of about 25–30:1. Measure by volume, by the bucket, not by weight — browns are light and bulky, so a weight-based ratio gives far too few browns.
What happens if I add too many greens?
The heap goes slimy, smelly and airless, because greens are wet and nitrogen-rich and the organisms run out of carbon and oxygen. Fix it by adding dry browns at roughly twice the volume of the smelly material and turning the heap to let air in.
What happens if I add too many browns?
The heap stays dry and does nothing for months, because there is not enough nitrogen to feed the organisms. Fix it by adding greens such as grass clippings or kitchen scraps, watering the heap until damp, and turning it to mix.
Do I need to measure the ratio exactly?
No. The 1:2–1:3 ratio is a guide, not a formula. A heap that is roughly right, damp like a wrung-out sponge, and turned every few weeks will compost well. The important habit is adding browns every time you add greens, not measuring precisely.
Can I compost with only browns or only greens?
Neither works well on its own. Browns alone rot very slowly for lack of nitrogen; greens alone go slimy and smelly for lack of carbon and air. A mix of both is essential, which is why keeping a supply of browns beside the heap matters so much.