Compost Brown to Green Ratio

Two numbers, one rule

	Ratio	Units
30 : 1	carbon to nitrogen	by weight
2–3 : 1	browns to greens	by volume

Both are correct. They are the same guidance expressed differently, and this is the single biggest source of confusion in composting.

30:1 is the real target — the mix that decomposes fastest and cleanest. But you cannot weigh carbon in your garden.

The volume rule approximates it, and it works because browns are typically dry and light while greens are wet and dense — so a much larger volume of browns supplies a comparable weight of carbon.

What's wrong is mixing them. A pile built at 30 volumes of browns to 1 of greens will sit there doing nothing for a very long time.

At 3:1, browns are three quarters of the heap

More than most people expect — and more than most people add.

A cubic yard (27 cu ft) at 3:1 is roughly 20 cu ft of browns to 7 of greens. The calculator converts that into bucket or barrow loads, which is what you can actually count while standing at the heap.

Browns and greens describe the role, not the colour

This trips people up constantly.

Browns (carbon): dry leaves, straw, cardboard, wood shavings, corn stalks, bark, paper. Bulky, dry, usually light.

Greens (nitrogen): grass clippings, kitchen scraps, fresh prunings, coffee grounds, manure. Wet, dense, usually heavier than they look.

Coffee grounds are brown and count as a green. Grass clippings that have dried out have effectively become a brown.

If in doubt, ask whether it's wet and living or dry and dead.

The heap tells you which way you're wrong

Far more reliably than any calculation:

Symptom	Meaning	Fix
Smells of ammonia, wet, slumped	too much nitrogen	add browns, turn it
Dry, cold, unchanged for weeks	too much carbon	add greens and water
Sour, soggy, matted	too wet / airless	turn it, add browns
Hot in the middle, steaming	working	turn it when it cools

The ammonia smell is worth understanding: excess nitrogen escapes as ammonia gas. That's both the smell and a waste of the fertility you were trying to keep.

Nobody making good compost is measuring precisely. They're correcting from those two signals.

The heap that does nothing is probably too small

A heap much smaller than a cubic yard — three feet in each direction — loses heat from its surface faster than the middle can generate it. So it never gets warm.

It'll still compost eventually, just slowly and cold, and it won't reach temperatures that kill weed seeds.

This is by far the most common reason a carefully balanced small heap appears to do nothing, and no amount of adjusting the ratio will fix it.

Moisture and air matter as much as the ratio

Aim for a wrung-out sponge — damp throughout, not dripping when you squeeze a handful.

Too dry and the process stalls entirely; the organisms doing the work need water to live in. Too wet and it goes anaerobic, which is slow, smelly and unpleasant to turn.

Turning addresses moisture and air at once, which is why it's the single most effective thing you can do to a stalled pile.

Stockpile your browns

Here's the practical problem nobody mentions: a kitchen produces greens continuously and browns almost never.

Which is why so many heaps drift wet, smelly and nitrogen-heavy — not because anyone got the ratio wrong, but because the inputs arrive unbalanced by default.

Autumn leaves are free, store in bags all year, and are the standard fix. Keep a sack beside the heap and add a few handfuls every time you add scraps.

It also helps to build in large layers rather than handfuls. A heap assembled in one go heats up properly and works through together; one that gets a few kitchen scraps every few days never accumulates enough at once to heat.

Print the heap sheet — the symptom table at the bottom is the part you'll actually use, because the ratio only gets you started.

Sources: Cornell — compost chemistry · UC Master Gardener — composting basics