Compost Brown-to-Green Ratio Calculator

How this is calculated

⚠️ THERE ARE TWO RATIOS AND THEY ARE NOT THE SAME NUMBER. This is the single biggest source of confusion in composting. 30:1 is the CARBON-TO-NITROGEN ratio BY WEIGHT, and it is the actual target - the mix that decomposes fastest and cleanest. But you cannot weigh carbon in your garden. 2:1 to 3:1 BROWNS TO GREENS BY VOLUME is the practical rule of thumb that approximates it. 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. So '30:1' and '3:1' are the same rule in different units, and neither is wrong. What is 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. BROWNS are the carbon: dry leaves, straw, cardboard, wood shavings, corn stalks, bark, paper. Bulky, dry, and usually light. GREENS are the nitrogen: grass clippings, kitchen scraps, fresh prunings, coffee grounds, manure. Wet, dense, and usually heavier than they look. The names describe the ROLE, not the colour. Coffee grounds are brown and count as a green; dried grass clippings have become a brown. ⚠️ THE HEAP ITSELF TELLS YOU WHICH WAY YOU ARE WRONG, and it is far more reliable than any calculation: SMELLS OF AMMONIA, wet, slumped - too much nitrogen. Excess nitrogen is lost as ammonia gas, which is both the smell and a waste of the fertility you were trying to keep. Add browns and turn it. NOT HEATING, dry, unchanged for weeks - too much carbon. Decomposition slows right down because the organisms have no nitrogen to build with. Add greens and some water. So treat the ratio as a starting point rather than a specification. Nobody achieving good compost is measuring precisely; they are correcting from those two signals. SIZE MATTERS FOR HEAT. A heap much smaller than about a cubic yard - three feet in each direction - struggles to hold the heat it generates, because it loses it from the surface faster than the middle produces it. It will still compost, just slowly and cold. This is the most common reason a carefully balanced small heap does nothing. MOISTURE should be that of a wrung-out sponge. Too dry stalls; too wet goes anaerobic and starts to smell. Turning adds air, which the process needs as much as it needs the ratio.

Common questions

Is it 30:1 or 3:1?

Both, and they are the same rule in different units - which is why this confuses so many people. 30:1 is the ratio of CARBON to NITROGEN by WEIGHT, and it is the real target. 2:1 to 3:1 browns to greens by VOLUME is the practical approximation of 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. Neither number is wrong. What is wrong is mixing them up - a heap built at 30 volumes of browns to one of greens will sit doing nothing for a very long time.

What counts as a brown and what counts as a green?

Browns are the carbon sources: dry leaves, straw, cardboard, wood shavings, corn stalks, bark, paper. Greens are the nitrogen sources: grass clippings, kitchen scraps, fresh prunings, coffee grounds, manure. The names describe the ROLE rather than the colour, which trips people up constantly - coffee grounds are brown and count as a green, while grass clippings that have dried out have effectively become a brown. If in doubt, ask whether it is wet and living or dry and dead.

How do I know if I got it wrong?

The heap tells you, and it is far more reliable than any calculation. If it smells of ammonia and looks wet and slumped, there is too much nitrogen - the excess is escaping as ammonia gas, which is both the smell and a waste of the fertility you were trying to keep. Add browns and turn it. If it is dry, cold and unchanged after weeks, there is too much carbon and the organisms have nothing to build with. Add greens and some water. Nobody making good compost is measuring precisely; they are correcting from those two signals.

Does the heap need to be a certain size?

Yes, if you want it to heat up. 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 will still compost eventually, just slowly and cold, and it will not 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.

How wet should it be?

Like a wrung-out sponge - damp throughout, but not dripping when you squeeze a handful. Too dry and the process stalls entirely, because the organisms doing the work need water to live in. Too wet and it goes anaerobic, which is slow, smelly and unpleasant to turn. Moisture and air are as important as the ratio, and turning the heap addresses both at once, which is why turning is the single most effective thing you can do to a stalled pile.

Do I have to add it all at once?

It works considerably better if you do, or at least in large layers. A heap built in one go heats up properly and works through together; one that gets a handful of kitchen scraps every few days never accumulates enough material at once to heat, and tends to sit at the greens-heavy end of the ratio because that is what a kitchen produces. The usual answer is to stockpile browns - autumn leaves are free and store all year in a bag - and add them in proportion each time you add greens.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Compost Brown-to-Green Ratio Calculator at https://www.bfcbrilliance.com/tools/compost-ratio-calculator.

What I entered:
- Compost you want to build (cu ft): ___
- Browns to greens, by volume (: 1): ___
- Size of your bucket or barrow (cu ft): ___

What it calculated:
- Browns: ___
- Greens: ___
- Browns — loads of your bucket: ___
- Greens — loads: ___
- Browns as a share of the heap: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

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