browns & greens compost ratio calculator
Add what you've got in parts — a part is any scoop you like, a bucket or an armful. The gauge shows your compost ratio: the carbon-to-nitrogen ratio of the whole mix. Compost cooks best between 25:1 and 30:1.
your mix
21 materials come with a packing density of their own. Search reaches another 268 items we have a carbon-to-nitrogen ratio for; those are treated as packing like dry leaves, so they show up marked (est.).
your compost ratio
Add a material to get started.
how it works
Compost ratios don't average like percentages — a mix's carbon-to-nitrogen ratio is total carbon over total nitrogen, which weights wet, dense greens far more than fluffy browns. The calculator's 21 core materials use published extension values (Cornell, NRAES-54), each paired with a packing density so that a scoop of it converts to dry weight — the number the chemistry actually runs on.
Search reaches further, into the 268 other items on this site with a published C:N ratio. Those ratios come from the same sources, but we do not have a packing density for every one of them, so the calculator treats them as packing like dry leaves and marks them (est.). A mix built on estimates points you the right way; it is not a measurement. Either way, parts are volume, so precision past half a part is false comfort: compost is forgiving, and anywhere near the green zone works.
common questions
- What is the right compost ratio of browns to greens?
- By carbon-to-nitrogen ratio the target is 25–30:1, which is what this calculator solves for. That is not the same as a 25:1 ratio of brown material to green material by volume — browns are far more carbon-dense per part, so in practice a rough 2:1 to 3:1 of browns to greens by volume usually lands in the target zone. Work in parts and let the gauge tell you.
- Why can I not just average the two C:N numbers?
- Because a mix ratio is total carbon divided by total nitrogen, not the average of the inputs. Mathematically it is a mass-weighted harmonic mean, which is always pulled toward the low-ratio material. One part of coffee grounds at 20:1 drags a mix down much harder than one part of cardboard at 350:1 pulls it up, because the grounds carry far more nitrogen per part than the cardboard carries carbon.
- What happens if my compost is too green or too brown?
- Too green — under about 20:1 — means surplus nitrogen, which leaves the pile as ammonia. That is the smell, and it is nitrogen you wanted in your compost. Too brown, over about 40:1, is not harmful, just slow: the microbes run short of nitrogen and the pile can sit for a year. Neither ruins anything, which is why "anywhere near the green zone" is honest advice rather than a cop-out.
- Are parts measured by weight or by volume?
- By volume — a "part" is whatever scoop, bucket, or bin you are using, as long as you use the same one throughout. Materials do not weigh the same per scoop, though, and the chemistry runs on dry weight, so for the 21 core materials the calculator corrects for how densely each one packs. That is why a scoop of fluffy dry leaves and a scoop of dense wet grass clippings do not count equally at one part each. Anything marked (est.) skips that correction and is treated like dry leaves, so read those mixes as a good steer rather than a measurement.
- How precise do I need to be?
- Not very, and precision past half a part is false comfort. Compost is forgiving: the 25–30:1 window is a target, not a threshold, and piles either side of it still work. Use the gauge to check you are in the right neighbourhood, not to chase a number.
Sized your pile yet? The bin size calculator picks the system this mix goes into, and the printable cheat sheet covers the everyday what-goes-in questions. For a single item, search the A–Z index.