Compost Calculator
Work out the carbon-to-nitrogen ratio of a compost pile from the volumes of browns and greens you are adding, and see exactly how much more of either you need to reach the ideal band.
🌱 What is the Compost Browns and Greens Ratio?
The compost browns to greens ratio is a shorthand for the carbon-to-nitrogen ratio of everything you pile up, written as C:N and read as "carbon to nitrogen". Carbon is the fuel that composting microbes burn for energy, and nitrogen is the building material they use to make new cells. Get the balance right and a bacterial population explodes, the pile self-heats to 55 or 60 degrees Celsius, and finished compost appears in a couple of months. Get it wrong in either direction and the pile stalls.
Gardeners use this calculation constantly, usually without formalising it. Someone clearing a lawn in June has a mountain of grass clippings at 17:1 and needs to know how much stored cardboard to shred into it. A small farm bedding cattle on straw wants to know whether the manure and straw mix will heat on its own or needs supplementing. A community garden turning a hundred kitchen caddies a week needs to know how many bags of autumn leaves to stockpile in October to carry it through to spring. A permaculture site building a hot bed for winter seedlings needs the ratio right to guarantee the heat is there when the seeds go in.
The critical misconception is that the ratio is about buckets. It is not. It is a ratio of carbon atoms to nitrogen atoms in the dry matter, so a feedstock that is 87 percent water contributes far less to the ratio per litre than a dry one. Fresh grass clippings weigh three and a half times as much per litre as dry leaves, yet after subtracting the water the two carry almost the same dry mass. Any recipe quoted purely in parts by volume is silently assuming particular materials, which is why one gardener's "three parts browns to one part greens" gives a perfect pile and another's gives a slimy anaerobic mess.
This calculator converts each feedstock from litres to dry kilograms using published bulk densities and dry matter fractions, blends the two ratios properly on a dry-mass basis, rates the result against the working bands, and then tells you exactly how many litres of browns or greens to add to land on your target. The chart shows how the ratio slides as you add greens, and the table lets you scan several mixes at once before you commit to a build.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Autumn Leaf Pile with a Little Grass
400 L of dry autumn leaves plus 100 L of fresh grass clippings, target 30:1
Example 2 — Kitchen Scraps and Shredded Cardboard
250 L of shredded cardboard plus 300 L of vegetable scraps, target 30:1
Example 3 — Grass-Heavy Summer Heap
100 L of straw plus 400 L of fresh grass clippings, target 30:1
❓ Frequently Asked Questions
🔗 Related Calculators
What is the ideal browns to greens ratio for compost?
Aim for a mix carbon-to-nitrogen ratio between 25:1 and 35:1, with 30:1 as the classic target. By volume that often works out near 2 or 3 parts browns to 1 part greens, but the exact volume ratio depends entirely on which materials you use, because their densities and moisture contents differ enormously.
How do I calculate the C:N ratio of a compost mix?
Convert each feedstock to dry mass (volume times bulk density times dry matter fraction), then divide the total dry mass by the sum of each mass divided by its own C:N ratio. For 34 kg of leaves at 60:1 plus 8.75 kg of grass at 17:1 the result is 42.75 divided by 1.081, which is 39.5:1.
What counts as a brown and what counts as a green?
Browns are dry, carbon-rich and usually woody or papery: autumn leaves, straw, cardboard, sawdust and wood chips. Greens are moist and nitrogen-rich: grass clippings, vegetable scraps, coffee grounds, manure and fresh weeds. Coffee grounds are brown in colour but are a green in composting terms.
Why is my compost pile not heating up?
The three usual causes are too much carbon, too little moisture and too little bulk. Check the ratio here first. If it reads above 45:1 the microbes are nitrogen-starved. If the ratio is fine, check that the pile is at least 1 cubic metre and damp like a wrung-out sponge.
Why does my compost smell of ammonia?
Ammonia means surplus nitrogen. When the C:N ratio drops below about 20:1 the microbes cannot use all the nitrogen and the excess is lost to the air as ammonia gas. Mix in browns straight away, since that nitrogen is being wasted rather than staying in your finished compost.
How big does a compost pile need to be?
About 1 cubic metre, roughly a metre cube, is the practical minimum for a hot pile. Smaller heaps lose heat through their surface faster than the microbes generate it. Piles larger than about 2 metres across can go anaerobic in the middle unless they are turned or actively aerated.
Does the C:N ratio use volume or weight?
Weight, and specifically dry weight. This is the single most common mistake in compost recipes. Fresh grass clippings are about 75 percent water, so a 100 litre bucket carries only around 9 kg of dry matter, while the same bucket of dry leaves carries about 8.5 kg despite being far lighter to lift.
What is the C:N ratio of grass clippings?
Fresh grass clippings run about 17:1, which is strongly nitrogen-rich. Every kilogram of dry clippings needs roughly two to three kilograms of dry leaves or one kilogram of straw alongside it to bring the blend up into the 25:1 to 35:1 working band.
Can I compost sawdust?
Yes, but sparingly. Sawdust sits near 500:1, the most carbon-rich common feedstock. A single 20 litre bucket carries about 3.7 kg of dry matter at 500:1, which needs a large volume of greens to balance. Spread it in thin layers and never as a thick blanket.
How long does compost take to finish?
A well-balanced hot pile turned every few days can be finished in six to eight weeks. A balanced pile left alone typically takes six to twelve months. A pile with the ratio badly wrong can sit for years, which is why getting the browns and greens balance right at the start pays for itself.
Should I add water to my compost pile?
Almost always, yes, unless it is built mostly from wet greens. Target 50 to 60 percent moisture by weight, judged by squeezing a handful: it should feel damp and hold together, with at most a drop or two of water appearing. Add water in layers as you build rather than pouring it over the top.
Does turning the pile change the C:N ratio?
No. Turning adds oxygen and evens out temperature and moisture, but it does not change how much carbon or nitrogen is present. The ratio does drift upward slowly on its own as microbes respire carbon away as carbon dioxide, which is why finished compost typically settles near 15:1 to 20:1.