Composting fails in two directions and both are ratio problems. Too much carbon and the pile sits inert for a year, because the microbes have plenty of fuel and not enough nitrogen to build the proteins they need to multiply. Too much nitrogen and the pile goes anaerobic and smells of ammonia, because the surplus nitrogen has nowhere to go but off into the air. The compost calculator above works out where your mix actually sits, on a dry-weight basis, and how much of one material to add to move it.
Arb Digital publishes free calculators for people who want the real arithmetic rather than a rule of thumb. The rule of thumb here — two parts brown to one part green by volume — is a reasonable starting point and it is also why so many piles stall. Volume is not mass, and mass is not dry mass, and the gap between them is often a factor of five.
What This Compost Calculator Does
In its main mode it takes two materials, each with a mass, a carbon percentage, a nitrogen percentage and a moisture content, and returns the carbon-to-nitrogen ratio of the resulting mixture. It also solves the inverse problem: how much of the second material you would need to add to bring the mix to your target ratio. It reports the mixture's moisture content alongside, because moisture is the second variable that decides whether a pile works.
In its buying mode it converts a bed area and an application depth into a volume in cubic metres, litres and cubic yards, and into a mass using a bulk density you can edit. That side deliberately overlaps with nothing else on this site: our mulch calculator covers mulch volume for surface dressing and has no ratio work at all, and the cubic yard calculator handles a raw volume conversion without the material assumptions.
The percentages are inputs rather than a built-in table on purpose. Carbon and nitrogen content vary enormously with the source, the season and how long a material has been sitting, and a table of single values would give a false impression of precision. Extension services publish measured figures for common feedstocks, and those are the numbers to type in.
How to Use It
- Weigh, or estimate by weight. Volume comparisons are misleading because a bucket of leaves and a bucket of grass differ several-fold in mass.
- Enter moisture honestly. Fresh grass clippings run around 80 percent water and dry autumn leaves nearer 15 percent. This is the input people skip and it changes the answer more than any other.
- Use published carbon and nitrogen percentages for your materials rather than guessing. They are dry-basis figures.
- Read the solver line under the headline ratio. It tells you how much more of the nitrogen material would bring the mix to your target, and the number is often surprisingly large.
- Check the moisture figure too. A perfect C:N ratio in a pile that is too dry will not compost either, because the microbes need water to move.
The Formula and How It Is Calculated
The ratio is a dry-weight calculation, so every mass has to be corrected for its water content first. Cornell Composting's C/N Ratio page gives the mixture equation used here, in which each material contributes its mass multiplied by its carbon or nitrogen percentage multiplied by its dry fraction:
R = Σ(Qₙ × Cₙ × (100 − Mₙ)) ÷ Σ(Qₙ × Nₙ × (100 − Mₙ)), where Q is mass, C is carbon percent, N is nitrogen percent and M is moisture percent. Cornell recommends a starting ratio of about 30 to 1, with finished compost settling near 10 to 1 as carbon is respired away as carbon dioxide. Penn State Extension's home composting guide gives the same target as a range, stating that the preferred C/N ratio is 25 to 30 parts carbon to one part nitrogen.
A worked example matching the defaults. Twenty kilograms of dry leaves at 50 percent carbon, 0.8 percent nitrogen and 15 percent moisture, plus ten kilograms of grass clippings at 45 percent carbon, 2.4 percent nitrogen and 80 percent moisture. The carbon term is 20 × 50 × 85 = 85,000 from the leaves and 10 × 45 × 20 = 9,000 from the grass, totalling 94,000. The nitrogen term is 20 × 0.8 × 85 = 1,360 plus 10 × 2.4 × 20 = 480, totalling 1,840. The ratio is 94,000 ÷ 1,840, which is 51 to 1 — far too carbon-rich to heat up. To reach 30 to 1 you would need about 82 kilograms of those clippings, not ten.
Why the Two-to-One Rule Misleads So Often
The worked example above is the reason. By volume, ten kilograms of grass clippings and twenty kilograms of leaves look roughly like the classic mix. On a dry-weight basis, the grass contributes only two kilograms of dry matter against seventeen from the leaves, so the nitrogen it brings is a fraction of what the volume suggests.
Water is the culprit and it is invisible in a wheelbarrow. Fresh green material is mostly water; dry brown material is mostly not. Any rule expressed in volumes is silently assuming a moisture content for both, and if your greens are wetter than the rule assumed, your pile is carbon-rich no matter how carefully you counted the shovelfuls.
This also explains why a pile that stalls in autumn often restarts in spring for no obvious reason. Nothing changed in the ratio; the leaves absorbed rain, the microbes finally had water to work in, and the same mixture started behaving differently. Ratio and moisture are separate levers and a pile needs both.
Moisture, Air and the Third Variable Nobody Counts
Carbon to nitrogen gets the attention, but composting has three requirements and the ratio is only one. Moisture should sit somewhere around half by weight — the standard test is that a squeezed handful feels like a wrung-out sponge and releases a drop or two but does not run.
Oxygen is the third. Decomposition that runs aerobically produces carbon dioxide, water and heat. Once a pile is compacted or waterlogged, the same material decomposes anaerobically instead, producing methane, organic acids and the smell everybody associates with a failed heap. That is why turning works: it is not the mixing that matters so much as reintroducing air. Grass clippings on their own are the classic failure case, because they mat down into an airless layer that no ratio calculation can rescue.
Particle size interacts with all three. Shredding increases the surface area available to microbes and speeds everything up, but shred too finely and the pile compacts and loses its air. Woody material shredded coarsely serves double duty as a structural bulking agent, holding channels open even as the softer material collapses around it.
What Happens to the Ratio as the Pile Works
The C:N ratio you start with is not the ratio you finish with, and the direction of travel is worth understanding. Microbes respire carbon away as carbon dioxide while retaining most of the nitrogen in their own biomass, so the ratio falls steadily. A pile starting near 30 to 1 typically finishes somewhere around 10 to 1, which is close to the ratio of stable soil organic matter.
That falling ratio is why finished compost is a soil conditioner rather than a fertiliser in any concentrated sense. Its nitrogen is largely bound in organic forms that release slowly, which is a feature for soil structure and a limitation if you are trying to correct a specific deficiency quickly. Applying an unfinished, carbon-rich compost has the opposite problem: soil microbes will scavenge nitrogen from the soil to break down the excess carbon, temporarily locking it away from plants. If you need to think about nutrient supply as a separate question, the fertilizer calculator works in the units fertiliser is actually sold in.
Working Out How Much to Buy
The buying mode is deliberately simple arithmetic, because the uncertainty is not in the geometry. Volume is area multiplied by depth, and a 12 square metre bed dressed at 5 centimetres needs 0.6 cubic metres, which is 600 litres or about 0.78 cubic yards.
The uncertainty is in bulk density. Compost is sold by volume and delivered by weight, or the reverse, and its density depends heavily on how wet it is and how much it has settled in transit. A figure around 600 kilograms per cubic metre is a reasonable working assumption for moist finished compost, but weighing a filled bucket of your actual material is worth more than any published number. Note also that a delivered volume settles: a cubic metre tipped from a bag occupies less once it is spread and firmed. The square footage calculator is useful if your bed is an awkward shape, and the cubic yard calculator converts between the volume units suppliers use.
Arb Digital publishes hundreds of free calculators across biology, maths, home projects and finance — no sign-up, no limits. If something you need is missing, tell us and we will look at building it.
Browse All Free Tools Suggest a ToolCommon Mistakes to Avoid
- Mixing by volume instead of dry weight — wet greens carry far less dry matter than their bulk suggests, which is why so many carefully measured piles turn out carbon-rich.
- Ignoring moisture in the ratio — the calculation is a dry-weight one, and leaving moisture out of a mix of dry leaves and fresh clippings can shift the answer several-fold.
- Chasing the ratio and forgetting air — a compacted or waterlogged pile goes anaerobic regardless of how well balanced it is.
- Piling grass clippings alone — they mat into an airless layer, which is a structural problem that no amount of nitrogen balancing will fix.
- Digging in unfinished compost — a carbon-rich material still breaking down will draw nitrogen out of the soil while it finishes.
Related Free Tools From Arb Digital
Work out surface dressing with the mulch calculator, convert volumes the way suppliers quote them with the cubic yard calculator, and measure an awkward bed with the square footage calculator. The fertilizer calculator handles nutrient supply as a separate question, the gravel calculator covers paths and bases, and the density calculator is useful for converting your own weighed sample into a bulk density. The full free online tools hub lists everything else.
Frequently Asked Questions
University extension guidance puts the preferred starting range at about 25 to 30 parts carbon per part nitrogen. Cornell Composting recommends around 30 to 1 at the start, with the finished material settling near 10 to 1 as carbon is respired away.
Because the C:N ratio is a dry-weight calculation. Fresh grass clippings are around 80 percent water and dry leaves nearer 15 percent, so ignoring moisture can misstate the ratio by a large factor.
It is a reasonable starting point expressed in volumes, but volumes hide moisture. The same mix worked out on dry weight often turns out considerably more carbon-rich than the rule implies, which is why piles stall.
Too much carbon and the pile decomposes very slowly because microbes lack the nitrogen to multiply. Too much nitrogen and the surplus is lost as ammonia, which is the source of the smell people associate with a failed heap.
Yes, and it falls. Microbes respire carbon away as carbon dioxide while retaining most of the nitrogen, so a pile starting near 30 to 1 typically finishes around 10 to 1, close to stable soil organic matter.
Multiply the bed area by the depth you want. Twelve square metres at five centimetres is 0.6 cubic metres, which is 600 litres or roughly 0.78 cubic yards, weighing about 360 kilograms at a typical bulk density.
The three usual causes are too much carbon relative to nitrogen, too little moisture for microbes to work in, and too little air because the material has compacted. The ratio is only one of the three and is not always the culprit.
University extension services and composting research programmes publish measured figures for common feedstocks. They are worth looking up rather than guessing, because values vary widely with the source, the season and how long the material has been sitting.
This calculator is provided for education and general reference. Figures depend entirely on the carbon, nitrogen and moisture values you supply, which vary widely between sources of the same material, so treat the result as a starting point to adjust from rather than a specification.
