Article — Compost C:N Ratio Calculator
A pile of pure grass clippings reeks of ammonia within days. A pile of pure dry leaves sits untouched for two winters. The reason is the same in both cases: the carbon-to-nitrogen ratio is wrong for the microbes doing the work. Those microbes consume carbon and nitrogen in roughly a 30-to-1 ratio. Match it, and a backyard pile finishes in three to six months. Miss it badly, and the pile either overheats and stinks or sits cold for years.
This guide walks through what the C:N ratio actually measures, the ratios of every common compost ingredient, why 25–30:1 is the sweet spot, and what to do when the pile drifts off the mark in either direction.
What the compost C:N ratio means
The compost C:N ratio is the weight of carbon in the pile divided by the weight of nitrogen in it. Express it as a ratio against 1, so 30:1 means 30 grams of carbon for every 1 gram of nitrogen. Every organic material has its own characteristic C:N — set by what the material actually is, not by how you handle it.
Microorganisms — bacteria, fungi, actinomycetes — drive composting. They eat carbon for energy and nitrogen to build proteins. The ratio they need is fixed by their own biology, and it sits at around 30:1. Drop a sample of pure carbon (sawdust) in front of them and they cannot build cell walls; drop pure nitrogen (urine, fresh manure) and they cannot fuel the reactions that decompose it.
The 25–30:1 rule comes from work by Clarence Golueke at UC Berkeley in 1972. He measured microbial uptake of carbon and nitrogen during active composting and showed that decomposition rate peaks when the input C:N matches the microbial demand. The standard has held for fifty years and is the basis for every modern composting protocol from the US Composting Council to Cornell's compost engineering courses.
Compost ingredient C:N ratios at a glance
Materials cluster into two camps. Browns — dry, woody, fibrous — sit between 50 and 500. Their structure is mostly cellulose and lignin, both heavy in carbon. Greens — fresh, wet, often green-coloured — sit between 10 and 30. Their tissue still contains proteins, sugars, and the nitrogen the plant pulled from soil while alive.
- Sawdust — about 400:1, the most carbon-heavy common ingredient
- Cardboard (shredded) — about 350:1, dries fast, structural
- Autumn leaves — about 55:1, balanced, free in fall
- Coffee grounds — about 20:1, surprisingly nitrogen-rich for a brown-looking material
- Grass clippings — about 20:1, the workhorse green
- Vegetable food scraps — about 17:1, wet and active
- Fresh manure — about 15–25:1 depending on species (chicken nitrogen-rich, horse bedding-diluted)
The exact numbers come from decades of laboratory work by US extension services. Cornell, LSU AgCenter, UC ANR, and South Dakota State University Extension all publish overlapping tables. There is variance — fresh manure from a corn-fed steer differs from grass-fed cattle — but the order of magnitude is reliable for home composting.
The ideal compost C:N ratio: 25–30:1
Aim for 25–30:1 to start the pile, and a hot pile (130–160 °F core temperature) follows within a week. The pile will shrink to roughly a third of its starting volume over three to six months. Carbon escapes as CO₂, water leaves as vapour, and the residue is the dark, crumbly humus that gardeners after.
The finished compost has a far lower C:N — usually 10:1 or 15:1 — because carbon left as gas while most nitrogen stayed put. This is normal and expected. The starting ratio is the variable you control; the ending ratio is what the microbes leave behind.
A forest floor composts naturally, but at a glacial pace. Fallen leaves arrive at C:N around 60–100:1 — far too brown to break down quickly. Nitrogen-fixing bacteria and fungi slowly pull nitrogen from the air to balance the carbon, and the process takes years. Home composting, by mixing browns and greens deliberately, accelerates the natural cycle roughly tenfold.
When compost has too much nitrogen
A pile dominated by greens — fresh grass, food scraps, manure — sits below 20:1 and quickly shows it. The first sign is heat: the core climbs above 160 °F (71 °C), which is enough to kill the very microbes doing the work. The second sign is smell. Nitrogen the microbes cannot use leaves as ammonia, the same compound that gives cat litter and stale urine their sharp odour.
The third sign is anaerobic collapse. A green-heavy pile compacts under its own weight, the airflow stops, and oxygen-loving microbes die off. Anaerobic bacteria take over and produce hydrogen sulphide (rotten-egg smell), methane, and a slick black mass instead of crumbly humus.
Pure grass clippings dump 50–100 lb of nitrogen-rich biomass at C:N 20:1 in a single load. Within 48 hours the pile heats violently, then collapses into a slimy mat. Always layer fresh clippings with at least an equal volume of dry leaves, shredded cardboard, or straw before the pile heats up.
Chicken manure can run as low as 7:1 — three times more nitrogen than a microbe can use. Mix it with carbon-rich bedding (straw, sawdust, shredded paper) at roughly 4 parts bedding to 1 part manure by volume before adding to the main pile. This is also why bagged "composted manure" at the garden centre is already mixed with bedding.
When compost has too much carbon
A pile dominated by browns — leaves, cardboard, sawdust, wood chips — sits above 40:1 and shows the opposite problem: nothing happens. The microbes are starved of nitrogen, they cannot multiply, and the pile sits cold all winter and all the next summer. A pile of pure cardboard at 350:1 can take five or more years to break down.
The fix is straightforward: add greens. Grass clippings, coffee grounds (free from any local café), vegetable trimmings, manure, or even diluted urine all work. The calculator above estimates how much to add to reach 30:1 — usually less than expected, because greens have such a low C:N that small additions pull the average down sharply.
- 10 lb cardboard alone — C:N 350:1, sits idle for years
- Add 5 lb coffee grounds — C:N drops to about 60:1, still slow
- Add 10 lb grass clippings — C:N drops to about 35:1, starts to work
- Add 15 lb grass clippings — C:N hits about 28:1, hot pile in a week
- Coffee grounds free from any café — most shops give grounds away in bulk
Wood chips at 150:1 and sawdust at 400:1 are useful as bulking agents and odour absorbers but should never dominate a pile. Limit them to one part in four by volume. Used as a thin top layer they trap heat and discourage flies; used as the main carbon source they stop everything.
Common compost ratio mistakes
The first mistake is following "1 part browns to 1 part greens" advice by volume. Browns are typically 10–50 times more carbon-dense than greens, so a 1:1 volume mix is wildly carbon-heavy in actual C:N terms. The classic working ratio is closer to 3:1 or 4:1 browns to greens by volume — or, more reliably, calculated by weight as in this tool.
The second mistake is treating coffee grounds as a brown. They look brown, smell earthy, and feel woody, but their C:N is 20:1 — squarely in the green camp. Adding several pounds of coffee grounds to compensate for too much grass actually makes the nitrogen excess worse.
The third mistake is ignoring moisture. The microbes need water to move and to dissolve nutrients. A pile at the right C:N but only 30% moisture sits as inactive as a carbon-heavy pile. Squeeze a handful — it should feel like a wrung-out sponge. The Cornell Cooperative Extension target is 50–60% moisture by weight.
These attract rats, raccoons, and flies, and their nitrogen is so concentrated that small amounts can push the pile anaerobic. Commercial in-vessel composters run above 60 °C for long enough to deal with them safely; home piles cannot. Bone, fish, and shellfish belong in green-waste collection, not the backyard bin.