Compost C/N Ratio Calculator
Enter what went into the pile — or what you plan to add — and this calculator blends the published carbon-to-nitrogen values of each material by weight. You get the resulting C:N ratio, an honest uncertainty range, and a solver that tells you exactly how much of any material to add to land in the 25–30:1 target band.
New to the idea? Start with what the C/N ratio actually means and why 25–30:1 is the target, or skim the full greens vs browns reference table.
Your mix
| Material | Weight (any one unit, used consistently) |
|---|
The math is unit-agnostic: it only compares weights, so kilograms, grams or pounds all work — just use the same unit everywhere (the label above only changes the wording of the results).
Result
- Blended C:N (typical): —
- Possible range: — (using each material's low/high published values)
- Status: —
- Total weight: —
- Greens/browns split:
Moisture & handling notes for this mix
What should I add?
Current mix off target? Pick a material and the solver computes the weight that moves the blend onto the 25–30:1 band, using the typical values:
Methodology (and why your compost book disagrees)
The calculator carries a typical value plus a low–high range for every material, taken from university extension and composting-science sources. The typical value is the midpoint of the published range — nothing more precise than that is honest, because published C:N values are lab-variable estimates:
- The same material tests differently by variety, maturity, storage and analytical method. Cornell's own datasets disagree with each other: shredded cardboard measures about 107:1 in one Cornell table, while Cornell's compost chemistry page cites newspaper/cardboard at up to 560:1.
- Cornell's C/N worked example uses 18.75:1 for grass clippings and 66.7:1 for leaves — both inside the wider 12–25:1 and 30–80:1 ranges in extension tables.
- Your kitchen scale weighs wet material; C:N is defined on dry mass. Wet greens (food scraps, grass) therefore contribute somewhat less than their wet weight implies.
The blend is a mass-weighted average of material C:N values. The rigorous version would sum measured carbon and nitrogen percentages per material (the approach of Cornell's own research calculator, which needs lab data per feedstock). For home mixing, the weighted average of published values is the standard practical estimate — treat the point value as a compass heading, not a lab report, and let the displayed range remind you of the error bars.
Sources behind the embedded table: Cornell Waste Management Institute (Compost Chemistry, C/N Ratio, Raw Materials Table 6) and Colorado State University Extension (GardenNotes #246, Making Compost).
Worked example
A classic first pile: 2 kg fresh grass clippings + 3 kg dry autumn leaves.
- Typical blend: (2 × 19 + 3 × 55) ÷ 5 = 40.6:1 — noticeably carbon-heavy.
- Range blend: (2 × 12 + 3 × 30) ÷ 5 = 22.8:1 at the green extreme, (2 × 25 + 3 × 80) ÷ 5 = 58:1 at the brown extreme.
- Solver: adding grass clippings (19:1) — x = M(T − R) ÷ (c − T) — needs about 4.8 kg to reach 30:1 and about 13 kg to reach 25:1. Anything in that window lands the pile in the band.
That "add more grass" answer is exactly what a smell-free, steady pile needs; see what to do when you're short on browns for the reverse problem, and the smelly-pile diagnostic flow if the pile is already built wrong.
FAQ
What C:N ratio should I aim for?
Most composting science references put the sweet spot at 25–30:1 at build time. Cornell's Waste Management Institute calls about 30:1 the usual recommended starting range; Oregon State University Extension materials note 20:1 or higher will still compost, just cooler and slower. Finished compost typically settles near 10–15:1 because microbes exhale about two-thirds of the consumed carbon as CO₂.
Why a range and not a single number?
Because the underlying data are ranges. The calculator blends the low ends and the high ends separately to show how far your mix could truly sit on either side. If the range straddles the target, you're close enough — composting forgives.
Ammonia smell — too green or too brown?
Ammonia = excess nitrogen escaping = too green. Fold in dry leaves, straw, shredded paper or cardboard. (A rotten-egg smell is a different problem: too wet and oxygen-starved — turn the pile and add bulky dry material.)
Can I use the 2:1 volume rule instead?
CSU Extension suggests roughly two parts green to one part brown by volume as a starting heuristic. It works until densities betray you — sawdust versus straw versus leaves. Weighing once per material teaches you what your "bucket" really weighs.
Does moisture change the ratio?
Not the chemistry, but your weighing: wet inputs overstate their dry contribution. Keep the pile at 50–60% moisture — a handful squeezed hard should release only a drop or two, like a wrung-out sponge (Cornell CWMI).
Educational tool, not a lab report
The calculator blends published typical values; your actual materials vary by season, species and storage. Use it to get into the right neighborhood, then judge the pile by smell, temperature and moisture.