C:N Blend Calculator
This compost carbon nitrogen ratio calculator works as a compost mix calculator, a browns to greens ratio calculator, and a compost greens to browns calculator: add each material with its amount, by weight or by volume. Dry weights are computed from moisture content, then blended with the exact harmonic formula — not a naive average of ratios.
Target Solver — “how much brown do I add?”
The killer feature — a browns to greens ratio calculator that works backwards: tell it what you already have and the C:N you want (30:1 is the classic hot-compost target), and it solves exactly how much brown to add to compost to get there.
Materials you already have (fixed)
Material Database — 26 ingredients
Typical C:N ratios, moisture and bulk density from published composting tables (Cornell, UC ANR, CDFA). Real values vary — every calculator row lets you override them.
| Material | C:N | Moisture | Bulk density | Tip |
|---|
Pile / Bin Volume Planner
This compost pile size calculator answers “how big will my pile be?”: volume comes from wet weight ÷ bulk density. Hot composting needs about 1 m³ (3×3×3 ft) minimum to hold heat.
Compost Diagnostics
Tick what you observe. If you ran the Blend tab, its C:N and moisture are used as extra context automatically.
Compost Quick Reference (print me)
nrtechstudio.com/compost-ratio-calculator — tape this to your bin.
Target zones
| Measure | Ideal | Too low | Too high |
|---|---|---|---|
| C:N ratio (starting mix) | 25–35 : 1 (aim 30) | < 20 → ammonia smell, slimy | > 40 → cool, very slow |
| Moisture | 40–60% (wrung-out sponge) | < 40% → dormant | > 60% → anaerobic, smelly |
| Core temperature (hot compost) | 55–65 °C (131–150 °F) | < 40 °C → add greens/water/size | > 71 °C → turn now |
| Pile size (hot compost min) | ≥ 1 m³ (3×3×3 ft) | Smaller = cooler & slower (still fine, just slow) | |
Greens vs browns cheat-sheet
Troubleshooting matrix
| Symptom | Likely cause | Fix |
|---|---|---|
| Ammonia smell | Too much nitrogen / too wet | Add dry browns, turn for air |
| Slimy, soggy | Over 60% moisture | Dry browns + turn, cover from rain |
| Won't heat up | Too dry, too much carbon, or pile too small | Water, add greens, build to 1 m³ |
| Dry, unchanged | Under 40% moisture | Water while turning (wrung-sponge feel) |
| Flies / pests | Exposed food scraps | Bury under 10–15 cm of browns |
| Over 71 °C | Over-active core | Turn now, add coarse browns |
Top C:N values to memorize
Grass 20 · food scraps 15 · coffee grounds 20 · chicken manure 10 · leaves 60 · straw 80 · newspaper 175 · cardboard 350 · sawdust 325 · wood chips 400 (all :1, typical — real values vary).
How the math works (methodology)
Some searchers call this a compost cn ratio calculator or c:n ratio calculator compost — whatever the name, the math below is what matters.
The exact blend formula — harmonic, not a simple average
A C:N ratio is a ratio, and ratios don't blend by simple averaging. The correct dry-weight math (as taught by the Cornell Waste Management Institute) is:
where mdry,i = wet weight × (1 − moisture)
This is exact whenever the materials share the same dry-basis carbon percentage — and in practice compost feedstocks cluster tightly around 40–55% carbon, so the approximation is very close. Check it against Cornell's own worked example (grass C/N 18.75 + leaves C/N 66.67, 10 kg grass + 6.8 kg leaves):
| Method | Result | vs Cornell's exact answer |
|---|---|---|
| This calculator (harmonic) | 35.5 : 1 | within ~3% of 36.6 — and correctly under Cornell's stated “a little less than 37:1” |
| Naive weighted average of ratios | 50.2 : 1 | 41% too high — would tell you to add far too many browns |
The same page's solver example (10 kg grass, target 30:1) needs 3.5 kg of leaves by Cornell's lab-exact %C/%N math; our table-based harmonic solver gives 3.9 kg — honest, close, and it round-trips to exactly 30.0. Lab-exact answers need measured %C/%N; published table values get you within ~10%.
The target solver algebra
With fixed materials totaling dry mass D and harmonic term K = Σ(dryi/CNi), the dry mass x of one more material needed for target T solves T = (D + x)/(K + x/CN) → x = (D − T·K) / (T/CN − 1), converted back to wet weight via its moisture content. If no positive x exists (target outside the span of your materials' ratios), the solver says so instead of inventing a number.
Why not just average the ratios? (a note on other calculators)
Many simple compost calculators blend C:N ratios with a weighted average. As the Cornell example above shows, that overstates the mix by ~40% — it systematically tells you the pile is more carbon-heavy than it is, so you under-add browns when you need them and over-add when you don't. This tool instead:
- uses the exact harmonic blend on a dry-weight basis (moisture-adjusted);
- adds a target solver (“I have 5 lb of kitchen scraps — how much dry leaves for 30:1?”);
- ships a 26-material database with moisture and bulk-density data, all overridable;
- plans pile volume from weight (or volume) inputs;
- includes symptom diagnostics tied to your actual blend; and
- prints a one-page quick reference for the bin.
Frequently asked questions
What is the ideal C:N ratio for compost?
Aim for about 30:1 when you build the pile, and anything in the 25–35:1 range works well — that is roughly 30 parts carbon to 1 part nitrogen by dry weight. Microbes burn carbon for energy and use nitrogen to build protein, and 30:1 is the sweet spot where they work fastest. As composting progresses, carbon escapes as CO₂ while most nitrogen stays, so finished compost naturally settles near 10:1. Start much above 40:1 and the pile stays slow and cool; start much below 20:1 and it risks ammonia odors.
What is the ratio of browns to greens for compost?
The classic rule of thumb is 2–3 parts browns to 1 part greens by volume — but that is only a starting guess, because a fluffy brown like dry leaves and a dense brown like wood chips have very different C:N values. For an exact answer, use the Blend Calculator above: enter what you have and it computes the true ratio, or use the Target Solver to get a precise “add this much” number for 30:1.
How does this compost ratio calculator work?
Enter each material and its amount. The calculator converts everything to dry weight using each material's moisture content, then blends the ratios with the exact harmonic formula (C/N = total dry mass ÷ Σ(dry mass ÷ C/N)) — the same math the Cornell Waste Management Institute teaches — instead of the incorrect simple average most calculators use. You can enter amounts by weight or by volume, so it doubles as a compost ratio calculator by weight and a compost ratio calculator by volume. Override any C:N or moisture figure, and see a per-material breakdown with a plain-English verdict.
Should I measure compost ingredients by weight or volume?
Weight is more accurate, because bulk density varies enormously — a bucket of dry leaves and a bucket of food scraps weigh very different amounts. That said, most people build piles by volume (buckets, wheelbarrows), so this calculator has a volume mode that converts using typical bulk densities for each material. Use weight mode when you have a scale; volume mode is fine for everyday pile-building.
Why is averaging C:N ratios the wrong way to blend them?
Because a ratio is not a quantity — you cannot average ratios directly. Take Cornell's own worked example: 10 kg of grass (18.75:1) plus 6.8 kg of leaves (66.67:1). A weighted average says the mix is 50:1; the correct harmonic blend is 35.5:1, agreeing with Cornell's exact answer of “a little less than 37:1”. The wrong method overstates the ratio by 41% and would tell you to add far too many browns. This calculator uses the harmonic formula, so it agrees with Cornell.
How much brown material should I add to my kitchen scraps?
It depends on the brown, but the Target Solver gives you the exact number. Example: 5 lb of kitchen scraps (C:N 15:1, 80% moisture) needs about 3.1 lb of dry leaves (60:1) to hit 30:1 — or only about 1.2 lb of shredded cardboard instead, since cardboard's ratio is much higher (350:1). Enter your own amounts in the Target Solver tab for a precise answer with your materials.
What counts as greens and what counts as browns for compost?
Greens are nitrogen-rich and usually moist: grass clippings, kitchen scraps, fruit and vegetable waste, coffee grounds, fresh manure, seaweed, fresh weeds. Browns are carbon-rich and usually dry: dry leaves, straw, shredded newspaper and cardboard, sawdust, wood chips, bark, pine needles, corn stalks. Color lies — coffee grounds look brown but count as greens (20:1). The Material Database tab lists all 26 ingredients with their ratios.
Are coffee grounds greens or browns for compost?
Greens. Despite their brown color, used coffee grounds test around 20:1 C:N with good nitrogen content, so they behave like grass clippings or vegetable scraps. Paper coffee filters, on the other hand, are browns. Add grounds freely but balance them with browns like dry leaves, and do not let thick layers of grounds mat down and go anaerobic.
How wet should my compost pile be?
40–60% moisture by weight — the classic test is the “wrung-out sponge”: squeeze a handful hard and only a few drops should come out. Below 40%, microbes go dormant and the pile stalls; above 60%, water fills the air pockets, the pile goes anaerobic, and you get slime and odor. The Blend Calculator estimates your mix's moisture from each ingredient so you can water — or add dry browns — before problems start.
Why does my compost smell like ammonia?
Ammonia smell means excess nitrogen relative to carbon — usually too many greens, a C:N ratio well under 20:1, or a pile that is too wet and compacted. Fix it by mixing in dry, carbon-rich browns (about 2–3 parts browns per part greens by volume), turning the pile to restore airflow, and holding off on new greens until the smell clears. The Diagnostics tab ranks this and other problems automatically.
Why isn't my compost pile heating up?
The four usual suspects: (1) too dry — below ~40% moisture microbes stall; (2) too much carbon — a ratio above ~40:1 starves microbes of nitrogen; (3) pile too small — hot composting needs about 1 m³ (3×3×3 ft) minimum to insulate the core; (4) needs air — turn it. Cold weather and big woody chunks slow things too. Work through the Diagnostics tab with your blend numbers for a ranked list of likely causes.
How big should a compost pile be?
For true hot composting — a 55 °C+ core that kills weed seeds and pathogens — build at least 1 cubic meter, roughly 3×3×3 ft. Smaller piles still compost, just cooler and slower (fine for a leaf-mold or slow cold pile). The Pile Size tab converts your ingredients' weight into volume, checks you against the 1 m³ threshold, and estimates finished compost at about one-third of the starting volume.
How accurate are the C:N numbers in the material table?
They are typical values compiled from published composting tables (Cornell, UC Agriculture and Natural Resources, California Dept. of Food & Agriculture) — good for planning, not lab measurements. Real materials vary: “dry leaves” can test anywhere from 30:1 to 80:1 by species and season, and sawdust ranges widely by wood type and freshness. That is why every row in the calculators lets you override the C:N and moisture with your own numbers, and why the honest advice is to watch the pile (smell, heat, moisture) rather than chase a decimal.
Does the C:N ratio change as composting progresses?
Yes — it steadily drops. Microbes burn carbon for energy and exhale it as CO₂, while most nitrogen stays in the pile (some is lost as ammonia if the pile is too nitrogen-rich or anaerobic). A pile that starts at 30:1 typically finishes near 10:1, which is one reason finished compost is such a gentle, balanced soil amendment.
Can I compost cardboard and sawdust?
Yes — both are excellent high-carbon browns (cardboard ~350:1, sawdust ~325:1), but their very high ratios mean you need nitrogen-rich greens alongside or the pile will sit cold for months. Shred or tear cardboard small and remove tape; use only untreated-wood sawdust and add it sparingly, in thin layers, since it compacts. The Target Solver tells you exactly how much green to pair with them.
Do I need to be exact about the C:N ratio?
No — and Cornell's composting scientists note that moisture is often the more critical variable. Landing roughly in the 25–35:1 zone with 40–60% moisture and decent airflow gets you 90% of the result. When in doubt, err toward slightly too much carbon (a high ratio): it composts a bit slower but stays aerobic and trouble-free, while too much nitrogen goes smelly fast. Use this calculator to get close, then let the pile's smell, heat, and moisture guide fine-tuning.