Compost C:N Ratio Calculator
The carbon-to-nitrogen ratio of a compost mix from its greens and browns, and how much brown material brings it to the 25–30:1 sweet spot.
Compost microbes work fastest at a C:N ratio of about 25–30:1.
How the compost c:n ratio calculator works
Compost microbes work fastest at a C:N ratio of about 25–30:1. Greens — grass clippings, food scraps, manure — are nitrogen-rich (10–25:1); browns — dry leaves, straw, cardboard — are carbon-rich (50–500:1). This calculator weights the ratios by mass, the usual back-of-envelope method; it is close enough for a garden heap, and it tells you how much brown to add to reach your target.
Formula: mix C:N ≈ (mass₁ × ratio₁ + mass₂ × ratio₂) ÷ (mass₁ + mass₂); browns to add = greens × (target − green ratio) ÷ (brown ratio − target)
Worked examples
| Inputs | Mix C:N ratio | Note |
|---|---|---|
| 10 kg grass, 5 kg leaves | 33.3 | 33.3:1 |
| Food scraps alone | 15 | 15:1 — add 4 kg of leaves |
| Straw-heavy heap | 62.9 | 62.9:1 — too slow |
FAQFrequently asked questions
What C:N ratio should compost have?
Start around 25–30:1. Higher is slow and cool; lower goes anaerobic and smells of ammonia as excess nitrogen escapes. Finished compost ends near 10–15:1.
What are typical C:N ratios?
Grass clippings 15–25, food scraps 15–20, coffee grounds 20, manure 15–25, leaves 40–80, straw 70–100, paper 150–200, sawdust 300–500, cardboard about 350.
Why is the calculation approximate?
It weights ratios by mass and ignores that materials differ in carbon and moisture content. The exact method uses each material’s %C, %N and moisture; for a garden heap the simple version is close enough.
Wet or dry weight?
Fresh weights as you would weigh them; the ratios quoted for materials are on the usual as-collected basis.
Where these figures come from
- NCBI Bookshelf — Molecular Biology of the Cell — growth kinetics and molecular conventions
- National Human Genome Research Institute — the US genomics research institute
Last checked: September 2026. Formulas are the standard textbook forms; assumptions are stated on each page because they are where these models break.