Compost Calculator — C:N Ratio for Fast Composting
Enter the weights of your brown (carbon-rich) and green (nitrogen-rich) materials to find the combined C:N ratio of your compost pile. The ideal range for fast, hot composting is 25:1 to 30:1. The calculator tells you if you need more browns or greens and by how much.
Brown material type
kg
Green material type
kg
Slightly high — composting will be slower. Add a bit more greens.
- 1
Carbon total
30 × 60/(60+1) + 10 × 17/(17+1) = 38.953Carbon fraction of each material ≈ C:N / (C:N + 1) by mass. - 2
Nitrogen total
30 × 1/(60+1) + 10 × 1/(17+1) = 1.0474 - 3
Combined C:N ratio
38.953 ÷ 1.0474 = 37.2
How does this calculator work?
The C:N ratio of a compost pile is calculated by dividing total carbon by total nitrogen across all materials. Target 25–30:1 for fast, hot composting. Enter your browns (leaves, cardboard) and greens (grass, food scraps) weights and the calculator shows the combined ratio and whether you need more of either.
Formula
How this is calculated
The carbon-to-nitrogen (C:N) ratio controls how fast microorganisms break down organic matter. They need roughly 25–30 parts of carbon for every 1 part of nitrogen to thrive. Too much carbon (high C:N, > 40) and decomposition slows dramatically; too much nitrogen (low C:N, < 20) and the pile turns anaerobic, producing ammonia and unpleasant odours.
For each material in the pile, the carbon fraction ≈ C:N/(C:N + 1) and the nitrogen fraction ≈ 1/(C:N + 1) by dry mass. The combined ratio is total carbon divided by total nitrogen across all materials. The C:N ratios used here are representative averages (2024) — they vary with moisture content, age, and source; values are editable if you know your material's actual ratio.
In practice, compost is rarely weighed precisely. Use the ratio as a guide: a roughly 3-to-1 pile of dry leaves (C:N ≈ 60) and grass clippings (C:N ≈ 17) by weight gives a combined C:N of about 30, which is ideal. Turn the pile regularly, keep it moist, and monitor temperature (55–65 °C in the core signals active composting).
Frequently asked questions
Browns are dry, carbon-rich materials: dead leaves, straw, cardboard, wood chips, newspaper, sawdust. Greens are fresh, nitrogen-rich materials: grass clippings, fruit and vegetable scraps, coffee grounds, fresh plant prunings, and manure. Despite the name, some "greens" can be brown in colour — it's the C:N ratio that matters, not the colour.
Decomposer microorganisms use about 30 parts of carbon for energy and 1 part of nitrogen for cell-building proteins. A slightly lower target (25:1) accounts for carbon lost as CO₂ during decomposition. This range maximises microbial activity, temperature, and decomposition speed.
Yes — a common rule of thumb is "3 parts browns to 1 part greens by volume". This calculator gives a more accurate picture when you have weights, but the volumetric rule is a practical starting point for backyard composting.
TG we-Calculate Editorial Team. (2026). Compost Calculator — C:N Ratio for Fast Composting [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/compost-calculator
TG we-Calculate Editorial Team. "Compost Calculator — C:N Ratio for Fast Composting." TG we-Calculate. 2026. https://we-calculate.com/calculator/compost-calculator.
TG we-Calculate Editorial Team, "Compost Calculator — C:N Ratio for Fast Composting," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/compost-calculator
@misc{wecalculate_compost_calculator, title = {Compost Calculator — C:N Ratio for Fast Composting}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/compost-calculator}}, year = {2026}, note = {TG we-Calculate} }
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