What is the ideal C:N ratio for compost?

The ideal carbon-to-nitrogen ratio is 25-30:1. This gives microbes enough energy (carbon) and protein (nitrogen) to decompose materials efficiently. Too high and it decomposes slowly; too low and it becomes slimy and smelly.

What C:N ratio do green materials have?

Green materials (kitchen scraps, grass clippings) are nitrogen-rich, typically 10-25:1. Per the On-Farm Composting Handbook, grass clippings average about 17:1 and food/vegetable scraps about 15:1. The calculator defaults green material to 20:1 to cover a typical mixed kitchen-and-garden bucket; lower it toward 15-17:1 if your greens are mostly fresh grass or food waste.

What C:N ratio do brown materials have?

Brown materials (dry leaves, cardboard, straw) are carbon-rich, typically 40-80:1. Autumn leaves are about 40-80:1, straw averages around 80:1, and corrugated cardboard is roughly 350-560:1. The calculator defaults brown material to 60:1 (mixed dry leaves); raise it for shredded cardboard or wood chips.

How does moisture affect composting?

Ideal moisture is like a wrung-out sponge (40-60%). Too dry and decomposition stalls because microbes need water. Too wet (dripping or waterlogged) and the heap excludes air, turns anaerobic and smelly, and slows down. Add dry browns to a soggy heap and water to a dry one. The time estimate here is a rough guide, not a cited standard.

How long does compost take?

Hot composting with good C:N ratio: 8-12 weeks. Cold composting: 6-12 months. Factors include C:N ratio, moisture, aeration (turning), and particle size.

Compost Ratio Calculator — C:N Ratio & Mix Balance

Calculate compost mix ratios for optimal decomposition.

Bin/Heap Volume

Capacity of your compost bin or bay. A standard dalek bin is ~330 L; a 1 m³ pallet bay is 1000 L. Used to show how full your materials would pack it.

Green Material

Nitrogen-rich, sappy waste by weight: kitchen scraps, fresh grass clippings, soft prunings. A full kitchen caddy is roughly 2-3 kg.

Brown Material

Carbon-rich, dry waste by weight: autumn leaves, shredded cardboard, straw, woody prunings. Browns are light — a packed bin bag is only a few kg.

Moisture LevelSqueeze a handful: damp like a wrung-out sponge (40-60%) is ideal. Dry stalls decomposition; wet or soggy excludes air and slows it down.

How We Calculate This

The calculator estimates your current carbon-to-nitrogen ratio from the weight of green and brown materials and their typical C:N values.

Formula: a mixture's C:N is total carbon mass ÷ total nitrogen mass, not the average of the two ratios (Cornell Composting). Carbon scales with weight while nitrogen scales with weight ÷ ratio, so C:N = (Green kg + Brown kg) ÷ (Green kg ÷ Green C:N + Brown kg ÷ Brown C:N). For the defaults — 10 kg green at 20:1 plus 15 kg brown at 60:1 — that is 25 ÷ (0.5 + 0.25) = 25 ÷ 0.75 ≈ 33:1, a good hot-composting balance.

If the ratio is outside the ideal range, it solves the same identity for the extra green or brown weight needed to hit your target ratio, and shows roughly what share of your bin the loaded materials fill.

Frequently Asked Questions

Last updated: March 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.

Compost Ratio Calculator — C:N Ratio & Mix Balance

How We Calculate This

Frequently Asked Questions

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