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) typically have a C:N ratio of 15-25:1. They are rich in nitrogen. Grass clippings are about 20:1, fruit scraps about 25:1.

What C:N ratio do brown materials have?

Brown materials (dry leaves, cardboard, straw) typically have a C:N ratio of 40-80:1. Cardboard is about 350:1, dry leaves about 60:1, and straw about 80:1.

How does moisture affect composting?

Ideal moisture is like a wrung-out sponge (40-60%). Too dry and decomposition stalls. Too wet and the heap becomes anaerobic (smelly). Add dry browns to soggy heaps, water to dry ones.

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.

GardenCalcs

Compost Ratio Calculator — C:N Ratio & Mix Balance

Calculate compost mix ratios for optimal decomposition.

Bin/Heap Volume

Bin/heap volume

Green Material

Kitchen scraps, grass clippings, fresh plant waste

Brown Material

Dry leaves, cardboard, straw, wood chips

Moisture LevelMoisture level

How We Calculate This

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

Formula: C:N = (Green kg x Green C:N + Brown kg x Brown C:N) / (Green kg + Brown kg).

If the ratio is outside the ideal range, it calculates how much additional green or brown material to add to reach your target ratio.

Frequently Asked Questions

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Last updated: March 2026

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