Compost Calculator (How Much to Spread and C:N Ratio)
How many cubic yards or bags of compost to spread, and the carbon-to-nitrogen ratio of your compost pile.
Compost to spread (yd³) = length × width × depth (in) ÷ 12 ÷ 27. A 20 × 10 ft bed covered 2 in deep needs 33 cu ft, about 1.2 cubic yards. For a pile, aim for a C:N ratio of 25–30:1; equal weights of fresh grass and dry leaves come to about 36:1.
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How to use it
- Choose how much to spread, or the C:N ratio of a pile.
- For spreading, enter each area and the layer depth in inches.
- For the pile, add each material and its weight.
What you enter
- Area and depth
- or materials and amounts
What you get
- Cubic yards, cubic feet and bags
- Pile C:N ratio with advice
Formula
yd³ = L × W × (D/12) / 27 | Mix C:N = Σ(dry mass × %N × C:N) / Σ(dry mass × %N)
Worked example: pile mix
Input: 10 kg fresh grass clippings + 10 kg dry leaves
Result: About 36:1
Slightly carbon-heavy; a little more grass or some food scraps brings it to the 30:1 target.
Questions
How much compost should I add to my garden?
1–2 in spread on top of beds each year, or 2–3 in worked into the top 6–8 in when making a new bed. Topdress lawns at 1/4–1/2 in.
What is the best C:N ratio for compost?
25–30 parts carbon to 1 part nitrogen. Much lower and the pile smells of ammonia; much higher and it breaks down slowly.
What are greens and browns?
Greens are nitrogen-rich, wet materials such as grass clippings, food scraps, coffee grounds and manure. Browns are carbon-rich, dry materials such as leaves, straw, cardboard and wood chips.
Is it 3 parts browns to 1 part greens?
As a rough volume rule, yes. Because materials differ so much in moisture and nitrogen, the calculator uses each material's values for a better estimate.
Limitations
- Material values are typical averages; real moisture and nitrogen vary.
References
- Rynk R et al. (1992) On-Farm Composting Handbook (NRAES-54). Natural Resource, Agriculture, and Engineering Service, Ithaca NY.
Related reading
- Poultry Manure Composting and Nutrient Stewardship
- Rabbit Manure Management and Composting on Small Farms
- Goat Manure Composting: Methods, Nutrient Content, and Field Application
- Livestock Waste Management: Composting, Anaerobic Digestion, and Nutrient Recovery
- Dairy Cow Composting: Manure Management and Bedding
- Duck Manure Management: Composting and Nutrient Recycling
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All free tools · By Dr. Zubair Khalid, DVM, MS, PhD.