Frost & Furrow

Compost C:N ratio calculator

Enter the weight of each ingredient (any unit, as long as it is the same for all). The calculator gives the mix C:N ratio and moisture, and suggests how much leaves or grass to add to reach your target.

Calculator

Ingredient 1
Ingredient 2
Ingredient 3
Ingredient 4

Results appear here.

How to use it

  1. Weigh (or estimate) each ingredient in the same unit.
  2. Keep the default moisture/carbon/nitrogen values for grass, leaves and food scraps (Cornell's worked example) or replace them with lab or published values for your materials.
  3. Aim for roughly 30:1 and about 60% moisture.

The formula

Mix C:N = Σ[Q × (100 − M) × C] ÷ Σ[Q × (100 − M) × N] Moisture % = Σ[Q × M] ÷ Σ[Q] Q = wet weight, M = moisture %, C = carbon % (dry basis), N = nitrogen % (dry basis) Check (Cornell): 10 grass + 13.31 leaves + 8.14 food scraps → 30:1 at 60% moisture

Cornell explains that about 30:1 is generally considered ideal: lower ratios lose nitrogen as ammonia (odors), higher ratios compost slowly. Typical published ranges: autumn leaves 30–80:1, straw 40–100:1, wood chips or sawdust 100–500:1, vegetable scraps 15–20:1, coffee grounds 20:1, grass clippings 15–25:1. Real materials vary a lot, so treat the result as a guide.

Gear for this job

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  • Compost bin or tumbler — [affiliate link placeholder]
  • Compost thermometer — [affiliate link placeholder]
  • Kitchen compost caddy — [affiliate link placeholder]
  • Leaf shredder — [affiliate link placeholder]

Frequently asked questions

What is the best C:N ratio for compost?

Around 30:1 by weight, according to Cornell Waste Management Institute. As composting proceeds the ratio falls to about 10–15:1 in finished compost.

Why not just average the ratios?

Because materials differ in water and nitrogen content. Cornell's method weights each ingredient by its dry matter and its actual carbon and nitrogen, which a simple average of ratios ignores.

My pile smells of ammonia. What do I do?

Too much nitrogen. Add carbon-rich browns such as dry leaves, shredded paper or straw, and mix.

How big should a compost pile be?

Cornell recommends at least 1 cubic yard (3 × 3 × 3 ft) for a pile to hold heat; smaller piles still compost, just more slowly.

Sources

Last reviewed 2026-10-22. Found an error? Tell us.