Compost Ratio Calculator

Mix two materials. Hit your target. Trust the math.

Gardener actively shoveling dark compost in outdoor pile, sunlight filtering through trees

Boundary Waters hinterland, October 2024. The shovel moves the balance.
This is where calculation becomes creation.

The Mixing Law

w₁ / w₂ = (R₂ − Rₜ) / (Rₜ − R₁)

w₁	weight of Material 1
w₂	weight of Material 2
R₁	C:N ratio of Material 1
R₂	C:N ratio of Material 2
Rₜ	Target C:N ratio

Derivation: Weighted average Rₜ = (w₁R₁ + w₂R₂) / (w₁ + w₂)
Sources: USDA NRCS On-Farm Composting Handbook • UMN Extension • Lam Field Notes (2023–2026)

Material 1 (High C:N) Total Weight Desired (lb or kg)

This is your final pile weight. Calculator splits it.

Material 2 (Low C:N) Target C:N Ratio

25:1 is the sweet spot. Peak microbial growth.

Your Perfect Blend

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Mix thoroughly. Moisture at 55%. Turn when steam stops rising.

Worked Example: Dry Leaves + Coffee Grounds

Step 1: R₁ = 60 (leaves), R₂ = 20 (grounds), Rₜ = 25 (target)

Step 2: w₁/w₂ = (20 − 25) / (25 − 60) = (−5) / (−35) = 1/7

Step 3: For 100 lb total pile: w₁ = 12.5 lb leaves, w₂ = 87.5 lb grounds

Step 4: Verify: (12.5×60 + 87.5×20) / 100 = (750 + 1750) / 100 = 25 ✓

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