Compost Calculator
Calculate compost bin volume, green-to-brown ratio, and estimated completion time from dimensions and ingredients.
Returns moisture and turning frequency.
Compost pile calculations determine the ideal volume and carbon-to-nitrogen ratio (C:N ratio) for rapid, efficient decomposition. The C:N ratio controls microbial activity — the right balance produces finished compost in 4–8 weeks; an imbalanced pile decomposes slowly or fails entirely.
Optimal C:N ratio: C:N ratio = Total Carbon Content ÷ Total Nitrogen Content
Target: 25:1 to 30:1 — the ideal zone for active microbial decomposition.
C:N contribution formula: C contribution = Weight of Material × Carbon % of Material N contribution = Weight of Material × Nitrogen % of Material
Blended C:N ratio: Blended C:N = Σ(C contributions) ÷ Σ(N contributions)
Pile volume formula: Volume = Length × Width × Height (all in feet or meters)
Minimum effective pile size: 3 ft × 3 ft × 3 ft (27 cubic feet / ~0.76 m³) Maximum manageable size: 5 ft × 5 ft × 5 ft before turning becomes impractical
What each variable means:
- Browns (high-carbon) — dried leaves (~60:1), straw (~80:1), cardboard (~350:1), paper (~175:1); contribute carbon
- Greens (high-nitrogen) — fresh grass clippings (~20:1), kitchen scraps (~15:1), coffee grounds (~20:1), fresh manure (~15:1); contribute nitrogen
- Moisture — the pile should feel like a wrung-out sponge: 40–60% moisture content. Too dry = slow. Too wet = anaerobic and smelly.
- Particle size — chop or shred materials to 1–2 inches for fastest decomposition; surface area accelerates microbial activity
Turning frequency for hot composting:
- Initial build: 0–7 days (microbial ramp-up)
- Active phase: turn every 3–5 days (pile should reach 130–160°F internally)
- Curing phase: turn weekly for 4–8 weeks until temperature stabilizes at ambient
Worked example: Pile materials: 40 lbs dried leaves (C:N 60:1) + 20 lbs fresh grass (C:N 20:1) + 5 lbs coffee grounds (C:N 20:1)
Carbon = (40 × 60) + (20 × 20) + (5 × 20) = 2,400 + 400 + 100 = 2,900 units C Wait — C:N ratio is used by weight fraction. Simplified: Effective C:N blend ≈ [(40 × 60) + (20 × 20)] ÷ (40 + 20) = (2,400 + 400) ÷ 60 = 46:1 — too much carbon; add more greens.
Adding 20 more lbs grass: blend C:N ≈ (2,400 + 800) ÷ 80 = 40:1 — close; add a little more nitrogen material.
Ideal: keep adding greens until the blended ratio falls into the 25:1–30:1 zone.