Compost Calculator

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, and 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), newspaper (~175:1), cardboard (~350:1), wood chips (~400:1); contribute carbon
• Greens (high-nitrogen): kitchen scraps (~15:1), fresh grass clippings (~17:1), coffee grounds (~20:1), cow or horse manure (~20:1); 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

Blending two C:N ratios, without the trap

You cannot average C:N ratios. Averaging treats a 60:1 material and a 15:1 material as if they contributed nitrogen equally, when the second carries four times as much. Total the carbon, total the nitrogen, divide. For materials of similar carbon content that comes out as:

Blended C:N = Total weight ÷ Σ(weight of each material ÷ its own C:N)

Worked example: 40 lbs dry leaves (60:1) plus 20 lbs fresh grass (17:1).

Nitrogen units = 40 ÷ 60 + 20 ÷ 17 = 0.667 + 1.176 = 1.843 Blended C:N = 60 ÷ 1.843 = 32.6:1

Slightly carbon-heavy, and close enough to work. Add 5 lbs of coffee grounds (20:1) and the nitrogen units become 2.093, giving 31.1:1.

Averaging instead would have said (40 × 60 + 20 × 17) ÷ 60 = 45.7:1, sending you off to find still more greens for a pile that is already near target. Our C:N ratio calculator does this arithmetic for up to three materials.