Calculate Carbon-to-Nitrogen ratio for compost piles.
The compost C:N calculator blends the carbon-to-nitrogen ratios of everything going into your pile into one weighted number, then tells you whether the mix will actually heat up. Hot composting works best when the blended ratio sits in the 25–35:1 window: piles above 35:1 are too woody and decompose slowly, while piles below 25:1 are too green and tend to go slimy and smell of ammonia as excess nitrogen escapes. Enter each material with its weight in pounds and its C:N ratio — reference tables put dry leaves near 60:1, fresh grass clippings near 20:1, and vegetable scraps around 15–25:1.
Blended C:N = Σ(weight × C:N) ÷ Σ(weight) — a weight-weighted average across all materials. The result is classified against the tool's thresholds: above 35 is flagged too woody, below 25 too green, and 25–35 ideal. Because the average is weighted by pounds, a small amount of a high-carbon material like sawdust shifts the blend more than its volume suggests, and the fix for an off-balance pile is always arithmetic: add enough of the opposite ingredient to pull the weighted average back into range.
10 lb of dry leaves (60:1) plus 10 lb of grass clippings (20:1) blends to (600 + 200) ÷ 20 = 40:1 — above the 35 threshold. Equal weights are not enough when one input is leaves.
Shift the same pile to 5 lb leaves and 15 lb grass: (300 + 300) ÷ 20 = 30:1, squarely in the 25–35 ideal band. The calculator makes these what-if adjustments instant.
20 lb of vegetable scraps at 17:1 alone reads too green. Adding 10 lb of dry leaves lifts the blend to (340 + 600) ÷ 30 ≈ 31:1 — ideal, and the pile stops smelling of ammonia.
Composting microbes consume roughly 25–35 parts carbon for each part nitrogen they assimilate. Inside that band the pile heats quickly; above it microbes run out of nitrogen, below it surplus nitrogen off-gasses as ammonia.
Extension-service composting tables list typical values: dry leaves ~60:1, straw ~75:1, fresh grass ~20:1, vegetable scraps ~15–25:1, sawdust several hundred to one. Enter the value alongside each material's weight.
This calculator weights by pounds. Volume-based 'browns to greens' rules are rough proxies because densities differ wildly — a bucket of wet grass outweighs a bucket of dry leaves several times over, which is exactly what a weighted average captures.
Add nitrogen-rich greens (grass clippings, coffee grounds, manure) and recalculate until the blend drops under 35:1. For a too-green pile, mix in dry browns like leaves or shredded cardboard to climb above 25:1.