Compost C:N Ratio Calculator — Browns to Greens by Weight

Balance browns (carbon-rich) and greens (nitrogen-rich) by weight to hit a healthy compost C:N ratio — pick your materials, enter how much of each you're using, and see where your mix lands.

Brown materialⓘCarbon-rich material — dry leaves, straw, cardboard, wood chips.Green materialⓘNitrogen-rich material — grass clippings, food scraps, coffee grounds, manure.

Brown weight (parts)ⓘAny consistent weight unit works (kg, lb, buckets of known weight) — what matters is the ratio between the two.Green weight (parts)ⓘSame unit as brown weight — the ratio between them is what determines your mix's C:N.

Your mix's C:N ratio48.8:1

For a 30:1 target, use0.50 : 1 (brown:green by weight)

How to measure your inputs

Brown / green weight: any consistent weight unit works (kg, lb, buckets of known weight) — what matters is the ratio between the two, not the absolute units.
Custom materials: if your material isn't in the presets, look up its typical C:N ratio from a reference table and use the closest preset as a stand-in for the calculation logic, adjusting your mental estimate by how far off the ratio is.

Methodology

Mass-weighted average of preset C:N ratios: mix ratio = (W_brown×R_brown + W_green×R_green) ÷ (W_brown + W_green). Preset ratios are representative midpoints of the ranges published by Cornell Waste Management Institute's composting reference — real materials vary within those ranges, so treat the result as a solid starting point, not a lab-precise figure.

Getting your compost C:N ratio right

Why 25-30:1 is the target

Microbes that break down compost need roughly 25-30 parts carbon for every 1 part nitrogen to work efficiently. Too much carbon (too many browns) and decomposition slows to a crawl; too much nitrogen (too many greens) and the pile turns anaerobic and smells — the classic “rotten eggs” compost problem.

This is a simplified approximation, not lab chemistry

Real composting science (the method Cornell Waste Management Institute's own calculator uses) weighs each material's exact %carbon, %nitrogen, and %moisture. This calculator uses representative C:N ratios for common materials instead, mass-weighted by how much of each you add — close enough for real-world composting, where the acceptable range is fairly wide, but not a substitute for a lab test if you need precision.

Why real material ratios vary so much

Published C:N ranges for the same material type can span a wide range — wood chips alone range from roughly 200:1 to over 1,000:1 depending on species and how fresh the wood is. If your pile isn’t behaving as expected, the actual material on hand may sit far from the preset used here.

Frequently asked questions

What happens if my compost has too much carbon?

Too many browns relative to greens slows decomposition dramatically — the pile can sit for a long time without heating up or breaking down, since microbes don’t have enough nitrogen to build the proteins they need to multiply and work.

What happens if my compost has too much nitrogen?

Too many greens relative to browns tends to turn the pile anaerobic and produce a strong ammonia or rotten-egg smell, since excess nitrogen is released as gas rather than being used by the decomposition process.

Do I need to weigh my compost materials precisely?

No — composting tolerates a fairly wide range around the 25-30:1 target. Use this calculator to get in the right ballpark by volume/weight rather than aiming for an exact figure; visual and smell cues (pile heating up, no strong odor) are the practical day-to-day check.

Can I use a material not listed in the presets?

Yes — look up a representative C:N ratio for your specific material from a reference table (Cornell Waste Management Institute publishes one) and treat it the same way as a preset when estimating your mix.