♻️ Compost C:N Ratio Calculator

Select your compost materials and their weights to estimate your pile's overall carbon-to-nitrogen (C:N) ratio. The ideal range for active composting is roughly 25:1 to 30:1.

Greens Weight (lbs) — e.g. grass clippings, food scraps

Browns Weight (lbs) — e.g. dry leaves, cardboard

How the blend is estimated

Greens are treated as 20:1 and browns as 60:1, and the two are averaged by weight:

blended C:N = (greens lb × 20 + browns lb × 60) ÷ (greens lb + browns lb)

Equal weights therefore give 40:1, which the tool flags as too carbon-rich. To land on 30:1 you need three times as much green as brown by weight: 30 lb of grass clippings to 10 lb of leaves gives (600 + 600) ÷ 40 = 30.0:1. Under 20:1 it warns of a nitrogen excess, 20 to 35 reads as good, above 35 as too woody.

Reading the estimate honestly

Two categories cover a huge spread. Published tables such as Cornell's composting reference put food scraps near 15:1 and grass at 15–25:1, but dry leaves at 40–80:1, straw at 75:1 and cardboard near 350:1. Shredded card behaves nothing like autumn leaves.
The weights are wet. Real C:N figures are quoted on dry matter, and kitchen scraps are around 80% water while dry leaves are near 15%. Weighing as delivered pushes the green side up.
Averaging ratios is not the exact maths. A true blend is total carbon over total nitrogen, which leans toward the nitrogen-rich side: equal carbon from 20:1 and 60:1 stock is nearer 30:1 than 40:1.

So read the output as a direction, not a measurement. The pile tells you more than the number does: ammonia smell means too much nitrogen, and a heap sitting cold for weeks means too much carbon or too little water.

Frequently asked questions

What is the ideal C:N ratio for compost?

Between 25:1 and 30:1. That is where the bacteria have enough nitrogen to build cells from the carbon they eat, so the heap heats fast without stalling or stinking.

How many browns to greens should I add?

By weight, this tool's figures want about three parts green to one part brown. The familiar advice to add far more browns is by volume, and a bin of dry leaves weighs a fraction of the same volume of wet grass.

Why does my compost smell of ammonia?

Excess nitrogen, or too little air. Mix in shredded card, straw or dry leaves and turn the heap; the smell is nitrogen escaping as gas, which is fertility you are losing.