Composting C:N Ratio Calculator
Calculate the blended carbon-to-nitrogen (C:N) ratio of a two-material compost mix to achieve the ideal decomposition range of 25:1 to 35:1.
Results
What is it?
The carbon-to-nitrogen (C:N) ratio determines how fast compost decomposes. Microorganisms that break down organic matter consume approximately 30 parts carbon for every 1 part nitrogen. The ideal composting ratio is 25:1 to 35:1. Too high (above 40:1) = slow decomposition. Too low (below 20:1) = ammonia smell and nitrogen loss. Brown materials (leaves, straw, cardboard) are carbon-rich; green materials (grass, food scraps, manure) are nitrogen-rich.
How to use
1. Select your carbon-rich (brown) material and enter its weight in kg. 2. Select your nitrogen-rich (green) material and enter its weight in kg. 3. The calculator shows the weighted C:N ratio of the blend. 4. The in-range indicator tells you whether the blend is in the ideal 25-35:1 zone.
Example scenario
Mixing 10 kg of dry leaves (C:N 50:1) with 10 kg of grass clippings (C:N 20:1). Blended C:N = (10x50 + 10x20) / 20 = 700/20 = 35:1. This blend is right at the upper edge of the ideal range. The pile should decompose well, but adding a bit more green material (or nitrogen) would speed decomposition.
Pro tip
When your pile is too high in carbon (slow, dry, not warming up), add nitrogen sources: fresh grass clippings, food scraps, coffee grounds, or diluted urine (C:N 1:1). When too low in carbon (slimy, smelly), add cardboard, shredded paper, or dry leaves. Aim for your pile to feel like a wrung-out sponge: moist but not dripping. Turn the pile weekly to aerate and speed decomposition.