About the Compost Calculator
This compost calculator works out the carbon-to-nitrogen (C:N) ratio of a mix of a "green" (nitrogen-rich) material and a "brown" (carbon-rich) material, and tells you how much brown material to add to reach your target ratio. Choose the two materials, enter how many parts of each you have, and the tool reports the blended C:N ratio and whether the pile is balanced.
Compost microbes work fastest at a C:N ratio of roughly 25–30 to 1. Too much nitrogen (low ratio) and the pile turns slimy and smells of ammonia; too much carbon (high ratio) and it sits cold for months. Because nitrogen content differs so much — grass clippings hold over twenty times more nitrogen than newspaper — simple "2 parts browns to 1 part greens" rules are often off.
Material values are typical dry-weight figures; real materials vary, so treat the result as a starting point. Enter parts by weight on a similar moisture basis, and adjust by feel: a balanced pile heats up within a few days and smells earthy.
With the default inputs, the mix c:n ratio is 32.9 : 1. Change any value above to recalculate instantly.
How to use the compost calculator
- 1Choose your main green (nitrogen) material.
- 2Choose your main brown (carbon) material.
- 3Enter how many parts of each you have, using the same unit for both.
- 4Read the mix ratio and the parts of brown needed for your target.
- 5Build the pile in layers, keep it as damp as a wrung-out sponge and turn it weekly.
Formula and method
Each material's carbon content is its nitrogen percentage times its C:N ratio. The blended ratio is total carbon divided by total nitrogen — a weighted average by nitrogen, not by weight, which is why a small amount of manure or grass shifts the ratio so much.
Solving the blend equation for the brown amount gives the parts of brown needed per G parts of green to hit target T, where Ng and Nb are the nitrogen percentages and Rg and Rb the C:N ratios. It only has a solution when the green is below the target and the brown above it. Values are typical dry-weight figures.
- G
- Parts of green material
- Ng, Nb
- Nitrogen % of green and brown (dry weight)
- Rg, Rb
- C:N ratio of green and brown
- T
- Target C:N ratio
Worked examples
1 part grass clippings to 2 parts dry leaves
One part of grass (2.4% N, C:N 17) supplies 0.024 parts of nitrogen and 0.024 × 17 = 0.408 parts of carbon; two parts of leaves (0.9% N, C:N 54) supply 0.018 nitrogen and 0.972 carbon. The mix is 1.38 ÷ 0.042 ≈ 32.9:1. About 1.44 parts of leaves per part of grass would hit exactly 30:1.
Kitchen scraps with cardboard
Three parts of food scraps (2.5% N, C:N 15) with one part cardboard (0.1% N, C:N 450) gives (1.125 + 0.45) ÷ (0.075 + 0.001) ≈ 20.7:1 — too wet and nitrogen-rich. About 2.68 parts of cardboard would bring it to 30:1.
Chicken manure and straw, equal parts
Equal parts of chicken manure (3% N, C:N 10) and straw (0.7% N, C:N 80) give (0.30 + 0.56) ÷ (0.03 + 0.007) ≈ 23.2:1. Using about 1.71 parts of straw per part of manure reaches the 30:1 target.
Frequently asked questions
What is the ideal C:N ratio for compost?+
Most guidance puts the ideal starting ratio at about 25–30 parts carbon to 1 part nitrogen. Piles between roughly 20:1 and 40:1 still compost, but more slowly or with more odor.
What counts as greens and browns?+
Greens are nitrogen-rich, usually moist materials such as grass clippings, food scraps, coffee grounds and manure. Browns are dry, carbon-rich materials such as leaves, straw, cardboard, paper and wood chips.
Is the 3:1 browns-to-greens rule correct?+
It is a rough volume rule of thumb. Because nitrogen content varies widely, the right ratio depends on the actual materials — three parts of sawdust to one of grass is far too carbon-rich, while three parts of leaves to one of manure can be about right.
Why does my compost smell like ammonia?+
An ammonia smell means excess nitrogen is escaping as gas, so the C:N ratio is too low. Mix in more browns such as shredded leaves or cardboard and turn the pile.
Why is my compost pile not heating up?+
A cold pile is usually too dry, too small (under about 3 × 3 × 3 ft) or too carbon-rich. Add greens, moisten it and turn it to add oxygen.