Composting Calculator
Equal weights of grass and straw is not a balanced pile.
Work out Composting. Equal weights of grass and straw is not a balanced pile. Free, with no account and nothing to install.
Carbon to nitrogen
37:1
20 kg total · 59% moisture · about 13 weeks
The C:N ratio is by dry weight of carbon and nitrogen, not by volume and not by "two parts brown to one green". That rule of thumb collapses the moment one material is sawdust at 400:1 and another is grass at 17:1 — the arithmetic and the eyeball give completely different answers. Moisture matters as much as ratio. Below about 40% microbial activity stalls; above about 65% water displaces air and the pile goes anaerobic, which is what smells. The target is the feel of a wrung-out sponge. Unfinished compost dug in robs nitrogen from whatever is growing there, because the microbes still working on the carbon take it first. A carbon-heavy pile used early does active harm rather than nothing.
How the Composting Calculator works
Carbon-to-nitrogen ratio for a mix of up to three materials, by dry weight rather than volume, with moisture assessed and the weight of browns or greens needed to hit your target.
Also known as: carbon to nitrogen ratio compost · how much brown to green compost · why does my compost smell · compost mix ratio calculator
Frequently asked questions
What C:N ratio should compost have?
Between about 25:1 and 35:1 by weight of carbon to nitrogen. Below 20 it goes slimy and smells of ammonia; above 40 it sits nearly inert.
Is two parts brown to one green right?
Only sometimes. That rule collapses when one material is sawdust at 400:1 and another is grass at 17:1 — equal weights of grass and straw lands well above 40:1, not in the middle.
Why is the ratio measured by dry weight?
Because water carries no carbon or nitrogen. Grass clippings are 82% water and straw 12%, so equal wet weights are nothing like equal amounts of material.
Why does my compost smell?
Either too much nitrogen, which leaves as ammonia, or too much water, which displaces air and turns the pile anaerobic. Both are fixed by adding dry browns and turning.
Can I use unfinished compost?
Not dug in. Microbes still working on the carbon take nitrogen from the soil first, so unfinished carbon-heavy compost actively robs whatever is growing there.
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