Compost C:N Ratio Calculator

Enter the litres of each green and brown going into your heap to see its carbon to nitrogen ratio, whether it is too wet, and how much of one material to add to bring it to about 27:1.

Every compost heap is a population of microbes that eat carbon for energy and need nitrogen to build their bodies. Give them about 25 to 30 parts carbon for every part nitrogen and they work quickly and the heap smells of forest floor. Give them too little nitrogen and nothing much happens; too much and the extra nitrogen escapes as ammonia, the sharp smell of a heap gone sour.

Gardeners shorthand this as “greens and browns”, but the real ratios vary enormously. Dry leaves are around 55:1, cardboard several hundred, kitchen scraps about 20:1 and chicken manure closer to 10:1. This calculator uses typical values for each material, adds up the carbon and nitrogen in what you put in, and tells you where the mix lands and what to add to fix it.

Compost C:N Ratio Calculator

Enter how many litres of each material you have; leave anything unused at 0. Values are typical figures only — real materials vary with season, source and moisture, so treat the answer as a guide and trust your nose and a handful test as well.

Full guide, more detail, and a printable version at https://hollyshomestead.com.au/tools/compost-cn-ratio-calculator.html

Building the heap? See our hot composting guide, the slower cold composting guide and building healthy soil.

How the Ratio Is Worked Out

Each material in the calculator has a typical carbon to nitrogen ratio, a typical nitrogen content, a typical moisture level and a typical weight per litre. From those the calculator turns your litres into kilograms, takes out the water to get dry weight, and works out the nitrogen in that dry weight. Carbon is nitrogen multiplied by the material’s own C:N ratio.

Add up the carbon and nitrogen across everything in the heap and divide: that is the mix ratio. It is not an average of the individual ratios, because a small amount of a very nitrogen-rich material has a big effect on the total nitrogen, and a large volume of low-density straw carries less carbon than it looks.

The fix suggestion solves for the number of litres of one chosen material that brings the whole mix to 27:1, the middle of the 25 to 30 range that most composting guides aim for. If your mix is already close, the calculator says so rather than inventing a task.

Reading Your Result

Between about 20:1 and 40:1 a heap will compost; 25 to 30 is the sweet spot for a hot, fast heap. Above 40 the heap is carbon-heavy and slow: it stays cool and takes months, which is fine for a lazy cold heap but not if you want compost by spring. Below 20 it is nitrogen-heavy and tends to turn wet, slimy and smelly as ammonia escapes.

Moisture matters as much as the ratio. A good heap feels like a wrung-out sponge, around 40 to 60 per cent water. Kitchen scraps and fresh grass are about 80 per cent water, so a heap of mostly greens runs wet even when the C:N looks right; mixing in dry brown material fixes both. If the moisture estimate is high, add the browns first and re-check.

Practical Tips

Because browns are bulky and light, the common rule of thumb is about two to three parts brown to one part green by volume. Wet greens like kitchen scraps need that much dry material to balance them, while a drier green such as lucerne hay or manure with bedding needs less, which is why the calculator works from each material rather than a fixed rule. Chop or shred big pieces, since surface area speeds everything up, and keep a stack of dry leaves, straw or cardboard beside the heap so you can add browns whenever a bucket of wet scraps goes in.

Do not put meat, dairy or cooked food in a home heap, and keep weeds that have gone to seed out of anything that will not get properly hot. The finished compost feeds your beds; see the soil volume calculator for working out how much you need to fill them, and the mulch calculator to cover them afterwards.