Compost mix ratio:
how many parts brown for each part green

Browns, greens and why the balance matters

A compost heap is a population of microbes, and they need a diet in the right proportions. Greens, such as vegetable scraps, fresh grass clippings and coffee grounds, supply nitrogen. Browns, like dry leaves, straw, shredded cardboard and sawdust, supply carbon and bulk. Too many greens and the heap turns wet, slimy and smelly. Too many browns and it sits cold and dry for months.

The textbook target is a carbon-to-nitrogen ratio of around 25 to 30 parts carbon to 1 part nitrogen by weight. Nobody weighs leaves, so gardeners use a volume shortcut instead: roughly two to three buckets of browns for each bucket of greens.

The scaling formula

A 2:1 mix means two parts brown to one part green. Dividing the brown parts by the green parts gives the multiplier: 2 ÷ 1 = 2. A 3:1 mix would multiply by 3, and a 1:1 mix by 1. Write the ratio as browns:greens so the order never flips.

Worked example: 30 litres of kitchen waste

That is three 10-litre buckets of greens and six of browns. If your dry leaves are very fluffy, they will look like more than 60 litres loose and settle once wet, so measure them lightly pressed rather than heaped.

Working backwards from a target heap works too. Suppose you want 150 litres in total at 3:1. Total parts are 4, so greens are 150 ÷ 4 = 37.5 L and browns are 150 × 3 ÷ 4 = 112.5 L.

Starting ratios for common materials

Because moisture and particle size change how materials behave, treat the ratio as a starting point. Squeeze a handful: it should feel like a wrung-out sponge, damp but dripping only a drop or two.

Reading the heap and correcting it

• Smells like ammonia or rot: too many greens or too wet. Mix in extra dry browns, about one part for each part of the original greens, and turn.
• Not heating and looks dry: too many browns or too little moisture. Add greens and water.
• Stays cool but damp with big pieces: shred or chop material smaller so microbes can reach it.
• Flies or rodents: bury kitchen scraps in the centre under a layer of browns, and avoid meat and dairy in open piles.

Moisture, size and temperature are part of the recipe

The ratio is only one of four levers. Moisture should feel like a wrung-out sponge, roughly 50 to 60 %. Particle size matters because microbes work on surfaces; shred woody material so that most pieces are under 5 cm. Air matters too, which is why you turn the heap or build it around a perforated pipe.

Temperature is the signal that the recipe is working. A well-balanced heap of about 1 cubic metre will warm to 55 to 65 degrees C within a week or two, hot enough to break down most weed seeds and pathogens if it holds there for several days. If your heap stays lukewarm, check the ratio first, then moisture, then size and volume; a small pile of under 0.5 m³ rarely gets hot because it loses heat too quickly.

Putting the recipe into buckets

In practice you will be measuring in buckets, not litres. A standard garden bucket holds about 10 litres when level, so the 30 L of kitchen scraps in the example is three buckets and the 60 L of browns is six. If a bucket is 12 litres, divide accordingly: 30 ÷ 12 = 2.5 buckets of greens means 5 buckets of browns at 2:1.

A handy way to remember is to keep a bag of dry leaves or shredded paper next to the kitchen caddy and add two handfuls for each handful of scraps. Over a season, a household producing about 10 litres of scraps a week at 2:1 would need 20 litres of browns weekly, which is 1,040 litres of browns over the year, a good reason to stockpile leaves in autumn.

Seasonal changes matter. Autumn gives you a surplus of leaves, which are brown, and little green material, while summer brings a flood of lawn clippings that are green and need much more carbon. Plan to store dry leaves in sacks or a spare bin through the year so that you always have browns to balance a summer heap.

A final point on safety and sense: leave out meat, fish, dairy and cooked food in open household heaps, because they attract pests and can smell. Pet waste from carnivores and diseased plants also do not belong in a domestic heap. Those materials need a hot, enclosed system or council collection.

Scaling a batch with a ratio tool

The same proportional maths is used when scaling any recipe up or down: you keep the ratio and change the batch size. A recipe-scaling tool can handle those proportions if you have a quantity you want to hit exactly.

Keep in mind that this is a volume rule of thumb rather than a lab measurement. Dense wet material is heavier per litre than dry leaves, so the true carbon-to-nitrogen balance in your heap will drift. Judge by smell, temperature and texture, and correct as you go.