Soil
Composting Calculator
Balance greens and browns for a working compost pile, and size your bin to the material you actually have.
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This bin is big enough to hold heat, so hot composting is realistic.
Greens, browns, and what the ratio really means
Composting microbes need carbon for energy and nitrogen for building proteins, at roughly thirty parts carbon to one part nitrogen. Nobody measures that directly, so gardeners use a volume proxy: one part green material to two or three parts brown.
Greens are fresh, wet, and nitrogen-rich. Browns are dry, structural, and carbon-rich. Browns also do a second job that gets overlooked — they hold the heap open so air can move through it, which is why a pile of grass clippings alone turns into a stinking mat rather than compost.
| Greens (nitrogen) | Browns (carbon) |
|---|---|
| Grass clippings | Dry leaves |
| Vegetable and fruit scraps | Shredded cardboard and paper |
| Coffee grounds and tea | Straw and hay |
| Fresh prunings and weeds | Woodchip and sawdust (untreated) |
| Manure from herbivores | Egg boxes and toilet roll tubes |
Size decides whether the heap gets hot
The heat in a compost heap is metabolic waste from thermophilic bacteria, and whether it accumulates depends on the ratio of heat-generating volume to heat-losing surface. Below roughly one cubic metre, heat escapes as fast as it is produced and the heap composts cold.
Cold composting is still composting. It simply takes six to twelve months instead of six to twelve weeks, and it never reaches the temperature that kills weed seeds and pathogens — so keep seeding weeds and diseased material out of a small bin.
Moisture and air, the two things people get wrong
- Target the dampness of a wrung-out sponge. A squeezed handful should release a drop or two, not a trickle.
- Too wet: add browns and turn. Too dry: water in layers as you build, not from the top.
- Turn every two to three weeks during the active phase if you want speed.
- Chop or shred material — surface area is the single biggest factor in decomposition rate.
How much finished compost you get
Expect roughly a third of your starting volume. Most of the loss is water and carbon dioxide leaving as microbes respire. A full 1 m³ bin therefore yields around 300-350 litres of finished compost — enough to top-dress a couple of raised beds.
Common mistakes
- Building the heap entirely from grass clippings, which compacts and goes anaerobic.
- Adding cooked food and meat to an open bin, which attracts rodents.
- Composting perennial weed roots in a heap that never gets hot.
- Letting the heap dry out completely in summer and stalling it.
- Judging progress by smell alone — a good heap smells of woodland soil, not of ammonia or vinegar.
What is the correct greens to browns ratio?
By volume, roughly one part greens to two or three parts browns. By carbon-to-nitrogen ratio the target is about 30:1, which those volumes approximate for typical garden material.
Why is my compost slimy and smelly?
Too many greens and not enough air. Mix in shredded cardboard, dry leaves, or straw and turn the heap to reintroduce oxygen.
Why is nothing happening in my heap?
Usually too many browns, too dry, or too small. Add greens, water to the dampness of a wrung-out sponge, and build the heap to at least a cubic metre if you want it to heat.
Can I compost cooked food and meat?
Not in an open heap — it attracts rodents and rarely reaches a safe temperature. Use a closed hot bin or a bokashi system for cooked waste.
Last reviewed February 2026