The question with a compost bin is never how much it holds, it is how long it takes to fill and whether there is somewhere to turn it into. A cubic metre sounds generous until you work out that a household of three generates about thirty litres of compostable material a week, which is a third of a cubic metre a month — and the heap shrinks as it rots, so a bin that looks full in three months is half that volume by the summer. This works out the capacity, the weeks to fill it, and how much finished compost comes out the other end.
Bin, household and waste
A bin is sized by how fast it fills, and it fills slower than people think.
A metre square is the practical minimum for a heap to heat up.
Two bays let you turn one into the other; three is the classic setup.
Kitchen scraps plus a share of garden waste. Ten litres a week is typical.
A well-managed heap loses around 60% of its volume. A cold one, less.
Result
Volume is the wrong way to size a bin
Bins are sold by volume and should be bought by throughput. A cubic metre is the smallest heap that reliably heats up, and that is a physical constraint rather than a convenience: below about a cubic metre the heap loses heat faster than the bacteria generate it, it never gets hot, and what you get is a slow cold pile of half-rotted material that takes two years instead of six months.
But the volume you need depends on what arrives, and what arrives is a rate. A household of three produces roughly thirty litres a week of kitchen and garden material — about 1,560 litres a year, or a cubic metre and a half. Two one-cubic-metre bays hold two thousand litres, so they fill in about seventeen months by input rate alone, and considerably longer in practice because the heap shrinks as it rots.
Why two bays is the minimum and three is the classic
A single bay is a heap you cannot turn. Turning is the single thing that most speeds up composting: it reintroduces oxygen, it mixes the dry outside into the wet middle, and it kills the anaerobic pockets that produce the smell. To turn a heap you need somewhere to put it while you turn it, which means a second bay.
Three bays is the standard arrangement for a reason. Bay one is being filled, bay two is rotting, bay three is finished and being emptied. Material moves one way round the system and you never have to dig into a hot heap to find the finished part at the bottom — which is the usual failure with a single bay, where the bottom is ready long before the top and nobody wants to unload the whole thing to get at it.
Most of the mass in a compost heap leaves as carbon dioxide and water vapour, which is why a full bin settles to a third of its height over a season.
Do not treat settling as a problem or keep topping the same bay up. Move the heap into the next bay and let it finish.
What goes in, and what to leave out
The working ratio is roughly half green and half brown by volume. Greens are wet and nitrogenous: kitchen scraps, coffee grounds, grass clippings, soft prunings. Browns are dry and carbon-rich: cardboard, paper, straw, autumn leaves, woody prunings that have been shredded. Too much green and the heap goes slimy and anaerobic; too much brown and it sits there doing nothing.
What to leave out: cooked food and anything with fat or dairy on it, which attracts rats; meat and fish, for the same reason; persistent weeds with viable seed or running roots; diseased material; and anything treated, painted or containing plastic. Cat and dog waste is best kept out of a home heap entirely.
| Approach | Turning | Time to finished | Quality |
|---|---|---|---|
| Cold heap, never turned | None | 12 to 24 months | Coarse, some seeds survive |
| Heap turned monthly | Monthly | 6 to 9 months | Good, usable |
| Heap turned fortnightly | Fortnightly | 3 to 4 months | Fine and consistent |
| Hot heap, built fast | Every few days | 6 to 8 weeks | Fine, weed seeds killed |
| Tumbler, turned daily | Continuous | 6 to 10 weeks | Fine, but often too dry |
A worked example
- CapacityA metre cube is a thousand litres; two bays is 2,000 L.
- Input rateThree people at ten litres a week is 30 L a week, or 1,560 L a year.
- Time to fillTwo thousand litres at thirty a week is about 67 weeks — and longer than that in practice, because it is shrinking all the while.
- Bays neededA year's waste at a thousand litres a bay is 2 bays, with nothing spare for turning.
- What comes outAt sixty percent shrinkage, 1,560 L of waste becomes about 624 L of finished compost — roughly fifteen bags.
Six hundred and twenty-four litres is a genuine amount of compost: enough to mulch a border or to top-dress a vegetable bed twice. It is also the argument for the third bay, because a year's output from two bays means the finished material has to go somewhere.
Using what comes out
Finished compost is dark, crumbly, smells like woodland and has no recognisable scraps in it. Sieve it if you want it fine for seed compost, and use the coarse fraction as a mulch. It is a soil conditioner rather than a fertiliser: it feeds the soil and the organisms in it, and its nutrient content is modest and slow-release.
Applied at about fifty millimetres once a year over a border, six hundred litres covers twelve square metres — the mulch volume figure turns that into bags if you need to buy more. Where the compost is going into beds rather than onto them, the raised bed figure sizes the soil it will be mixed into, and the planter figure does the same for containers.
Do
- Aim for at least a cubic metre, so the heap can heat up.
- Build two bays minimum, three if you have the room.
- Balance greens and browns at roughly half and half by volume.
- Turn it — that is the biggest single lever on how fast it finishes.
- Keep cooked food, meat and dairy out of a home heap.
Avoid
- Building a heap smaller than a cubic metre and wondering why it never heats.
- Using a single bay and having nowhere to turn it into.
- Adding mostly grass clippings, which go slimy and anaerobic.
- Letting it dry out, which stops everything.
- Expecting the output volume to match the input volume.
Where the numbers stop being true
The input rate is the weakest figure on the page and the easiest to measure: weigh or bucket your kitchen scraps for a week and you will have a real number. Ten litres per person per week is typical for a household that eats fresh food, and it varies by several times depending on how much is cooked from scratch and how much garden waste is included.
Shrinkage depends entirely on management. A hot, well-turned heap loses more volume than a cold one, which sounds perverse but is simply because it is working: the material that leaves is carbon going off as gas. A heap that shrinks slowly is a heap that is rotting slowly.
Seasonality is not modelled at all. Garden waste arrives in great bursts — a hedge cut, the autumn leaves, the spring clearance — and a bin sized for the average week will overflow in October. If you prune a lot, the pruning waste figure is the one that tells you how big those bursts are.
How the figures are produced
Bin volume is width times depth times height, converted to litres and multiplied by the number of bays. Weekly input is people times litres per person, and annual input is that times fifty-two. Weeks to fill is capacity divided by weekly input, and bays needed is annual input divided by the volume of one bay, rounded up. Finished compost is annual input reduced by the shrinkage percentage.
Questions people ask
How big should a compost bin be?
At least a cubic metre, because smaller heaps cannot hold heat. Two bays of a cubic metre suit most households; three is better.
How long does compost take?
Six to nine months turned monthly, three to four turned fortnightly, and a year and a half if you never touch it.
Why is my heap not heating up?
Usually too small, too dry, or too much brown material. Add greens, add water, and build it bigger.
Can I compost grass clippings?
Yes, in thin layers mixed with dry browns. On their own they compact into a slimy anaerobic mat.
Do I need an activator?
No. A good mix and some air is enough. Activators help a badly balanced heap, which is better fixed by balancing it.
What do I do with the finished compost?
Mulch with it at about fifty millimetres, or mix it into beds at planting time. It is a conditioner rather than a feed.