A compost bin is a container that holds garden waste and kitchen scraps while they rot down into compost — dark, crumbly, earthy-smelling material that feeds the soil. A bin keeps the heap tidy, holds in heat and moisture, keeps out rats, and makes composting possible in a small garden where an open heap would be impractical. This guide covers the types of compost bins, how to choose and build one, what to put in it, how to tend it, and how to know when the compost is ready.
Composting is rotting, managed well. Bacteria, fungi, worms and other organisms break down organic matter, and in a good bin they do it fast enough to generate heat — a hot heap can reach 55–65°C in the middle. The organisms need four things: food, air, water and warmth. A bin helps with all four: it holds the material together so it heats up, it shelters the heap from drying wind and heavy rain, and it keeps the material in one place so it can be turned. A bin needs to be at least a cubic metre — roughly 1 m × 1 m × 1 m — to heat up properly; smaller bins work, but slowly. Two bins are better than one: one filling, one maturing. Our composting basics guide explains the science in detail, and the compost ratio calculator helps with the green-to-brown mix. If you are unsure about composting in your area, check your local extension service for local advice.
Types of compost bins
There are four main types of compost bins. Wooden bins, made from pallets or boards, are cheap, easy to build, and breathe well — air moves through the gaps between boards, which is good for the organisms. Plastic bins, often sold as a tapered cylinder with a lid and a small hatch at the bottom, are neat, keep in heat and moisture, and suit small gardens, but they can be hard to turn and may need extra air holes. Wire mesh bins are the cheapest and airiest, but they dry out fast and look untidy. Tumbler bins are sealed drums on a frame that you rotate to mix the contents — they are fast and tidy, but small, and they need regular turning to work. Choose based on your garden size, your budget and how much material you produce. A large garden with lots of waste needs a large wooden bin or two; a small garden with mainly kitchen scraps suits a plastic bin or a tumbler. Whatever you choose, it needs air holes in the sides, an open base on the soil so worms can enter, and a lid or cover to keep rain off and warmth in.
Building a wooden compost bin
A wooden bin from pallets is the most popular home-made option, and it takes less than an hour to build. You need four pallets, a few screws or wire, and a level spot on bare soil in a sheltered corner. Stand three pallets on edge to form a U-shape, screw or wire them together at the corners, and use the fourth pallet as a front that can be removed for turning and emptying — or leave the front open and cover the heap with old carpet or a lid. Line the inside with wire mesh if the gaps between pallet slats are wide, to keep material in and rats out. The bin should be at least a cubic metre inside. Put a 15 cm layer of coarse browns — twigs, straw, shredded cardboard — at the bottom for drainage and air. Then add alternating layers of greens and browns, roughly one part green to two or three parts brown by volume, watering each layer lightly as you go. Add a few spadefuls of garden soil or finished compost to introduce the organisms that start the process. Cover the top with old carpet, a lid or a piece of plastic to keep rain off and warmth in.
What to put in — and what to leave out
Good for the bin: vegetable and fruit peelings, tea bags (plastic-free only), coffee grounds, eggshells (crushed), grass clippings in thin layers, leaves, straw, shredded cardboard and paper, prunings chopped small, spent bedding from herbivore pets, and well-rotted manure. Chop or shred large material first so it breaks down faster, and add grass clippings in thin layers mixed with browns — a thick layer of grass alone goes slimy and smelly. Do not put in: meat, fish, dairy, fats and oils, cooked food in large amounts, pet waste from dogs or cats, diseased plants, weeds with mature seeds, invasive weeds such as bindweed or couch grass roots, treated timber, glossy paper, or anything with chemicals on it. Meat and dairy attract rats and smell; pet waste can carry pathogens; diseased plants and seeding weeds can survive a cool bin and spread when the compost is used. Our composting basics guide covers this in more detail, and compost tea explains how to make liquid feed from finished compost.
Tending the bin
A compost bin needs tending, but not much. Turn the contents every few weeks with a fork, moving the outside material into the middle, which reintroduces air and mixes the contents. Check moisture each time: the heap should be damp like a wrung-out sponge, never wet and never dry. If it is dry, water it; if it is wet and smelly, add dry browns and turn it. A hot bin will steam on a cool morning and feel warm when you push your hand into the middle. If the bin is not heating up, check the size — it may be too small — the green-to-brown ratio, and the moisture. If it smells of ammonia, there are too many greens; add browns and turn it. If it smells sour, it is too wet and airless; add browns and turn it. Keep the lid on to hold in warmth and keep rain off, and keep the base open to the soil so worms can enter. Worms are a sign of a healthy bin — see our worm composting guide for a system built entirely around them.
When compost is ready and how to use it
Finished compost is dark brown or black, crumbly, and smells like earth — sweet and fresh, not sour or ammonia-like. You should not be able to recognise the original materials in it, apart from the odd twig or eggshell. A hot bin can produce finished compost in two to three months in summer; a cool bin takes six to twelve months. To test readiness, put a handful in a sealed plastic bag for a few days: if it smells sour or like ammonia when you open the bag, it is not ready yet. Or sow a few fast seeds such as cress in a pot of the compost: if they germinate and grow well, the compost is ready. Use finished compost as a soil improver, a mulch, or an ingredient in mixes: work a 5–8 cm layer into the top 20–30 cm of a bed each year, spread it as a mulch, or mix it into potting mix at about a third of the volume. Do not use unfinished compost on seedlings or in seed trays, because it continues breaking down in the soil and temporarily uses nitrogen. Wear gloves when handling compost and wash your hands before eating — see our garden safety guide.
Common mistakes
The most common mistake is too many greens and not enough browns, which makes a slimy, smelly bin — add dry browns and turn it. The second is a bin too small to heat up; build it at least a cubic metre. The third is never turning the bin, so the middle goes airless. The fourth is letting it dry out, so nothing happens for months. The fifth is adding meat, dairy or pet waste, which attracts rats. The sixth is a bin with no lid, so rain soaks the heap and washes nutrients away. And the seventh is using unfinished compost, which robs nitrogen from plants. A bin that is the right size, tended every few weeks, and fed with the right mix will produce good compost with very little effort.
Frequently asked questions
What size should a compost bin be?
At least a cubic metre — roughly 1 m × 1 m × 1 m — to heat up properly. Smaller bins work, but slowly. Two bins are better than one: one filling, one maturing. If you have a small garden, a plastic bin or a tumbler is a good compromise.
Where should I put a compost bin?
On level, bare soil in a sheltered corner, where worms can enter from below and excess water can drain away. Avoid a spot hard against a fence or wall, which restricts air, and avoid a spot where water pools. Put it where it is easy to reach with a wheelbarrow.
How long does compost take in a bin?
A hot, well-managed bin can produce finished compost in two to three months in summer. A cool bin takes six to twelve months. Turning regularly, keeping the bin damp, and getting the green-to-brown ratio right all speed it up.
Can I compost in winter?
Yes, but the process slows down in cold weather. Keep the bin covered and insulated with old carpet or straw, keep adding material, and turn it less often in winter. It will heat up again in spring. Check your local extension service for winter composting advice in your area.
Do compost bins attract rats?
They can, if they contain meat, dairy or cooked food, or if the bin has gaps rats can enter. Keep meat and dairy out, line wooden bins with fine wire mesh, keep the lid on, and turn the bin regularly. A well-managed bin with the right materials rarely attracts rats.