Compost needs two kinds of input: "greens," wet and nitrogen-rich, and "browns," dry and carbon-rich. Coffee grounds are a green, despite their color. Add only greens and the pile turns into a wet, compacted, oxygen-starved mess that smells and takes months longer to break down, if it finishes at all.
What counts as carbon
The most useful browns for a household composter are things most people already throw away:
- Dry leaves, shredded or crumbled so they do not mat into a solid layer
- Cardboard, torn into small pieces, tape and glossy print removed
- Egg cartons and toilet roll tubes, torn up the same way
- Sawdust or wood shavings, from untreated wood only
- Straw or dried garden trimmings, if you have access to them
The ratio to aim for
By volume, aim for roughly two to three parts browns for every one part coffee grounds. In practice: a full mug's worth of used grounds wants a couple of handfuls of shredded leaves or cardboard mixed in at the same time, not sprinkled on top afterward.
You do not need a scale for this. A pile that smells earthy and stays crumbly is balanced. A pile that smells sour or looks like wet sludge needs more browns, immediately, mixed in rather than layered on.
Layer, then mix
Adding ingredients in alternating layers makes it easy to keep the ratio honest as you go, a layer of grounds, a layer of shredded cardboard or leaves, repeat. But layers alone do not compost well, air cannot move through flat, distinct strata. Turn the pile after every couple of additions to fold the layers into each other.
Cardboard is a good browns bank
Cardboard boxes are one of the few browns available year-round in most households, unlike leaves, which are seasonal. Keep a torn-up stash in a dry box near your compost bin so you always have carbon ready the moment you add grounds, rather than scrambling to find some later.
If you have already got a smelly or slimy pile from too much coffee and not enough carbon, see the troubleshooting guide for how to fix it.