In short: Coco coir is the medium most compact composting toilets are designed around — the makers of the Cuddy toilet call it the best compost medium for their unit — because it absorbs several times its own weight in liquid, stays open so air gets through, smells of nothing, and packs flat as a brick. Prepare it damp, not wet (about 15 litres of water to a 5 kg brick, then a squeeze test), use roughly a cup after each use in a bucket-style toilet or fill an agitator-style toilet to just above the paddle, and choose washed, low-salt coir so the finished compost is not salty. One 5 kg brick expands to about 75 litres — around 300 cupfuls, or several months in a van, boat or cabin.
Which kind of composting toilet do you have?
The two families use coir differently.
| Toilet type | Examples | How coir is used |
|---|---|---|
| Urine-diverting with an agitator (solids tank with a handle you turn) | Nature's Head, Air Head, Cuddy, Trobolo and similar compact units for vans, boats and cabins | A charge of damp coir is put in the empty solids tank before use — to just above the agitator paddles — and mixed in with each use. Topped up occasionally; the whole tank is emptied every few weeks. |
| Bucket, "loveable loo" and DIY separating toilets | Home-built plywood boxes, wheelie-bin systems, Kildwick-style separators | A scoop of cover material over the solids after each use. Sawdust is traditional; coir does the same job with less smell and dust. |
CompoCloset, who make the Cuddy, describe both: fill the unit to about an inch above the agitator prongs, or, for layering toilets, use around a cup of medium per use, and empty every two to four weeks. Free Range Designs, who have built compost toilets in Wales for 25 years, recommend a small scoop of cover material after each use and default to sawdust, leaf mould or soil for larger DIY systems. Coir is a drop-in replacement for any of those.
Why coir works (and why washed coir matters)
- Absorbency. Coco coir holds several times its own weight in liquid, which is what keeps the solids tank from going wet and anaerobic — the cause of smells.
- Air. It stays open and spongy rather than compacting, so the aerobic composting a toilet relies on keeps going and the agitator turns easily.
- No smell of its own, no dust. Unlike sawdust it does not carry resinous smells or fine wood dust into a small living space.
- Packs flat. A 5 kg brick is the size of a large loaf and expands to 75 litres — the reason it suits vans and boats.
- Salt. Coir is processed either with fresh water or with brackish water. CompoCloset specifically advise choosing coir manufactured using fresh water, because salt-processed coir carries salt into the finished compost. Ours is washed to an electrical conductivity below 0.5 mS/cm and every batch is tested — what our washing and buffering means.
- No additives. No fertiliser, wetting agents or chemicals — nothing to interfere with the compost or with the garden it ends up in.
How to prepare a coir brick for a composting toilet
- Use less water than for gardening. The growing-medium instruction is 20 litres per brick; for a toilet you want it damp and crumbly, so start with about 15 litres in a large tub or bucket and leave it 20–30 minutes. Expanding a brick.
- Squeeze test. A handful should hold together and feel moist but release no water. If it drips, spread it out for a few hours or mix in some dry coir.
- Break up any lumps so the agitator does not have to.
- Charge the toilet. Agitator units: fill to just above the paddles — typically 8–10 litres. Bucket units: keep a lidded tub of the damp coir beside the toilet with a cup or scoop.
- Store the rest dry. Let unused expanded coir dry out and keep it in a breathable sack or a lidded tub; it keeps indefinitely and re-wets in minutes. If 75 litres is too much to store in a van, a dry brick can be sawn into halves or quarters with an old hand saw before expanding.
How much coir does a composting toilet use?
| Set-up | Typical coir use | One 5 kg brick (≈75 L) lasts |
|---|---|---|
| Bucket toilet, one scoop (≈250 ml) per use | ≈ 1.75 L per person per week | ≈ 300 uses — 10 months for one person, 5 months for two |
| Agitator toilet, two people full-time | ≈ 8–10 L per fill, emptied every 2–4 weeks | 7–9 fills — roughly 4–8 months |
| Agitator toilet, weekend use | One fill lasts many weekends | A year or more |
These are working estimates; diet, ventilation, temperature and how much paper goes in all change the rate. Nature's Head note that composting slows below about 13 °C (55 °F) and stops in freezing conditions, so a winter tank fills faster than a summer one.
Managing the tank
- Too wet, smelly or the handle stiff: the mix needs more dry coir and more air. Add a few handfuls of dry coir and run the fan.
- Too dry (handle hard to turn, nothing breaking down): a splash of water or, as Nature's Head suggest, used coffee grounds add moisture and keep it loose.
- Flies: keep the lid closed, never let urine into the solids tank, and dust the surface with a little dry coir or diatomaceous earth.
- Paper: single-ply breaks down fastest.
What to do with the finished material
What comes out of a compact toilet after a few weeks is partly composted, not finished compost. The accepted practice in the UK is to transfer it to a dedicated, covered compost bin or bag and leave it for at least a further twelve months before using it around trees, hedges and ornamental plantings — not on food crops — and to empty on campsites and marinas only into facilities that permit it. Check the rules of any site you are on. The coir itself breaks down into a dark, crumbly soil improver along with everything else.
Frequently asked questions
Can I use coco coir in a composting toilet?
Yes — it is the medium most compact composting toilets recommend. Prepare it damp rather than wet and use washed, low-salt coir.
Coco coir or sawdust?
Both work. Sawdust is free if you have a source but can be dusty, resinous and variable; coir is consistent, odourless, absorbs more liquid and stores as a brick. Many owners of agitator toilets find sawdust compacts and clogs the paddle, which is why the manufacturers specify coir or a peat-free medium.
Does the coir need to be buffered?
Buffering is a horticultural treatment for plant nutrition; it neither helps nor hinders a toilet. What matters for a toilet is that the coir is washed to a low salt level and has no additives — both true of our bricks.
How wet should the coir be?
Like a wrung-out sponge: moist to the touch, holds together when squeezed, no drips.
Can I compost the contents at home?
In a dedicated bin, for at least a year, and used only around non-edible plantings — the standard UK guidance. Never add it to a general garden compost heap that goes on vegetables.
Is coco peat the same as coco coir?
Yes — the two names describe the same material.
Washed, low-salt coco coir with no additives — 75 litres per 5 kg brick, months of use in a van or cabin, free UK delivery. Three bricks for a season off-grid.
Shop the 5 kg brick → Or the 15 kg 3-pack