Water autonomy: how many days you really get between taps
· 6 min read
“I have a 100 L tank” tells you nothing. Nobody plans a trip in litres — you plan it in days between taps. Litres are the unit you buy; days are the unit you live in, and the conversion between them is one line of arithmetic: days = litres ÷ (people × litres per person per day). Every argument about tank size is really an argument about the denominator. This post is about that denominator, and about the tank that usually runs out first — which is not the one you were watching.
If you are still choosing a capacity and deciding where the tank physically goes, start with the companion piece on tank sizing, placement and axle load. This one assumes the tank exists and asks how long it actually lasts.
Three honest usage profiles
Consumption is not a personality trait, it is a set of habits, and they cluster into three bands. These are the figures the VanPlot calculator runs on, per person per day:
| Profile | L / person / day | What it means in practice |
|---|---|---|
| Frugal | 6 L | No on-board shower. Dishes in a bowl, teeth at the tap outside, wet wipes doing quiet work. |
| Normal | 12 L | Cooking, proper washing-up, hands and faces, the occasional solar-shower rinse. |
| Comfort | 25 L | A shower on board every day, plus everything above. |
The jump from frugal to comfort is a factor of four. That is the entire range of van water planning, and you move through it by changing one habit rather than by changing vans.
The habit is the shower. A realistic on-board shower, taken by somebody who knows the tank is finite, is about 11 L — wet down, pump off, soap, rinse. Notice the internal consistency: normal (12 L) plus one 11 L shower is 23 L, which is essentially the comfort profile. The comfort number is not a different lifestyle, it is the normal lifestyle with a daily shower bolted on. If you are still deciding whether to fit one at all, that arithmetic is half the answer — the rest is in whether a van shower earns its space.
Litres into days
Here is the whole conversion, for the tank sizes people actually build. Fresh water only, assuming you empty the tank to the pump pickup:
| Crew | Fresh tank | Frugal (6 L) | Normal (12 L) | Comfort (25 L) |
|---|---|---|---|---|
| Solo | 60 L | 10.0 days | 5.0 days | 2.4 days |
| Solo | 100 L | 16.6 days | 8.3 days | 4.0 days |
| Solo | 150 L | 25.0 days | 12.5 days | 6.0 days |
| Two | 60 L | 5.0 days | 2.5 days | 1.2 days |
| Two | 100 L | 8.3 days | 4.2 days | 2.0 days |
| Two | 150 L | 12.5 days | 6.3 days | 3.0 days |
| Four | 100 L | 4.2 days | 2.1 days | 1.0 day |
| Four | 150 L | 6.3 days | 3.1 days | 1.5 days |
Read the “Two / 100 L” row twice. On the normal profile that couple gets about 4 days — a comfortable long weekend with a margin. On the comfort profile, the same 100 L tank in the same van gives 2 days, and once you account for grey (below) it is closer to 1.6. Adding one shower each per day to the normal profile takes daily use from 24 L to 46 L and cuts the trip from 4.2 days to 2.2. Nothing about the van changed. You did not buy a smaller tank; you bought a shower habit, and it cost you half your range.
Run it the other way and it gets blunter. To hit a week off-grid, the tank you need is daily use × 7: a couple on the normal profile needs 168 L, which is buildable. The same couple on the comfort profile needs 350 L, which is not — that is 350 kg of water in a van with maybe 800 kg of payload. Past a certain point, autonomy stops being a plumbing problem and becomes a habits problem, and no tank fixes it.
The tank that actually ends the trip
Almost every water guide sizes fresh and treats grey as an afterthought. In practice the grey tank is very often the binding constraint. Roughly 85 % of the fresh water you use comes back out as grey — the missing 15 % is what you drink, what boils away, what leaves with the coffee, and what you spit out at an outside tap. Everything else lands in the grey tank.
That gives a hard threshold. Grey fills before fresh empties whenever grey capacity is under 85 % of fresh. Since nearly every build sizes grey smaller than fresh, nearly every build is grey-limited and most owners have never done the sum. The practical line builders draw is around 70 %, and it is worth knowing what that costs: at 70 % you keep about 82 % of your fresh-tank days. Below that it falls off fast.
Concretely, one couple, normal profile, 100 L fresh — 24 L in per day, 20.4 L of grey out:
| Grey tank | Share of fresh | Days before grey is full | What ends the trip |
|---|---|---|---|
| 85 L | 85 % | 4.2 days | Dead heat — both at once |
| 70 L | 70 % | 3.4 days | Grey, by most of a day |
| 50 L | 50 % | 2.5 days | Grey — you lose 40 % of the trip |
| 30 L | 30 % | 1.5 days | Grey, decisively |
| 20 L canister | 20 % | 1.0 day | You are emptying it nightly |
The reason this matters more in Europe than the arithmetic suggests: you usually cannot legally tip grey water on the ground. On most aires, Stellplätze and sosta networks, grey goes down a designated drain or it does not go anywhere, and a bowl of greasy washing-up water poured into a hedge is exactly the behaviour that gets wild spots closed to vans. So a full grey tank does not mean a mild inconvenience — it means driving to a service point, which is the same as running out of fresh water, except you also still have fresh water and feel stupid about it. Size grey at 70 % of fresh or better, or accept that your real autonomy is the grey number in the table above.
Winter halves everything again
Below zero, autonomy is not limited by capacity but by which parts of the system are still liquid. Two rules do most of the work.
Keep tanks and pipe runs inside the heated envelope. An underslung fresh tank sits in the airstream and freezes while you sleep; a tank under the bed, inside the insulation, is fine as long as the van is heated. This is the one design choice that decides whether you are a three-season or four-season build, and it is free at the layout stage.
It is almost never the tank body that freezes first. A big mass of water has enormous thermal inertia — 100 L takes a long, cold night to drop through zero. What freezes is the thin stuff: the tap, the last 30 cm to the outlet, exposed pipe against a cold wall, the pump head, the shower mixer. You wake to a full tank you cannot reach. Route pipes on the warm side of furniture rather than against the outer skin, and never let a run cross an uninsulated void to save 20 cm of hose.
Then there is the ritual. Fit a low-point drain at the bottom of the run while the build is still open — retrofitting one means pulling furniture. If the van sleeps below freezing unheated, drain everything: tank, pump, both sides of the water heater, and the low points, then leave the taps open so any residue has somewhere to expand. A cracked calorifier is the classic spring discovery, and it costs more than the whole water system did.
Range extenders that beat a bigger tank
Before you specify another 50 L, notice that autonomy is about refill opportunities as much as capacity, and opportunities are cheaper.
A wide, jerry-compatible fill point. A 150 L tank with a narrow inlet assumes you can park beside a tap with a hose. Often you cannot — the tap is in the middle of a village square, or it is a petrol station forecourt, or the pitch is 40 m from the service point. An inlet you can pour a 20 L can into, without a funnel and without a contortion, means every fountain in France is a refill. That converts a fixed tank into a topped-up one, which in practice extends real-world range further than 30 L of extra capacity ever does.
A separate small drinking container. Ten or twenty litres, removable, filled from a source you trust. It decouples the water you are fussy about from the water you wash with, lets you keep using a tank you are no longer certain about, and gives you an honest reserve you cannot accidentally spend on dishes. It also carries into the tent, the café, or the hire car.
The weight you carry for days you never take
One litre is one kilogram, exactly. Autonomy you never use is payload you always carry: the 350 L that would let a couple shower daily for a week is 350 kg driven around for the fifty weeks a year you are not doing that. Size for the trips you actually take, add one day of margin, and put the rest into a wide fill point. Where those kilos sit — how far from the rear axle, how high off the floor — is its own argument, made in full in the tank sizing and placement post.
Check it against your own layout
The useful version of all this is not a table, it is your van. VanPlot totals the fresh and grey tanks you have actually placed in the layout, applies your crew size and usage profile, and reports the number that matters — days of autonomy, and which tank is the one ending the trip. Change the shower from “fitted” to “not fitted” and watch the days move. If you just want the arithmetic without a build, the free water tank calculator runs the same numbers in the browser: put in litres, get back days.