Water tank & autonomy calculator
Tank size only matters as days between taps. This converts litres into days for your crew and usage style — and checks whether your grey tank, not your fresh one, is what actually ends the trip.
0 if waste goes into a portable canister.
Autonomy
3.4 days
Daily use
24 L
Fresh tank for your target
120 L
Your grey tank fills before the fresh one empties — you'll be hunting for a drain, not a tap.
About 85 % of what goes in comes back out as grey. A grey tank under ~85 % of the fresh one runs out of room before the fresh tank runs dry.
Winter travel: keep tanks and pipework inside the heated envelope. Underslung fresh tanks freeze, and outlets freeze before tanks do.
Planning figures, not certification. Standards are summarised for builders; your inspector, your van's plate and your own measurements always win.
How this is calculated
- Daily use = people × (profile litres + 11 L per extra shower). 11 L is a realistic on-board shower once you're rationing.
- Fresh days = tank ÷ daily use. Grey days = grey tank ÷ (daily use × 0.85), since drinking and cooking losses never reach the waste tank.
- Autonomy is whichever runs out first.
Sources
- FarOutRide — water system guide (shower and daily-use figures)
- Build A Green RV — plumbing, tank placement and freeze protection
Questions builders ask
What size water tank does a campervan need?
Size it in days between taps, not litres. At normal use — 12 L per person per day for cooking, dishes and washing up — two people burn 24 L a day, so a 100 L tank is about four days. Frugal use (6 L) doubles that; a daily on-board shower (25 L) cuts it to two.
How much water does one person use per day in a van?
About 6 L a day frugal, with no on-board shower and dishes done in a bowl; about 12 L a day for normal cooking, dishes and washing up; about 25 L a day once you shower on board daily. One rationed on-board shower is roughly 11 L of that.
How big should a grey tank be compared to the fresh tank?
At least about 85 % of the fresh tank. Roughly 85 % of what goes in comes back out as grey — the rest leaves as drinking and cooking losses — so a smaller grey tank fills before the fresh one empties and you end the trip hunting for a drain rather than a tap.
Keep reading
Plan the whole van, not just this number
VanPlot is a 3D layout planner for camper conversions. Place real components in your real van and watch weight, axle loads, power and cost update live — this calculator is one panel of it.
Who makes this
I'm Vladimir. I'm planning my own conversion, and none of the tools I tried could tell me whether a layout would fit or what it would do to the rear axle, so I made this one. I work on it alone. If something's wrong or missing, write to me — the address goes to my inbox.
Vladimir Šterjoski