Water Autonomy Calculator
Estimate how many days a van's water system will last — and whether the fresh water tank or the grey/black waste tank runs out first. The maths runs entirely in your browser — nothing you type is sent or stored.
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Methodology
Van water systems have TWO tanks that both limit autonomy, not one: the fresh tank empties while the grey/black tank fills, and whichever hits its limit first ends your autonomy — often the waste tank, since it's usually smaller.
- Daily use (L/day) = occupants × usage profile. Usage profile is the litres/person/day directly: minimal ≈5, moderate ≈10, comfortable (with showering) ≈20 — typical ranges cited in off-grid RV water-usage guidance, not a fixed law of physics.
- Fresh water days = fresh tank (L) ÷ daily use (L/day).
- Waste water days = grey/black tank capacity (L) ÷ (daily use × fraction of fresh water that becomes waste water).
- That fraction (default 85%) is a rule of thumb: not all fresh water consumed becomes stored waste water immediately (some is drunk, cooked with, or used outdoors) — adjust it to your own setup.
- Autonomy shown = the SMALLER of the two — the true limiting factor, not an average.
This calculator gives a planning estimate based on typical usage assumptions, not a measured reading of your specific habits. Real consumption varies widely by shower frequency, dishwashing method, and climate. Track your actual use for a few days to refine the usage-profile figure for your household.
Sources
What this page is based on — every source is verified and links out so you can check it yourself.
- Verified / official
- Provisional
- Out of date