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Van Fresh & Grey Water Tank Calculator
Work out how much fresh water to carry between refills, and how big a grey water tank to catch what drains away.
Sizing fresh water for your refill cycle
Daily water use per person (drinking & cooking plus washing) is multiplied by the number of occupants to get total daily use, then multiplied by the number of days you plan to go between refills. That gives a fresh water tank size that comfortably covers a full cycle without running dry early.
Estimating grey water capacity
Grey water is whatever goes down the sink or shower drain rather than being drunk or cooked with. This tool applies a grey water ratio to your fresh water total — the share of fresh water you actually expect to end up as waste water — to estimate the grey tank size you'll need to avoid overflowing before your next dump.
Frequently asked questions
Why is grey water tank capacity usually smaller than fresh water?
Not all fresh water used ends up as grey water — some is consumed (drinking, cooking) or evaporates, and many builders size the grey tank a bit smaller on purpose to force more frequent, deliberate dumping rather than carrying extra dead weight.
What's a realistic daily water use figure per person?
Frugal van dwellers often get by on 5–10 liters per person per day for drinking, cooking and quick washing; if you shower inside the van regularly, daily use can easily rise to 20–40 liters per person.
Should I round my tank size up from the calculated number?
Yes — tanks come in fixed sizes, so round up to the nearest size actually available, and leave a little headroom for guests, hot weather, or days when refill access isn't convenient.
Does this account for the weight of a full water tank?
Not directly — water weighs roughly 1 kg per liter, so a 100-liter fresh tank adds about 100 kg when full. Carry that figure into the weight budget calculator so your tank size doesn't push you over your GVWR.