RV Tank Duration Calculator
How many days your fresh, grey and black tanks last — and which one actually stops you, which is rarely the one people watch.
Where the water goes
| Use | Gallons a day | Ends up in |
|---|
Water in equals water out
Every gallon you draw from the fresh tank does one of three things: it goes into the grey tank, it goes into the black tank, or somebody drinks it. There is no fourth option, and that identity is the entire model here.
It also explains the surprise most people hit on their first long boondock. Since almost everything you draw comes back as grey, and grey tanks are usually smaller than fresh ones, the tank that fills is the constraint, not the tank that empties. Watching the fresh gauge tells you the wrong thing.
And it is heavy
Water weighs 8.34 pounds a gallon. A 60-gallon fresh tank is 500 pounds, straight out of your payload, and a full grey and black tank on top of that is several hundred more. If you are near your weight limit, filling water at the campground instead of at home is one of the few levers that moves hundreds of pounds at once.
What actually extends a stay
- Water weighs 8.34 pounds a gallon. A 60-gallon fresh tank is 500 pounds, which comes straight out of your payload — and it is why filling up before a climb is a worse idea than filling up after it.
- Grey capacity is the constraint people are least prepared for. Watch the tank that fills, not the one that empties.
- A navy shower — wet down, water off, soap, rinse — is two gallons against nine for a normal one. Nothing else on this page changes the answer as much.
- Wiping dishes before washing them saves more grey water than any low-flow fitting, because the fitting only slows the rate at which you use water you were going to use anyway.
- Catching the cold water while the shower warms up gets you a jug of perfectly good drinking water instead of a gallon of grey.
How to use
- Enter the party size, trip length and your three tank sizes.
- Pick a shower style and set flushes and dishwashing.
- Read which tank runs out first and how many days that gives you.
Frequently asked questions
Which tank runs out first?
Usually grey, and that catches people out. Almost every gallon you draw from fresh comes straight back as grey — only drinking water does not — so a grey tank smaller than the fresh tank fills before the fresh one empties.
How much water does one person use per day?
Boondocking carefully, about five to eight gallons including a short shower. With normal household habits it is easily twenty-five. The shower is the difference: two gallons for a navy shower against nine for a normal one.
Where does the water actually go?
Showers, sinks and dishwashing go to grey. Only the toilet goes to black. Drinking and cooking water leaves in you and never reaches a tank at all, which is why the three numbers never quite add up the way people expect.
How much does a full water tank weigh?
Water is 8.34 pounds a gallon, so a 60-gallon fresh tank is about 500 pounds — a serious bite out of payload. If you are near your weight limit, filling at the campground rather than at home is one of the few levers that moves hundreds of pounds at once.
What is the single best way to stretch a stay?
A navy shower. Wet down, water off, soap, rinse. Two gallons instead of nine, and it saves grey capacity and fresh capacity at the same time. Nothing else on the page comes close.
Can I dump grey water on the ground?
Almost never legally, and never near water or a campsite. Rules vary by land agency and state, so check the specific place rather than assuming — the assumption is what gets people fined.
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