Holding tank fill / empty planner

Check the handbook or the spec sheet for your van. There is often a figure near the fresh filler, or on the maker’s website. Grey is less often printed; the cassette usually has the litres on the tank itself (Thetford-style ones are often about 17–20 L). The numbers on this page are typical UK leisure figures — type your real litres when you know them.

The grey tank is often a bit smaller than the fresh tank, and most of what you use — showers, washing up, laundry — becomes grey water. Drinking and cassette flushes come from the fresh tank but do not go in the grey tank. If grey is the smaller tank, it can still fill first. That is why a wild-camping stretch often ends with “need to empty grey”, not “out of fresh water”.

It depends on tank sizes versus how you use water. Most washing becomes grey, and many UK vans have a slightly smaller grey tank than fresh — so grey fills first. If you drink a lot, flush a lot, or have a large grey tank, fresh can run out first. “Stretch lasts” is whichever happens sooner on your numbers.

No. On this site, drinking and cooking come out of the fresh tank but do not go into grey. Cassette flushes don’t either — they fill the cassette. Showers, washing up and laundry do fill grey. That is why grey can fill before fresh is empty, even when the tanks are a similar size.

A cassette is a removable toilet tank, usually about 15–20 litres on UK leisure vans (Thetford-style). You slide it out and empty it at a service point. A fixed black tank is plumbed in, often much larger, and emptied through a dump valve. Most UK coachbuilt and van conversions use a cassette. This page supports both — pick the one you have and type the litres.

There is no single rule. This page uses the flush litres from Water — a planning figure, not how full the cassette “feels”. A typical 15–20 L cassette on a careful weekend for two can last several days; a busy week may need more empties. Empty at a proper service point when you can, and before it is full. Smell and site rules matter as much as the litre count. The cassette empty planner focuses on empty stops on their own.

This page only plans the onboard tanks. Extra water in an Aquaroll, jerry can or from a site tap does not add to “fresh runs dry” until you pour it in (or use it instead of the tank). It can stretch fresh, but grey and the cassette still fill as you wash and flush. A separate top-up planner will come later.

Look at the “stretch lasts” figure — that is the first tank that stops you. If grey is the limit, use site showers when you can, wash up in a bowl, and empty grey whenever you pass a service point. If fresh is the limit, keep showers short. Leave a day’s margin; real use is messier than a plan.

They can. Rain into a grey tank, a leaky tap, or water you tip away all change the real days. This calculator does not model weather or leaks. Treat the figures as a starting plan and leave a margin.

Use it as a planning estimate, not a promise. A week off-grid only works if your tanks and habits add up to about seven days — and real use is messier (longer showers, extra people, a leak). Leave a day’s margin, and know where you would fill or empty if you need to.

No. This is a planning estimate only. It is not a design certificate, a payload calculation, or a promise that your tanks will last. Check the sizes stamped on your van, and empty grey and cassette waste at proper points — never on the ground.