Choosing the Correct Water Tank

Water tank listings tend to sell on capacity. A 40 litre tank becomes a 60 litre tank, then 75, 100 or more, and it is easy to assume that carrying more water must give you more freedom. It can, but every extra litre needs somewhere to live and adds roughly another kilogram when the tank is full.

The right tank follows from how long you want to spend between refills, where there is actually space to fit it, and how much payload the vehicle can spare. Work through those three things first and the biggest tank that will physically fit is rarely the automatic winner.

Start with how you actually travel

A campervan that spends most nights on campsites has very different water requirements from one regularly parked off-grid for several days.

With easy access to fresh water, carrying a huge reserve makes little sense. You are using payload and space to transport water that could have been topped up at the next stop.

Off-grid travel changes the calculation. If the tank has to cover drinking, cooking, washing up and showering for several people over a few days, additional capacity becomes genuinely useful.

The important distinction is between water you might use and water you routinely need to carry.

If a 100 litre tank is filled before every journey but you normally return with half of it untouched, the extra capacity has achieved little beyond carrying 50kg around the country.

Tank capacity follows people and days, not the size of the van

A large motorhome does not automatically need a large water tank, and a compact campervan can sometimes justify one.

What matters is the number of people using the water and how long they expect to go between refills.

Drinking and cooking use relatively little. Washing up adds more. Showering is usually where consumption climbs quickly, particularly if the water system has a higher-flow pump and conventional shower head.

This also means that increasing the water tank is not the only way to stay off-grid for longer. Managing how quickly the water leaves it can be just as effective.

A couple who use water carefully can sometimes travel longer on a modest tank than a single person with a powerful shower can on a much larger one.

Every litre of water has to be carried

Water weighs approximately one kilogram per litre. Fill a 40 litre tank and you have added around 40kg to the vehicle. Fill a 100 litre tank and it is around 100kg before counting the tank itself, brackets, pipework and fittings.

That matters in any conversion, but particularly where payload is already tight.

Batteries, awnings, gas cylinders, heating systems, furniture, passengers and luggage are all competing for the same available weight. Water needs to be considered alongside them rather than after everything else has been fitted.

There is little advantage in having enough water for a week off-grid if filling the tank pushes the vehicle beyond its permitted weight.

The honest aim is enough capacity for the way the vehicle is used, with some reserve, rather than simply the largest tank that will fit underneath it.

Internal and underslung tanks solve different problems

Once capacity is roughly established, the next question is where to put it.

An internal tank lives within the insulated part of the vehicle, often beneath a seat, bed or inside a cupboard. It is protected from road dirt and debris and, when the living space is being heated, is considerably less vulnerable to freezing.

The price for that protection is interior space.

In a compact campervan, giving up a large section of a bed box to water can mean losing the storage needed for everything else carried on a trip.

An external or underslung tank turns that around. It uses otherwise wasted space beneath the vehicle and leaves cupboards, seats and bed boxes available inside. It can also make larger capacities practical without sacrificing the living area.

The compromise is exposure. The tank, pipework and drain fittings sit underneath the vehicle where temperature, road spray and positioning all matter.

Neither arrangement is inherently better. They simply spend space in different places.

Winter use changes the decision

For a van used mainly from spring through autumn in the UK, an underslung fresh water tank is often a very practical solution. It keeps valuable space free inside and freezing temperatures are less likely to be a regular concern.

Regular winter touring deserves more thought.

An internal tank benefits from the warmth of the living space and is therefore the natural starting point for a genuinely all-season conversion.

External tanks can be insulated, and there are ways of protecting exposed water systems in colder conditions, but it is important to think about the whole installation. Protecting the tank while leaving pipes, valves and fittings exposed simply moves the point at which the water can freeze.

For occasional cold nights, protection may be enough. For sustained sub-zero touring, bringing the fresh water system inside the heated envelope of the van is the more dependable approach.

Fresh and waste water do not have to live together

There is no rule saying both tanks need to be installed in the same way.

In fact, a useful compromise for many conversions is an internal fresh water tank with an external waste tank.

The clean water stays protected inside the vehicle while the grey water uses otherwise unused space underneath. Since waste water is normally emptied regularly rather than stored for long periods, the balance often works well.

Other builds put both tanks underneath because interior storage matters more. Some keep both inside because winter capability takes priority.

The vehicle and the trips should dictate the arrangement rather than convention.

Shape can matter more than capacity

Underneath a van is rarely a large, convenient rectangle waiting for a water tank.

Exhausts, fuel tanks, spare wheels, handbrake cables, chassis members and suspension components all compete for space. Two tanks with the same capacity can therefore be completely different propositions when it comes to fitting them.

A long, shallow tank may disappear neatly alongside a chassis rail where a deeper tank cannot. Vehicle-specific tanks can make use of awkward spaces that a simple rectangular tank would miss altogether.

This is why measuring the available space properly is more useful than deciding on a capacity first and then trying to make the chosen tank fit.

Allow room not just for the tank, but for the brackets and fittings, pipe connections and somewhere sensible for the water to enter and leave it.

The right tank is the one the vehicle can comfortably carry

A campsite-based camper may only need a modest fresh water supply. A couple spending several days off-grid may genuinely benefit from 60, 75 or 100 litres. A winter van may sacrifice some cupboard space to keep that water safely inside, while a summer conversion can put the tank underneath and keep the interior free.

There is no single capacity that makes a good campervan water system.

The best tank carries enough water for the trips you actually make, fits the space available without compromising other components, and leaves enough payload for everything and everyone else travelling in the vehicle.

If you're unsure where to start, measure the space you have available and consider how many people will be travelling and how long you normally want to stay between refills. From those three pieces of information, the sensible tank options usually become much easier to see.