What Size Water Tank for a Campervan? A Sizing Method

Tank size should come from a worksheet, not from the largest moulded tank that fits between the wheel arches. Oversizing adds mass, cost and stale-water risk; undersizing turns every trip into a search for a potable tap.

The sizing formula

Use:

fresh capacity = people × measured litres per person per day × days between reliable fills + reserve

Then apply four limits:

1. usable payload and axle capacity;
2. physical mounting and service access;
3. water turnover and hygiene;
4. matching grey-water and disposal plan.

The Drinking Water Inspectorate advises that a stored-water tank should not be unnecessarily large because water quality can deteriorate when turnover is low. Its tank guidance also stresses approved material, a close-fitting cover, protection from heat and regular inspection.

Measure instead of guessing

Run a realistic one-day test at home or in a campervan rental:

• measure drinking and hot drinks;
• cook the meals you would actually make;
• wash hands and dishes using the intended tap flow;
• include tooth brushing and pet needs;
• time any shower and catch or meter its volume;
• note water wasted while waiting for hot water.

Repeat on two different days. Add the results, not a generic “vanlife” number. If the test is performed at home with a high-flow kitchen tap, restrict it to the planned campervan flow so it reflects the actual system.

Useful starting scenarios

These are design-test allowances, not promises:

A five-minute shower is meaningless as a sizing input without flow. At 3L/min it uses 15L; at 6L/min it uses 30L. Measure the actual shower head and heater at a comfortable setting.

Account for payload and axles

One litre of water weighs about one kilogram. Therefore:

Add the tank, brackets, pump, heater contents and grey water. A full 100L fresh tank plus 50L of grey water represents roughly 150kg of liquid. That can materially change a 3,500kg conversion.

Place the tank between the axles where practical and check the effect on each axle. Use the payload process in the campervan conversion planning guide, then visit a weighbridge after installation and again when the build is loaded for travel.

Do not assume you must drive with the tank full. If reliable potable fills exist near the destination, partial fill can preserve payload. The tank and mounting must still be engineered for the worst case: completely full on a rough stop, corner or emergency manoeuvre.

Internal versus underslung tank

Internal

Advantages:

• easier frost protection inside the heated envelope;
• shorter pump inlet and easier inspection;
• less exposure to debris and road salt.

Costs:

• consumes cabinet or garage volume;
• a leak occurs inside the conversion;
• needs a drained, inspectable containment strategy.

Underslung

Advantages:

• preserves interior storage;
• low location can help centre of gravity;
• exterior drain and fill routing may be simpler.

Costs:

• vulnerable to freezing and road damage;
• brackets, clearance and exhaust/heat sources need careful design;
• servicing sensors and fittings can be unpleasant.

Insulation cannot guarantee an underslung tank will not freeze. If winter travel is real, use the winterisation guide and plan the tank with the heating and insulation system.

Shape, baffles and usable volume

A tank's catalogue capacity is not always usable capacity. Outlet position, vehicle slope and pump pickup can leave water behind. Allow space for fittings, vent rise and access rather than treating the external rectangle as pure storage.

Large, wide tanks allow water to surge. Use a tank intended for mobile use with the maker's baffling and mounting requirements. Secure the tank over a suitable supported area; do not rely on a thin cabinet wall or one strap with no stated rating.

The vent must admit and release air at least as fast as filling and pump draw demand. Route it to resist contamination and water traps. A blocked or undersized vent can deform a tank, cause pump starvation or make the fill spit back.

Fresh and grey capacity together

Build a simple water balance:

fresh used = consumed water + sink grey + shower grey + small losses

Drinking and cooking water do not reach the grey tank, but most washing does. If two people use 40L/day and consume 6L, about 34L may become grey. A 25L grey container would need servicing before the fresh tank is empty.

Grey storage needs a vent, drain, cleaning access and authorised disposal method. Portable containers can be easy to empty but must be secured for driving and handled hygienically. Fixed tanks save lifting but require a usable dump arrangement.

Reserve without permanent overcapacity

A separate 5–10L potable container is often a better emergency reserve than making the installed tank much larger. It remains available if the pump, main tank or plumbing becomes contaminated or fails. Rotate it, keep it cool and clean it using the container maker's method.

For remote trips, identify refill points before departure and confirm they are potable. Natural surface water is not safe merely because it looks clear. Use an appropriate treatment process when source safety is uncertain; do not assume a taste filter makes unknown water drinkable.

Final selection checklist

• measured litres per person per day;
• longest realistic interval between potable fills;
• reserve strategy;
• full water mass and axle effect;
• mobile-use tank and stated mounting method;
• potable-material evidence;
• fill, vent and lowest drain sizes;
• inspection/cleaning access;
• internal or underslung frost plan;
• grey-water capacity and disposal interval;
• level sensor that can be maintained;
• route from fill point to tank without contamination traps.

Choose the smallest capacity that passes all of those checks. Then confirm the pump, heater and pipe system around it in the complete campervan water-system guide.