How to Size a Water Tank

What you need

A tape measure, the tank's dimensions (or the space it must fit), and the water tank calculator. Know what the tank is for before you size it: drinking water, rainwater harvesting, a heating system or a backup supply all size differently, and the purpose decides the capacity.

Step 1 — Work out how much water you need

Start from the use, not the space. A household of four uses about 304 litres of hot water a day (see the hot water guide), and a rainwater tank is sized by the roof area and the rainfall it must catch. Size for the use first — a tank that fits the space but holds half of what you need is a tank that runs dry.

Step 2 — Measure the space

Measure the length, width and height available, and check the access: can the tank get through the door, up the stairs or into the loft? A tank that cannot reach its spot is not a tank — measure the route, not just the destination. Leave room around it for the pipes, the overflow and access for cleaning.

Step 3 — Work out the capacity

The water tank calculator takes rectangular or round tanks. A rectangular tank 2 m × 1 m × 1 m holds 2 m³, which is 2,000 litres (528.34 US gallons); at 50% fill it holds 1,000 litres usable. A round tank 1.2 m across and 1.5 m high holds 1,696.46 litres (448.16 gallons). The fill percentage matters: a tank is rarely full, so size on the usable litres, not the headline capacity.

Step 4 — Check the support

One litre of water weighs one kilogram, so a 2,000 litre tank weighs 2 tonnes when full — plus the weight of the tank itself. The floor or base must be rated for it: a concrete slab usually is, a timber floor or a loft usually is not. Get a structural check for any large tank, and never put a full tank on a floor that was not designed for it.

Step 5 — Plan the plumbing

Plan the inlet, the outlet, the overflow and a drain for cleaning. The overflow must go somewhere safe — a drain or a soakaway, never onto a neighbour's land or against the building's walls. The pipe volume calculator tells you how much water the pipes themselves hold, and the water flow calculator tells you how fast the tank fills and empties.

Worked example

A rectangular tank 2 m × 1 m × 1 m has a capacity of 2 × 1 × 1 = 2 m³, which is 2,000 litres (528.34 US gallons); at 50% fill it holds 1,000 litres usable. A round tank 1.2 m in diameter and 1.5 m high holds 1,696.46 litres (448.16 gallons). Those are exactly the numbers the water tank calculator gives you — and the rectangular tank weighs 2 tonnes when full.

Common mistakes

• Sizing by the space instead of the need. A tank that fits the corner but holds half of what you use runs dry every week. Size by use, then find the space.
• Forgetting the full weight. An empty tank is light; a full one is tonnes. The support must be rated for the full weight, not the empty one.
• No overflow route. A tank without a safe overflow floods the first time it overfills. Plan the overflow before you install the tank.

Safety

A water tank is heavy, and its installation is plumbing work: where it connects to the mains supply, it must be installed by a qualified plumber and meet your local building code — including backflow prevention, so tank water can never flow back into the mains. Check the structural support for the full weight before you fill it, and check your local rules on rainwater harvesting and storage. Never work on a tank above head height without proper access and support.

Try these calculators

• Water Tank Calculator — capacity in litres, gallons and cubic metres.
• Pipe Volume Calculator — water held in any pipe.
• Water Flow Calculator — flow rate from pipe size and velocity.