How to Calculate Water Tank Size for a House: Simple Formula and Worked Example

A tank that is too small means the taps run dry at the first delay in the network or the water tanker. A tank that is far too big means water sitting for many days, higher construction cost and more sediment at every cleaning. The right capacity is not the biggest possible; it is what covers your real need with a sensible margin.

In this guide we build the calculation on figures published by official sources, show where we assume and where we rely on a source, then solve an example for a typical Saudi family.

What is the formula for household water tank capacity?

The formula is simple, but its accuracy depends on three inputs that you choose: how much the household really uses, how many days it must last without a refill, and what margin suits your home's conditions.

How many litres does one person use per day? (input Q)

Saudi Gazette reported, citing the 2023 water statistics bulletin from the General Authority for Statistics, that household water use per person reached 102 litres per day in 2023, down from 113 litres in 2022, a fall of about 10%.

Internationally, the WHO document on domestic water quantity, service level and health links the amount available to the level of service: basic access is often no more than about 20 litres per person per day, intermediate access about 50 litres, and optimal access through multiple taps is 100 litres or more, the level at which all hygiene needs are met.

If you have a meter and bills, the most accurate method is to calculate your actual use: divide the month's consumption in cubic metres by the number of days and then by the number of people, and multiply by 1,000 to get litres.

How many days should the tank last? (input D)

There is no single correct figure for every home. The number of days depends on how regular supply is in your district: a house on a regular network needs a shorter reserve than one that relies on tankers or sits in a district supplied on an intermittent schedule.

• Regular network: a reserve that covers a short, occasional interruption.
• Weekly or intermittent supply schedule: a reserve covering the gap between two deliveries plus an extra day.
• Reliance on tankers: a reserve covering the usual period between orders and the delivery delay.
• Istiraha or a house used seasonally: a smaller tank, emptied and cleaned before use.

Worked example: a family of 6 in Riyadh

Suppose a family of 6 in a villa connected to the network, with an occasional supply delay that calls for a 3-day reserve, and a 20% safety margin for guests and summer days. We calculate first with the General Authority for Statistics average, then with a higher assumption for comparison.

1. 1

   Set the inputs

   P = 6 people, Q = 102 litres, D = 3 days, S = 0.2.

2. 2

   Work out the family's daily use

   6 × 102 = 612 litres per day.

3. 3

   Multiply by storage days

   612 × 3 = 1,836 litres.

4. 4

   Add the safety margin

   1,836 × 1.2 = about 2,203 litres, or roughly 2.2 cubic metres.

5. 5

   Compare with a higher assumption

   If we assume 150 litres per person (an assumption for comparison, not an official figure): 6 × 150 × 3 × 1.2 = 3,240 litres, or about 3.2 cubic metres.

The practical result is that this family needs a total capacity of roughly 2.2 to 3.2 cubic metres, depending on its usage pattern. You then round up to the nearest commercially available size, taking account of space and the way the tank is installed.

How do you split capacity between the ground tank and the roof tank?

In many Saudi houses the ground tank receives network or tanker water, and a pump then lifts it to a rooftop tank that feeds the house by gravity. The calculation above gives total capacity; the split depends on what the roof can carry, the ground space available and the pump's capability.

Remember that each litre weighs about a kilogram, so a 2,000-litre rooftop tank adds roughly two tonnes of water above the slab. Do not place a large rooftop tank without confirming the slab's structural capacity. To choose the tank material see our comparison of a plastic or concrete water tank, and for its position see the best location for a ground water tank.

Common mistakes when sizing a tank

• Copying the neighbour's tank size without counting your own household and its use.
• Forgetting the housemaid, driver or regular guests in the head count.
• Adding a large safety margin on top of an inflated number of days, so the error is doubled.
• Ignoring that not all of the tank volume is usable: part sits below the outlet and part above the float level.
• Sizing a rooftop tank without checking the slab's capacity for the weight of water.

How tank size affects cleaning frequency and water quality

A tank matched to the household's size turns its water over constantly, so there is less chance of stagnation and sediment builds up more slowly. A huge tank that is always half full collects mud on the floor and needs a deeper clean. Either way, regular tank cleaning and disinfection remains necessary, but a correctly sized tank is easier and quicker to clean.

If the rooftop tank is exposed to summer sun, tank insulation, thermal and waterproofing reduces the rise in water temperature and helps residual chlorine last longer.