Water heater size calculator
Hot water is used mixed at around 40 °C, but the tank keeps it at about 60 °C — so a litre from the tank gives about 1.7 litres at the shower. The calculator takes that into account and picks the first standard tank size above the need.
Your measurements
Result
- Daily need (water at ~40 °C)
- 160 L
- From the tank, at 60 °C
- 96 L
- Recommended tank size
- 100 L
The first standard size above the hot water drawn from the tank; if the whole family showers in the same hour, go one size up.
How the calculation works
- Daily need = number of people × litres of hot water per person (at about 40 °C, shower temperature).
- Tank water at 60 °C is mixed with cold water at about 10 °C: need × (40 − 10) ÷ (60 − 10).
- Recommended size = the first standard capacity (50, 80, 100, 120, 150, 200 or 300 L) above that volume.
Worked example
Four people using 40 litres each per day use 160 litres of water at 40 °C. From a 60 °C tank that is 160 × 30 ÷ 50 = 96 litres, so a 100-litre water heater.
What the calculator assumes
- 40 litres per person is one ordinary shower a day; filling a bath takes 100–150 litres on its own.
- The tank reheats between uses; if the whole family showers in the same hour, go one size up.
When to adjust the result
- Setting the tank above 60 °C increases the reserve but also the losses; below 55 °C the risk of Legionella bacteria rises.
- Energy use depends on the hot water you heat, not on the tank size — an oversized tank just loses more heat.
Choosing a water-heater size
Size a cylinder on how many people shower in a row and how big the showers are: a powerful rain shower can use twice the water of an economy head. Too small means cold showers at peak times; too large means higher standing losses.
Set the temperature around 50–60°C and run a periodic higher-temperature cycle where recommended to control legionella. Check the anode every few years in hard-water areas, and consider a heat-pump water heater or a timer for off-peak or solar hours.
Frequently asked questions
What size water heater for a family of four?
Around 80–100 litres for showers only; more if you use a bath or everyone showers at once.
What temperature should a water heater be set to?
Around 60 °C is a common recommendation: hot enough to limit Legionella, without excessive losses.
How big a cylinder for four people?
Often around 150–200 litres for a family with standard showers; more with large showers or a bath used daily.
Tank or instantaneous water heater?
Instantaneous heaters avoid standing losses but need a high-power connection; tanks work with smaller connections and can store cheap energy.