Hot Water Cylinder Sizing Calculator

Work out hot water cylinder sizing instantly with clear inputs, formula shown and shareable results.

About Hot Water Cylinder Sizing Calculator

Cylinder volume is daily demand per person multiplied by occupancy. Reheat energy is the familiar m x c x ΔT with water at 4.18 kJ/kg·K, converted to kilowatt hours by dividing by 3600. Because stored water at 60 °C is blended down at the tap, the usable volume at 40 °C is larger than the cylinder itself.

Formulas

Reheat energy

kWh = litres x 4.18 x ΔT / 3600

Blended volume

Usable at 40 °C = volume x (store - cold) / (40 - cold)

Worked example

With the default values below, Hot Water Cylinder Sizing Calculator returns Cylinder volume required of 180. Change any input above and every figure here is recalculated in your browser.

Inputs used

Occupantsminimum 1: 4
Hot water per person per dayminimum 5 L: 45 L
Store temperatureminimum 30 °C: 60 °C
Cold feed temperatureminimum 1 °C: 10 °C

Results

Cylinder volume required: 180 L
Energy for full reheat: 10.45 kWh
Coil duty for 90-minute reheat: 6.97 kW
Usable water blended to 40 °C: 300 L

Step by step

1Values used
2Reheat energy
3Blended volume
4Cylinder volume required= 180 L
5Energy for full reheat= 10.45 kWh
6Coil duty for 90-minute reheat= 6.97 kW
7Usable water blended to 40 °C= 300 L

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Hot Water Cylinder Sizing figure worked out?

kWh = litres x 4.18 x ΔT / 3600. Cylinder volume is daily demand per person multiplied by occupancy.

Why store at 60 °C rather than 45 °C?

Storage above 60 °C controls legionella. The water is then blended at outlets by a thermostatic valve so the delivered temperature stays safe to touch.

What values do I need to enter?

This calculator takes 4 inputs: Occupants, Hot water per person per day, Store temperature, Cold feed temperature. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.