Water Heater Sizing Calculator
Size a tank or tankless water heater from peak household demand.
About Water Heater Sizing Calculator
Tankless heaters are sized on instantaneous power for the peak simultaneous flow, while tanks are sized on stored volume plus recovery. Temperature rise is the dominant variable, so a cold winter inlet can halve tankless output. A tankless unit adequate in summer can fail to reach temperature in winter, because the same power produces a smaller rise as the inlet cools.
Formula
Water Heater Sizing
Power in kW = flow (L/min) × ΔT × 4.186 ÷ 60
- 4.186
- Specific heat capacity of water in kJ per kg per kelvin
- first hour rating
- Hot water a tank can deliver in the first hour, including reheat
Worked example
With the default values below, Water Heater Sizing Calculator returns Tankless Power Required of 50.23. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Occupantsminimum 1
- 4
- Fixtures used at onceminimum 1
- 2
- Flow per fixtureminimum 0.1 L/min
- 9 L/min
- Cold inlet temperatureaccepts -5–40 C
- 10 C
- Delivered hot temperatureaccepts 20–90 C
- 50 C
Results
- Tankless Power Required
- 50.23 kW
- Peak Flow
- 18.00 L/min
- Temperature Rise
- 40.0 C
- Equivalent Tank Volume
- 140 L
- First Hour Rating Needed
- 220 L
Step by step
- 1Values used
- 2Water Heater Sizing
- 3Tankless Power Required= 50.23 kW
- 4Peak Flow= 18.00 L/min
- 5Temperature Rise= 40.0 C
- 6Equivalent Tank Volume= 140 L
- 7First Hour Rating Needed= 220 L
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges 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 Water Heater Sizing calculated?
Power in kW = flow (L/min) × ΔT × 4.186 ÷ 60 Tankless heaters are sized on instantaneous power for the peak simultaneous flow, while tanks are sized on stored volume plus recovery. Temperature rise is the dominant variable, so a cold winter inlet can halve tankless output.
Why does Water Heater Sizing matter?
A tankless unit adequate in summer can fail to reach temperature in winter, because the same power produces a smaller rise as the inlet cools.
What values do I need to enter?
This calculator takes 5 inputs: Occupants, Fixtures used at once, Flow per fixture, Cold inlet temperature, Delivered hot temperature. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.