Plumbing
Water Heater Size Calculator
Size a water heater from household size and peak demand, for both tank and tankless.
Result
Pipe and drain sizing is governed by plumbing code (IPC/UPC or local equivalent) and must be designed and installed by a licensed plumber. This is an educational estimate, not a design. Tank sizing depends on first-hour rating rather than capacity alone. A well-recovering 40 gallon heater can outperform a slow 50. Incoming water temperature drops sharply in winter, which is when tankless units disappoint if undersized.
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About this calculator
What size water heater for a family of four?
Typically a 50 gallon tank, or 40 with a high recovery rate. What matters is the first-hour rating, not the tank volume on its own.
Why does my tankless run cold in winter?
Because incoming water is colder, so the required temperature rise is larger and the unit's GPM capacity at that rise falls. Tankless units must be sized for winter, not summer.
The formula
tankless BTU/h = GPM × temperature rise × 500
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