Water Heater Size Calculator
Size a water heater from peak-hour demand rather than guesswork.
How this calculator works
first hour rating = (showers × 20) + other peak hour use
First hour rating, not tank size, is the number that matters — it combines stored hot water with the recovery rate during that hour. A well-recovering 40 gallon gas heater can outperform a 50 gallon electric.
Worked example
With people in the household 4 , showers in the peak hour 2 , other peak hour use 15 gal, this calculator returns 55 gal.
Frequently asked questions
What size water heater do I need for a family of four?
Work out the peak hour, not the daily total. Two showers plus a dishwasher run typically means a 55 gallon first hour rating, which a 40 gallon gas heater delivers comfortably.
What is first hour rating?
The gallons of hot water a heater can supply in one hour starting from full — stored water plus what it reheats during that hour. It is the figure to match against your peak demand.
Is gas or electric better for recovery?
Gas, substantially. A 40 gallon gas heater typically has a higher first hour rating than a 50 gallon electric because it reheats two to three times faster. Electric needs more storage to compensate.
Should I switch to tankless?
Tankless never runs out but is limited by flow rate rather than volume. Size it in gallons per minute at your winter incoming water temperature — a unit that handles two showers in Florida may manage only one in Minnesota.