CalcStack — free, instant, no signup. The interactive calculator loads automatically.
Cold shower math, before it happens to you.
Water heaters fail households at 7 AM, not on the showroom floor — the question is whether the first-hour rating beats your family's peak hour. This calculator totals your peak demand from actual uses, computes recovery rates from burner input and temperature rise, picks the smallest tank that covers the peak for gas or electric, and separately sizes a tankless unit from simultaneous flow and incoming water temperature.
Plumbers use it to size replacements against the family's actual peak hour instead of like-for-like swapping the old tank — and to show customers why electric needs a bigger tank than gas. Homeowners quoted a tankless upgrade use it with winter inlet temps to see whether the unit feeds two showers or one.
Recovery speed. A 40,000 BTU gas burner pours back about 55 gallons per hour at a 70°F rise; a standard 4.5 kW electric element manages about 26. Since first-hour rating is roughly 70% of the tank plus one hour of recovery, the gas unit's burner does the work of 15 extra gallons of storage.
It is the hidden variable in every sizing chart. Heating 40°F groundwater to 120°F is an 80°F rise; the same tankless unit that feeds two showers in Florida feeds barely one in a Minnesota January. Re-run the calculator with your winter inlet temperature — that is the number to size to.
Up to your actual peak — past it, you are paying standby losses to keep water hot nobody uses. A household peaking at 42 gallons gains nothing from an 80-gallon tank over a 50, except a higher gas bill and a shorter floor life from the bigger footprint.
They recover slower than gas but faster than standard electric in efficient modes, and most carry hybrid elements for high-demand hours. Size them like electric with the hybrid element active, then enjoy using about a third of the energy the rest of the year.