Water heater sizing calculator

Household → GPM or first-hour rating → the cheapest heaters that keep up → what they cost to run. A tankless heater is sized by the showers and taps running at once and how cold your water arrives; a tank by your busiest hour. This works out both from your ZIP code.

1. Where is it?

ZIP codeorClimate zoneWinter inlet °FAverage inlet °F

On a private well? Well water stays close to the annual average all year: enter that for both.

2. Your household

PeopleBathroomsShowers running at onceShower head

At the same moment, also: kitchen tap dishwasher or washer filling bathtub filling

More: busiest hour, shower length, setpoint, daily use

Showers in the busiest hourMinutes per showerSetpoint °FHot water a day, gal

3. Fuel, power and rates

Gas at the houseSpare panel ampsHeat pump must plug into 120 V

Electricity $/kWhNatural gas $/thermPropane $/gal

How the sizing works

Inlet water. Your ZIP sets a climate zone, and the zone sets the mains water temperature with the NREL model DOE uses: the annual average air temperature plus 6 °F, swinging colder in winter. A tankless heater is sized for the coldest month.

Tankless. We add up the heat for everything running at the busiest moment: each shower at 105 °F, a kitchen tap at 110 °F, a tub at 104 °F, an appliance fill at the setpoint. A heater fits if its output (input × UEF for gas, kW × 3,412 for electric) covers that, without exceeding its maximum flow.

Tank (heat pump). We add up the busiest hour in gallons of setpoint water, as in the DOE sizing worksheet, and look for a first-hour rating at least that high. FHRs are published for the default hybrid mode; in heat-pump-only mode a tank recovers more slowly.

Running cost. Daily gallons × 8.34 × (setpoint − average inlet) ÷ UEF, at your rates. Heat pump units also cool the room they stand in, a bonus in summer and a small extra heating load in winter if they sit in heated space; that is not counted here. Installation is not included in the ten-year figure: a tankless swap with new venting or gas line often costs more than the heater.