Size a tank by peak-hour demand or a tankless unit by flow and temperature rise
Updated October 2026
For a storage tank, think of your household’s busiest hour, usually the morning, and count how many showers, washer loads, dishwasher loads, hand-dishwashing or food-prep sessions and shaves happen in it. For a tankless unit, add the flow rates in gallons per minute of every fixture that could run at the same moment, then enter the incoming water temperature (assume 50°F if you do not know it) and the temperature you want, usually 120°F. The results show peak-hour gallons, the first-hour rating to shop for, and for tankless the flow, temperature rise, heat output in BTU per hour and the electric equivalent in kilowatts.
| Use | Gallons of hot water per use |
|---|---|
| Shower | 20 |
| Shaving | 2 |
| Hand dishwashing or food prep | 3 |
| Automatic dishwasher | 7 |
| Clothes washer, top-load | 25 |
| Clothes washer, front-load | 15 |
The first-hour rating, printed in the top left of the EnergyGuide label as “Capacity (first hour rating)”, already accounts for tank size and how fast the burner or element recovers, which is why a 40-gallon gas tank can out-deliver a 50-gallon electric one.
| Busiest hour | First-hour rating needed |
|---|---|
| One shower and a dishwasher load | 27 gallons |
| Two showers, a front-load wash and a dishwasher load | 62 gallons |
| Two showers, a top-load wash and a dishwasher load | 72 gallons |
| Three showers, a front-load wash, hand dishwashing and a shave | 80 gallons |
Tankless units are rated by how much they can heat at a given flow. Required output = gallons per minute × 500 × temperature rise, in BTU per hour; the 500 is 8.33 pounds per gallon times 60 minutes, because one BTU raises one pound of water by 1°F. Worked example: a 2.5 gpm shower and a 0.75 gpm faucet together draw 3.25 gpm. Raising 50°F water to 120°F is a 70°F rise, so the heater must deliver 3.25 × 500 × 70 = 113,750 BTU/h, or 113,750 ÷ 3,412 = 33.3 kW for an electric unit.
| Simultaneous use | 70°F rise | 80°F rise |
|---|---|---|
| One standard shower (2.5 gpm) | 87,500 BTU/h (25.6 kW) | 100,000 BTU/h (29.3 kW) |
| One WaterSense shower (2.0 gpm) and a faucet (0.75 gpm) | 96,250 BTU/h (28.2 kW) | 110,000 BTU/h (32.2 kW) |
| Shower (2.5) and faucet (0.75) | 113,750 BTU/h (33.3 kW) | 130,000 BTU/h (38.1 kW) |
| Two standard showers (5.0 gpm) | 175,000 BTU/h (51.3 kW) | 200,000 BTU/h (58.6 kW) |
DOE notes a typical gas tankless unit gives a 70°F rise at about 5 gallons per minute and an electric unit at about 2. Colder winter groundwater in northern states raises the rise and lowers the flow a given unit can deliver. Standard showerheads use 2.5 gpm and WaterSense-labeled models no more than 2.0 gpm, which lowers the size a tankless unit needs.
Shopping by tank gallons. Two 50-gallon tanks can have very different first-hour ratings. Sizing for an average day instead of the busiest hour leaves you with cold showers on the mornings that matter. Using summer water temperature to size a tankless unit. Picking a whole-house electric tankless unit without checking the electrical panel: at 240 volts, 139 amps of heating load is more than many homes can add. Oversizing a tank, which costs more and loses more heat while standing. Setting the thermostat too high: DOE’s sizing method uses 120°F, and hotter water raises scalding risk.
This calculator sizes capacity only. A licensed plumber or installer should handle gas line sizing, venting, electrical service for electric units, expansion tanks, earthquake strapping where required, and any permits your jurisdiction requires.