WATER HEATER SIZE
About this calculator
This water heater sizing calculator returns the recommended residential water heater capacity for either a tank-style heater (rated in gallons) or a tankless heater (rated in GPM at a 70°F temperature rise). The math follows DOE Energy Saver guidelines based on household size, the strongest predictor of peak-hour hot water demand. For tank heaters, the recommendation accounts for First Hour Rating (FHR) — the gallons of hot water available in the busiest hour of the day. For tankless, the GPM target assumes one shower plus a bathroom sink running simultaneously at the household's typical peak. Water heater capacity sizing follows IRC P2801 and ASHRAE 90.1 efficiency tables.
How to use this calculator
Enter the number of people regularly using the home's hot water and pick tank or tankless. Tank heaters store hot water at temperature and are sized in gallons; tankless heaters heat on demand and are sized in GPM (gallons per minute) at a 70°F temperature rise.
The calculator returns both recommendations so you can compare. Tank sizing follows DOE Energy Saver guidelines based on First Hour Rating (FHR) — the gallons of hot water available in the home's busiest hour. Tankless GPM assumes one shower (2.5 GPM) plus a bathroom sink running simultaneously at peak. For very cold inlet water (under 50°F, common in northern winters), bump the tankless GPM recommendation up 20–30%.
Worked example
A 4-person household:
Tank recommendation: 60 gallons. Tankless: 8 GPM at 70°F rise.
Common tank sizes: 30, 40, 50, 65, 75, 80 gal. The 60-gal recommendation rounds to 65-gal tank. Cost: ~$600–$1,200 installed for electric, $900–$1,800 for gas.
Common tankless sizes: 5.5, 6.5, 8.5, 9.5, 11 GPM. The 8 GPM recommendation rounds to 8.5 GPM. Cost: ~$1,500–$3,500 installed for gas tankless, $2,000–$4,500 for electric.
Tankless saves 20–30% on energy if hot water demand is moderate (single shower at a time, no overlapping appliance use). For high-simultaneous-use households (teenagers, multiple showers running, dishwasher + washing machine + shower), tank often wins on simplicity and avoiding "drop-in cold water" complaints when demand exceeds GPM.
Common mistakes & waste factors
Sizing tankless too small. A 6.5 GPM tankless can run a single shower (2.5 GPM) plus a sink (1 GPM) but bogs down on a shower + dishwasher (2.5 + 1.5 = 4) plus a sink. Underspecified tankless is the #1 install regret.
Forgetting cold-climate inlet temperatures. Northern winter inlet water is 40–45°F; sun-belt inlet is 65–70°F. Tankless GPM ratings assume 70°F rise; cold inlet drops effective GPM significantly.
Buying tank for "hot water security" when the home is small. A 1–2 person household with low simultaneous use does great on tankless — and the energy savings pay back the price difference in 8–12 years.
Skipping the recirculation question. Tank heaters with built-in recirculation give instant hot water at distant fixtures. Tankless can do recirculation but needs a dedicated pump and wiring. Important for large homes with the heater far from the master bath.
Rules of thumb
Tank sizing by household: 1–2 people = 40 gal; 3 = 50 gal; 4 = 60 gal; 5 = 75 gal; 6+ = 80 gal.
Tankless GPM by household: 1–2 = 5; 3 = 6; 4 = 8; 5 = 9; 6+ = 11 (at 70°F rise).
Cold inlet (<50°F): bump tankless GPM 20–30%.
Tank life: 8–12 years standard; 12–15 years with annual flushing.
Tankless life: 15–20 years with descaling every 1–2 years in hard-water areas.
Energy savings: tankless saves 20–30% over tank if usage is moderate; less savings or even break-even for very high-use households.
Common questions
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