How to size a water heater by household (first-hour rating and GPM)
Updated · Reviewed against the current HotWaterAtlas dataset
Short answer: size from your busiest hour, not the tank sticker. For a storage heater, match the first-hour rating to DOE's bands (about 45+ gallons for 1–2 people, 62+ for a family of four, 75+ for five or more). For a tankless, add up the fixtures that genuinely run at once and demand that flow at your winter temperature rise.
Step 1: describe your peak hour honestly
Every sizing mistake starts with a polite fiction about the morning routine. Write down what actually happens in your busiest hour — who showers, whether the dishwasher gets started, whether laundry overlaps. That one hour is the entire sizing problem: the other 23 are easy.
Step 2 (tank path): first-hour rating bands
The DOE bands map household size to the first-hour rating a tank should meet. These are the same thresholds our best-by-household pages filter with:
| Household | Min. first-hour rating | Min. tankless flow (70 °F rise) |
|---|---|---|
| a Family of 2 | 45 gal | 2.5 GPM |
| a Family of 3 | 55 gal | 4.5 GPM |
| a Family of 4 (2 bathrooms) | 62 gal | 5.5 GPM |
| a Family of 5 | 72 gal | 6.5 GPM |
| a Family of 6 or More | 80 gal | 8 GPM |
First-hour rating already includes recovery, which is why it beats tank volume as a sizing number — see 40 vs 50 gallon for how much two same-size tanks can differ. 416 certified models rate 75 gallons or more first-hour for large households.
Step 2 (tankless path): add up simultaneous fixtures
Sum the hot-water flow of everything that genuinely runs at the same time:
| Fixture | Hot-water flow |
|---|---|
| Shower | 2.0 GPM |
| Bathroom sink | 0.5–1.0 GPM |
| Kitchen sink | 1.5 GPM |
| Dishwasher | 1.5 GPM |
| Clothes washer | 2.0 GPM |
| Tub filler | 4.0 GPM |
Two showers plus a bathroom sink ≈ 5 GPM. Then demand that number at your winter temperature rise — a unit's certified GPM is measured at a 77 °F rise, and marketing figures assume far milder conditions. The tankless sizing guide has the climate math, and every tankless model page on this site shows its real flow at four temperature rises.
Step 3: sanity-check the categories
- Undersized tank? The symptom is the last shower going cold on busy mornings, then a long recovery wait. Move up a first-hour band, or pick a heat pump model one nominal size larger — in hybrid mode the backup elements cover surges.
- Undersized tankless? The symptom is everything getting lukewarm when a second fixture opens in January. There is no recovery wait — just a hard flow ceiling.
- Oversized anything? Mostly wasted capital. A much-too-big tank also adds standby losses, though certified models keep those small.
With the threshold in hand, your household's best-water-heater page lists every certified model that clears it, ranked by efficiency and running cost.