Water heater size by household
Want the quick starting point before you do the full peak-hour math? Pick your household size and read the LABELED tank-gallons, First-Hour Rating and tankless-GPM band — then beat it with your real routine.
1 Enter your numbers
For 3–4 people, a 50–60-gallon tank (FHR 60–75) or a 6–8 GPM tankless is the labeled starting point — always beat it with the peak-hour-demand method for your actual routine.
The household-size chart is the answer everyone reaches for first, and it is a fine starting point — but treat it as a bracket, not a verdict. Two households of the same size can differ by a full tank size depending on whether their showers overlap, how long they run and how cold the incoming water is.
This tool gives you the LABELED band for tank gallons, First-Hour Rating and the tankless-GPM equivalent, drawn from the signature size-by-household dataset. Use it to sanity-check a quote, then confirm the exact size with the peak-hour-demand method for your own morning.
Formula
This is a LABELED lookup, not a calculation. Each household bracket maps to a planning band:
household → [gallons band, First-Hour Rating band, tankless GPM band]
For the precise figure, compute peak_hour_demand = Σ(uses × gallons_per_use) and match a First-Hour Rating at or above it. The band tells you the neighborhood; the peak-hour method picks the house.
Worked example
Household of four (bracket 3–4 people):
50–60 gallon tank | FHR 60–75 | 6–8 GPM tankless
That lines up with the peak-hour example: a busy morning of three showers peaks near 75 gallons, landing a 50-gallon tank at the top of the FHR band or a 6–8 GPM tankless. If your four people stagger showers across two hours, drop to the bottom of the band; if you have teenagers who all leave at 7 a.m., size to the top and add headroom.
Measure first, avoid a wrong size
The band is a bracket, not a spec. It assumes an average routine and a middle-of-the-road inlet temperature. Your own peak hour and winter inlet are what pin down the size.
- Same size, different peaks. Overlapping showers, long showers and a cold northern inlet all push you toward the top of the band or a size above it.
- Gas vs electric shifts the gallons. A slow-recovering electric tank often needs the upper end of the band to match a gas unit’s first-hour delivery.
- Tankless reads across to GPM. The GPM column is the flow-equivalent; confirm it against your simultaneous-flow number and temperature rise in the tankless tool.
Reference table
LABELED water-heater size by household — the signature planning dataset. Your selection is highlighted.
| Household | Storage (gal) | First-Hour Rating (gal) | Tankless (GPM) |
|---|---|---|---|
| 1–2 people | 30–40 | 40–55 | 3–5 |
| 2–3 people | 40–50 | 50–65 | 5–7 |
| 3–4 people | 50–60 | 60–75 | 6–8 |
| 5+ people | 60–80 | 75–90 | 8–10 |