Tank sizing: think first-hour rating, not just gallons
Tanks are sold by nominal gallons (30, 40, 50, 66, 80), but the number that decides whether the third shower goes cold is the first-hour rating (FHR) — how much hot water the unit can deliver in its busiest hour, tank plus recovery. Rule of thumb: FHR should meet or beat your peak-hour demand (~12 gallons per shower, 7 per person for everything else). This calculator maps household size and peak habits onto standard tank sizes accordingly.
Tankless sizing: GPM at YOUR temperature rise
Tankless heaters are rated in gallons per minute at a given temperature rise. A unit advertising "9 GPM" delivers that in Florida (72°F groundwater), but the same unit in Minnesota (47°F in, 120°F out = 73°F rise) may manage barely 4 GPM. Two simultaneous showers need ~5 GPM — check the spec sheet at your actual rise, which this calculator computes from your groundwater temperature.
| Household | Tank | Tankless (mixed climate) |
|---|---|---|
| 1–2 people | 30–40 gal | ~3.5 GPM |
| 3–4 people | 40–50 gal | 5 GPM |
| 5+ people | 50–80 gal | 6–7 GPM or two units |
Frequently asked questions
What size water heater for a family of 4?
A 50-gallon tank (or 40-gallon with a strong first-hour rating) covers most 4-person households. For tankless, look for ~5 GPM at your local temperature rise.
Is a 40 or 50 gallon water heater better?
Pick by peak hour: if two showers plus laundry can overlap, the 50-gallon is worth it. If usage is spread out, a 40-gallon with FHR ≥ 60 gallons is fine and slightly cheaper to run.
What size tankless water heater do I need?
Add the GPM of fixtures that can run at once (shower ~2.5, faucet ~1) and check the unit's rating at your temperature rise — 120°F output minus your groundwater temperature. Cold climates need much bigger units for the same flow.
Are heat pump water heaters worth it?
Usually yes: they use 3–4× less electricity than resistance tanks. Size them like a normal tank (often one size up, e.g. 66 gal) because recovery is slower.