What size water heater do I need?
By household size for a tank, by flow rate for tankless — how to get it right.
By The T.A.P. Team · October 2024 · 6 min read
For a tank water heater, size by household: roughly 30–40 gallons for 1–2 people, 40–50 for 3–4, and 50–75+ for larger homes — but the more useful number is the "first-hour rating," which reflects real morning demand. For a tankless heater, you size by flow rate (gallons per minute) for the fixtures you'll run at once, adjusted for our incoming water temperature. Here's how to size either so you never run out of hot water.
- 1–2 people: ~30–40 gal tank
- 3–4 people: ~40–50 gal tank
- 5+ people: ~50–75+ gal tank
- Tankless: size by GPM (fixtures at once)
- Check the "first-hour rating," not just gallons
Sizing a tank heater
Tank capacity is the headline, but two tanks of the same size can deliver different amounts of hot water depending on how fast they reheat. That's what the "first-hour rating" (FHR) captures — how many gallons of hot water the unit can supply in a busy first hour. Match the FHR to your peak demand (back-to-back morning showers plus a dishwasher), not just the gallon size.
Sizing a tankless heater
A tankless heater has unlimited capacity but a limited flow rate — it can only heat so many gallons per minute. Add up the fixtures you'd realistically run at once (a shower is ~2 GPM, a kitchen faucet ~1.5) to get your peak GPM, then choose a unit that delivers that. Compare the approach in tankless vs tank.
Why Texas water temperature matters for tankless
A tankless unit's output depends on how much it has to raise the water's temperature (the "temperature rise"). Our incoming water is warmer than up north for much of the year, which helps a tankless keep up — but in a cold snap the colder inlet water lowers its effective GPM, so size for the worst case, not the average.
Getting it right
Undersize and you run out of hot water; oversize a tank and you pay to keep extra water hot. The sweet spot matches the unit to your household's real peak use. If you're replacing an old unit that always ran out (or never did), that's useful data. See water heater lifespan and cost.
| Household | Tank size (starting point) |
|---|---|
| 1–2 people | 30–40 gallons |
| 2–3 people | 40 gallons |
| 3–4 people | 40–50 gallons |
| 5+ people | 50–75+ gallons |
| Tankless | Sized by peak GPM |
The right water heater is sized for your busiest moment — the weekday-morning rush of showers and appliances — not your average use. For a tank, that means matching the first-hour rating; for a tankless, the peak simultaneous flow rate at winter inlet temperatures. Size to the peak and nobody gets a cold shower; size to the average and someone always does.
Sizing for a North Texas home
Between household size, fixture count, and our seasonal water temperatures, sizing is worth getting right the first time. We size and install tank and tankless heaters across McKinney, Frisco, and Plano, matched to how your household actually uses hot water. See our plumbing team.
The short version
- Tank size by household: ~30–40 gal for 1–2 people, 40–50 for 3–4, 50–75+ for larger.
- The "first-hour rating" matters more than raw gallons for a tank.
- Size a tankless by peak flow rate (GPM) — the fixtures you'd run at once.
- Cold-snap inlet water lowers a tankless's output, so size for the worst case.
- Undersizing means running out; oversizing a tank wastes standby energy.
- Match the unit to your busiest moment, not your average use.
Water heater sizing — frequently asked questions
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