Why square footage isn't the right starting point
A common but misleading shortcut is sizing a water heater off home square footage. What actually matters is peak simultaneous hot water demand — how many fixtures could realistically run hot water at the same time on your busiest morning, not how big the house is.
For tank water heaters: first-hour rating, not just gallons
A tank's "gallons" number describes storage capacity, but the more useful spec is First Hour Rating (FHR) — how much hot water the unit can deliver in a single hour accounting for both stored capacity and recovery rate while it's being used. Two 50-gallon tanks can have meaningfully different FHRs depending on their burner or element wattage.
A practical sizing starting point
- 1–2 people, 1 bathroom: 30–40 gallon tank, or a smaller tankless unit sized to roughly 3–4 GPM peak.
- 3–4 people, 2 bathrooms: 40–50 gallon tank, or a mid-size tankless unit sized to roughly 5–7 GPM peak.
- 5+ people, or a home with a soaking tub or multiple simultaneous showers: 50–75 gallon tank, or a larger tankless unit (or two units) sized to 8+ GPM peak.
For tankless water heaters: GPM, not gallons
Tankless units are rated by gallons-per-minute (GPM) output at a given temperature rise. The key calculation is adding up the flow rate of every fixture that might run simultaneously (shower ≈ 2.0–2.5 GPM, kitchen faucet ≈ 1.5–2.0 GPM, dishwasher ≈ 1.0–1.5 GPM) and matching that combined peak to the unit's rated output — corrected for how cold your incoming water is, since a bigger temperature rise lowers a tankless unit's effective GPM.
Why undersizing is the most common mistake we see
It's far more common to find an undersized unit than an oversized one — usually because a tank was replaced like-for-like with the same size as the original, even though the household's usage grew in the meantime (more people, a finished basement bathroom, a new soaking tub). A replacement is a good moment to re-run the sizing rather than assume the old number is still right.
Frequently asked questions
What size water heater does a 3-bedroom house need?
It depends more on bathroom count and household size than bedroom count, but a typical 3-bedroom, 2-bathroom Federal Heights home with 3–4 occupants usually lands on a 40–50 gallon tank or a mid-size tankless unit rated around 5–7 GPM.
Is it bad to oversize a water heater?
A modestly oversized tank mostly just costs more upfront and slightly more in standby energy loss — it's a much smaller problem than undersizing, which shows up daily as running out of hot water.
How do I know my current water heater is undersized?
The clearest sign is consistently running out of hot water during normal daily use — for example, the second shower in a row running cold, or hot water not lasting through a full load of dishes plus a shower.