How to Size a Water Heater: The Peak-Hour Method
Quick answer: add up the hot water your household uses in its busiest hour (showers + dishwasher + laundry), then pick a heater whose first-hour rating beats that peak. A family of 4 with 2 baths lands around 50–60 gallons.
The quick sizing table
Start here, then refine with the peak-hour method if your household has unusual habits (long showers, a soaking tub, or a heavy laundry morning).
| Household | Bathrooms | Tank size | FHR to look for |
|---|---|---|---|
| 1–2 people | 1 | 30–40 gal | 30+ gal |
| 2–3 people | 1–2 | 40–50 gal | 40+ gal |
| 3–4 people | 2 | 50–60 gal | 50+ gal |
| 5 people | 2–3 | 60–80 gal | 60+ gal |
| 6+ people | 3+ | 80 gal or tankless | 75+ gal |
The peak-hour method
Your heater only fails during the busiest hour of the day, so size for that hour:
- Showers: 10–20 gal each (standard head 10–12, rain head 15–20).
- Dishwasher: 6–10 gal per load.
- Laundry: 20–30 gal per load (front-loaders less).
- Soaking tub: 20–40 gal to fill.
A family of 4 with everyone showering in the morning hour: 4 × 12 = 48 gal, plus the dishwasher at 8 = 56 gal peak. That needs a heater with a 55+ gal FHR.
First-hour rating explained
FHR = the gallons of hot water a heater delivers in the first hour while it reheats. It's tank volume plus recovery. A 40-gal gas tank can have a 65-gal FHR because gas recovers fast; a 50-gal electric tank often has a 50-gal FHR because elements recover slowly. That's why two "same-size" heaters can perform completely differently — compare FHR, not gallons.
Tank vs tankless
- Tank: stores 30–80 gal, reheats between uses. Cheaper upfront, needs a ventilated closet or garage space.
- Tankless: heats on demand — never runs out, saves floor space and ~20–30% energy. But it needs a big gas line or a heavy electric draw (up to 120 amp) and service access.
- Heat pump: the most efficient option, but recovery is slow — size by FHR, and it works best in warm basements or garages.
If your busiest hour repeats back-to-back (kids' sports season, guests), tankless or a big tank with high FHR both work — tankless never runs out, a big tank just gets larger.
Sizing mistakes
- Shopping by tank size alone — FHR is the real spec.
- Oversizing for the guest bath — you pay to reheat water you rarely use.
- Ignoring the recovery rate — electric tanks recover slower than gas at the same size.
- Assuming tankless is always better — the fuel supply decides; check before you commit.
- Sizing for the average day — you run out exactly on the busiest morning.
Get your number
Run the water heater size calculator with your household and peak-hour habits, then compare FHR ratings on the models you like. The one that beats your peak is the one that never makes you take a cold shower.