Water Heater Size Calculator — What Size Tank Do I Need?
Enter your household size and bathrooms to get the recommended tank gallons and first-hour rating. No sign-up, no data stored.
Tank recommendation
How water heater size is determined
- Peak demand = 10 gal per person (showers) + 8 if dishwasher + 25 if laundry + 20 if a tub, all in the same hour.
- First-hour rating — pick a heater whose FHR meets or beats that peak. Gas tanks typically deliver 30–75 gal in hour one; electric tanks often match their tank size.
- Tank size: 1–2 people → 30–40 gal; 3 people → 40–50; 4 people → 50–60; 5+ → 60–80 (or a tankless).
- Tankless if your peak hour repeats back-to-back or you never want to wait — but it needs big gas lines or high electric draw.
The tank size is a shortcut; the first-hour rating is the truth. A 40-gal gas heater with a 65-gal FHR beats a 50-gal electric with a 50-gal FHR for a busy family.
Household → tank size reference
Standard sizing for conventional tank heaters.
| 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 |
Last updated: October 2026 · Estimates use standard residential usage (10 gal/person peak hour). Big tubs, high-flow showerheads and simultaneous usage push you up a size.
Water heater FAQ
Usually yes — if the model's first-hour rating is 55+ gallons. Without that rating, four back-to-back showers will run the tank cold.
Tank size is the stored water; FHR is how much hot water the heater delivers in the first hour while reheating. FHR = tank + recovery, and it's the number your peak-hour demand must beat.
When you never want to run out, use hot water in bursts all day, or want to free up floor space. Tankless needs a bigger gas supply or a 120-amp electric draw — check your service before falling in love with one.
Heat pumps recover more slowly, so size them by peak demand, not tank volume — a 50-gal heat pump may have a lower FHR than a 40-gal gas tank. They win on energy cost, not on first-hour output.
You get lukewarm showers at peak hour and faster tank wear from constant full-reheat cycling. Sizing up one tank size costs little; sizing down costs comfort.
Related calculators & guides
How to Size a Water Heater
The peak-hour method, FHR explained, and tank vs tankless for every household size.
Read the guide →