What Size Water Heater Do I Need?
Size a storage (tank) water heater from your peak-hour hot-water demand and match it to a tank whose first-hour rating keeps up.
Calculator
Your peak-hour hot-water demand is about 46 gallons, so aim for a tank whose first-hour rating (FHR) meets or beats it — a 50-gallon class tank here. FHR is not the same as tank volume; check the yellow EnergyGuide label. A licensed installer sizes the final unit.
Storage tanks are not sized by household size alone — they are sized by the peak hour: the single busiest 60 minutes of hot-water use in your day (usually the morning rush). Add up the gallons that get drawn in that hour, then pick a tank whose first-hour rating (FHR) meets or beats it.
Formula
Peak-hour demand is a simple sum of the draws in your busiest hour:
peak-hour demand (gal) = showers × gallons-per-shower + shaving + kitchen
Then choose the next standard tank whose first-hour rating covers it:
recommended tank = smallest standard size (30 / 40 / 50 / 66 / 80 / 100 gal) with FHR ≥ peak-hour demand
First-hour rating is not the same as tank volume. FHR is how many gallons of hot water the unit can deliver in one hour starting full — tank gallons plus what the burner reheats during that hour. It is printed on the yellow EnergyGuide label.
Worked example
A family with a busy morning: 2 showers at 20 gal each, plus 2 gal for shaving and 4 gal at the kitchen sink.
peak-hour demand = 2 × 20 + 2 + 4 = 46 gallons
The smallest standard tank whose first-hour rating clears 46 gallons is the 50-gallon class, so that is the recommendation. A tankless unit is an alternative that never runs out but must be sized on simultaneous flow and temperature rise instead.
Background & practice
Use your real peak hour, not the daily total. A house of four that showers across two hours needs a smaller tank than a house of three that showers back-to-back in twenty minutes. Long, hot, or multiple simultaneous showers push the number up fast — a 20-gallon-per-shower figure assumes an 8–10 minute shower at a roughly 2 GPM head.
Gas tanks recover (reheat) much faster than electric, so a gas 40 can have a higher first-hour rating than an electric 50. If you are borderline, check the recovery rate and lean to the larger unit or a higher-recovery model. A licensed installer confirms the final size, venting and connections.
Reference table
| Standard tank (gal) | Labeled typical household (confirm by FHR) |
|---|---|
| 30 | 1–2 people, single bathroom |
| 40 | 2–3 people, one busy bathroom |
| 50 | 3–4 people, two bathrooms |
| 66 | 4–5 people, heavy morning use |
| 80 | 5–6 people or a large soaking tub |
| 100 | 6+ people / very high simultaneous demand |
Labeled planning typicals — always match the first-hour rating on the EnergyGuide label to your peak-hour demand, not the household size.
Frequently asked questions
What size water heater do I need for a family of 4?
For a typical family of four with a busy morning — about two back-to-back showers plus sink use — peak-hour demand lands near 45–50 gallons, which points to a 50-gallon tank (or a gas 40 with a high first-hour rating). Run your own numbers above; simultaneous showers or a soaking tub push it higher.
Is first-hour rating the same as tank size?
No. Tank size is the stored volume; first-hour rating (FHR) is the stored volume plus what the burner or element reheats during that first hour of heavy draw. Size to FHR, which is printed on the yellow EnergyGuide label.
Should I get a bigger tank to be safe?
Oversizing wastes standby energy keeping water you never use hot, and costs more up front. Size to your real peak hour with a small margin. If you regularly run out, a higher-recovery unit or a tankless heater is often better than a much larger tank.
Tank or tankless?
A tank is cheaper to install and simple; a tankless never runs out but must be sized on simultaneous flow and temperature rise. Compare the two on lifetime cost with the tank vs tankless tool.