Water Heater Size Calculator
Find the right storage water heater tank size from your household size and hot-water usage level.
How it works
tank gallons = ceil(people × gallons per person) · first-hour rating ≈ tank gallons × 1.6
Storage water heaters are sized to a household’s peak-hour hot-water demand — the busiest hour, like everyone showering before work. Multiplying the number of people by a per-person peak-hour figure (10–15 gallons depending on habits) gives the working tank size. The first-hour rating, roughly 1.6× the tank for a typical unit, is how much hot water it can deliver in that first hour by reheating as it draws down, and is the number to match against your peak demand.
Worked example
A household of 4 with average usage (12 gal/person): 4 × 12 = 48 gallons — about a 48-gallon (182 L) tank, with a first-hour rating near 77 gal/hr.
Frequently asked questions
Should I size by tank size or first-hour rating?
For storage heaters, the first-hour rating matters most — it reflects how much hot water you actually get during a busy morning as the burner or element reheats incoming water. Two 50-gallon tanks with different recovery rates can serve very different peak demands.
What about a tankless water heater?
Tankless units are sized by flow rate (gallons per minute) and temperature rise, not tank volume, since they heat on demand with no storage. This calculator sizes storage tanks; for tankless, add up the GPM of fixtures you would run at once instead.
Is a bigger tank always better?
No. An oversized tank keeps more water hot around the clock, wasting standby energy, and costs more up front. Size to your real peak-hour demand plus a small margin rather than buying the largest tank that fits.
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Estimate for planning. Add a waste margin and confirm with your supplier and local codes.