Water Heater Size Calculator

What size water heater do you need?

The right size depends on the type. A storage tank is sized by how much hot water your household draws in its busiest hour — its First-Hour Rating. A tankless unit is sized differently: by the flow rate it can heat at the same time, at the temperature rise your climate demands. This calculator does both.

Sizing a storage tank: First-Hour Rating

The U.S. Department of Energy recommends sizing a tank to your peak-hour demand — the most hot water you use in one hour, usually a weekday morning — then buying a tank whose First-Hour Rating (FHR) is within a gallon or two of it. The FHR is printed on the yellow EnergyGuide label, and it matters more than raw gallons: a fast-recovery gas 40-gallon tank can deliver more hot water in the first hour than a slow-recovery electric 50.

• 1–2 people: often a 30–40 gallon tank.
• 3–4 people: typically 40–50 gallons.
• 5+ people or heavy simultaneous use: 50–80 gallons, or a high-recovery gas unit.

Sizing a tankless unit: GPM and temperature rise

A tankless heater never runs out, but it can only heat so much water at once. Size it on two numbers together: the combined flow rate (gallons per minute) of every fixture that might run simultaneously, and the temperature rise it has to add. A shower is roughly 2 GPM and a faucet about 1.5 GPM, so two showers and a sink is around 5.5 GPM.

Temperature rise is the catch. Incoming groundwater is near 4 °C across the northern US and closer to 21 °C in the south, so a unit that delivers 6 GPM in Florida might only manage 3–4 GPM in Minnesota for the same 49 °C output. Always read the manufacturer's flow-versus-rise chart at your rise, not the single headline GPM.

Frequently asked questions

Is a tankless water heater always better? Not always. Tankless units never run out and save standby energy, but they cost more up front and can bottleneck when several fixtures run at once in cold climates. A well-sized tank is often the simpler, cheaper choice for a household with a clear morning peak.

What temperature should my water heater be set to? 49 °C is the standard recommendation — hot enough to limit bacteria and comfortable for use, while avoiding the scald risk and wasted energy of 60 °C.

Why is my hot water sized by the busiest hour, not the whole day? Because a tank refills and reheats continuously. It only ever falls behind during a short peak — several showers plus the dishwasher on a weekday morning — so that peak hour is what sets the size.

Can one tankless unit run two showers at once? Only if it's rated for the combined flow at your temperature rise. Two 2-GPM showers need ~4 GPM; in a cold-water region that pushes toward the top of a single residential gas unit, and past most electric ones.

These are planning estimates from published DOE guidance. Fixture flow rates and household habits vary — for an unusual layout or a whole-home renovation, confirm with a plumber. See our plumbing guides for more.