How to size a water heater: tank & tankless
Too small and you run out mid-shower; too big and you pay to keep water hot you never use. Here is how to size a water heater right, whether you are going tank or tankless.
Knowing how to size a water heater comes down to one idea: match the heater to your peak demand — the busiest hour of hot-water use in your home, not the average. A tank and a tankless unit answer that demand in completely different ways, so the sizing method is different for each. Get it right and hot water simply never runs out; get it wrong and you feel it every morning. Results here are estimates for guidance, not a substitute for local code or a licensed install.
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Start with your peak-hour demand
Before you pick a heater, estimate how much hot water your household pulls in its busiest hour — think back-to-back morning showers plus a dishwasher. A shower runs about 10 gallons, shaving 2, a dishwasher 6, a load of laundry 7. Add up the worst-case hour; that number drives everything.
Sizing a tank water heater (first-hour rating)
For a tank, the number that matters is not the raw gallon size — it is the first-hour rating (FHR), how many gallons of hot water the tank can deliver in a busy hour starting full. Match the FHR to your peak-hour demand and you will not run dry. As a rough starting point by household size:
| Household | Typical tank size |
|---|---|
| 1–2 people | 30–40 gallons |
| 3–4 people | 40–50 gallons |
| 5+ people | 50–80 gallons |
Always confirm the FHR on the label rather than trusting gallons alone — two 50-gallon tanks can have very different first-hour ratings.
Sizing a tankless water heater (GPM + temperature rise)
A tankless unit heats on demand, so you size it by flow, not storage. Add up the GPM of every fixture that might run at once — a shower is about 2 to 3 GPM, a kitchen faucet about 1.5 — to get your peak simultaneous flow. Then find your temperature rise: the gap between your incoming water temperature and your target (about 120F). A tankless unit is rated to deliver so many GPM at a given rise, and cold-climate homes need a bigger unit for the same flow because the rise is larger. Cross-check flows with the water flow rate calculator, and size the supply lines feeding it with the water line sizing guide. For the underlying heat-and-flow relationships, references like the Engineering ToolBox tabulate the same values.
Tank vs tankless: which to size for
Tanks cost less up front and deliver a big burst easily; tankless costs more but never runs out and saves standby energy. Either way the discipline is the same — size to peak demand. Undersize either one and you will feel it every morning; oversize and you pay for capacity you never touch.
Work your hot-water numbers in seconds
PlumbCalc Pro helps you nail flow, demand, and pipe sizing — free, in your browser.
Open PlumbCalc Pro ▶ Subscribe on YouTubeHow to size a water heater — FAQ
What size water heater do I need?
Match a tank first-hour rating to your peak hour — roughly 30 to 40 gal for 1 to 2 people, 40 to 50 for 3 to 4, 50 to 80 for 5 plus. Tankless is sized by GPM and temperature rise.
How do I size a tankless water heater?
Total the GPM of fixtures that run at once, find the rise from incoming temp to about 120F, and pick a unit that delivers that GPM at that rise.
What is first-hour rating?
How many gallons a tank delivers in a busy hour starting full — match it to your peak demand, not just the gallon size.
Is PlumbCalc Pro free?
Yes — the core calculators are free with no signup; an optional one-time PRO purchase unlocks advanced tools.