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Water Heater Sizing Calculator
Find the right water heater size for your home — tank size by first-hour rating, or the tankless flow rate (GPM) you need at your temperature rise, based on household size and fixtures.
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Choose tank or tankless, enter your household details, and see the recommended size or flow rate.
📋 Important: All calculator results are ballpark estimates
The figures shown are approximate estimates based on typical averages and should be used for general planning purposes only. They are not a substitute for a professional assessment or written contractor quote. Actual costs, savings, and results will vary significantly based on your specific circumstances, local market conditions, equipment choices, and contractor pricing. Always confirm any estimate with a licensed contractor, financial advisor, or qualified professional before making purchasing or financial decisions.
How to Size a Water Heater
Tank vs. tankless: two different sizing methods
Tank and tankless water heaters are sized in completely different ways. A tank is sized by first-hour rating (FHR) — the gallons of hot water it can deliver in the busiest hour. A tankless unit is sized by flow rate in gallons per minute (GPM) at your required temperature rise, because it heats water on demand with no stored reserve. The calculator above switches between the two methods based on the type you choose.
| Household size | Typical tank size | Typical tankless GPM |
|---|---|---|
| 1–2 people | 30–40 gal | 3–4 GPM |
| 3–4 people | 40–50 gal | 5–7 GPM |
| 5+ people | 66–80 gal | 8–10 GPM |
These are rules of thumb — a home with several bathrooms in simultaneous use may need a larger unit than household size alone suggests.
First-hour rating explained
First-hour rating (FHR) is the single most important number when sizing a tank water heater. It combines the stored volume plus the recovery capacity during one hour of heavy use. Match the FHR to your peak-hour demand — roughly 12 gallons per person during the busiest hour of the day (think back-to-back morning showers plus a dishwasher). A tank's FHR appears on its yellow EnergyGuide label, so you can compare units directly.
Why temperature rise matters for tankless
A tankless heater has a fixed heating capacity, so the colder your incoming water, the fewer gallons per minute it can heat. Temperature rise is the gap between your incoming groundwater temperature and your target output (usually 120°F). A unit rated 6 GPM at a 45°F rise may deliver only 4 GPM at a 70°F rise. Homes in northern climates with ~40°F groundwater need higher-capacity units than warm-climate homes to hit the same flow rate — always check the manufacturer's GPM rating at your specific rise.
Recovery rate and fuel type
Recovery rate is how many gallons a tank can reheat per hour. Gas units generally recover faster than electric units of the same size, which is why a 50-gallon gas tank often has a higher first-hour rating than a 50-gallon electric tank. High recovery lets a smaller tank keep up with steady demand; low recovery means you lean more on the stored volume before running out. A licensed plumber can help you weigh tank size against recovery for your fuel type and usage pattern — the kind of load-out planning tools like Plumbing CRM help contractors document on every job.
Household-size rules of thumb
- Estimate about 12 gallons of peak-hour hot-water demand per person
- More bathrooms increases the odds of simultaneous hot-water use
- For tankless, count the fixtures likely to run at once (each shower ≈ 2 GPM)
- Oversizing a tank wastes standby energy; undersizing means cold showers
- When in doubt, size up slightly for tankless to preserve flow at peak rise
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Book a Free DemoFrequently Asked Questions
What size water heater do I need for my household?
For a tank water heater, size by first-hour rating (FHR), which should meet your peak-hour demand — roughly 12 gallons per person during the busiest hour. A family of 4 typically needs a 50-gallon tank; larger households or homes with multiple simultaneous showers may need 66–80 gallons. For a tankless unit, size by flow rate (GPM) at your required temperature rise instead.
What is first-hour rating (FHR) and why does it matter?
First-hour rating is the number of gallons of hot water a tank can deliver in one hour starting with a full tank. It combines the stored volume plus the recovery (reheat) capacity during that hour. FHR matters more than raw tank size because it reflects real-world peak demand — two households with the same tank size can have very different FHRs depending on the burner or element power.
How do I size a tankless water heater?
Tankless units are sized by flow rate in gallons per minute (GPM) at a specific temperature rise. Add up the GPM of the fixtures you expect to run at the same time (about 2 GPM each for showers), then check the unit's GPM rating at your temperature rise — the difference between your incoming groundwater temperature and your desired output (usually 120°F). Colder incoming water means a bigger rise and lower available GPM.
Why does temperature rise affect a tankless water heater's capacity?
A tankless heater has a fixed amount of heating power. The colder your incoming water, the more energy it takes to raise each gallon to the target temperature, so the unit can heat fewer gallons per minute. A unit rated 6 GPM at a 45°F rise might only deliver 4 GPM at a 70°F rise. Homes in northern climates with ~40°F groundwater need higher-capacity units than warm-climate homes.
What is recovery rate on a water heater?
Recovery rate is how many gallons a tank water heater can reheat per hour. A gas unit typically recovers faster than an electric one of the same size, which is why gas tanks often have a higher first-hour rating. High recovery lets a smaller tank keep up with steady demand, while low recovery means you rely more on the stored volume before running out of hot water.
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Get a Free DemoAll calculations are ballpark estimates based on typical water heater sizing guidelines and should be verified with licensed plumbing contractors in your area before making any purchasing decisions.