Water Heater Sizing Calculator — Tank & Tankless
Size tank and tankless water heaters by peak hot-water demand, first-hour rating (FHR), and recovery rate so the unit keeps up with simultaneous fixtures.
- List peak-hour fixtures Add the fixtures likely to run at the same time (shower, dishwasher, laundry) and their hot-water GPM. This sets your peak demand.
- For a tank, match first-hour rating Choose a tank whose first-hour rating (FHR) meets or exceeds peak-hour demand — FHR combines storage plus one hour of recovery.
- For tankless, size by GPM and temperature rise Sum simultaneous fixture GPM and find the unit’s flow at your required temperature rise (incoming water up to about 120°F).
- Verify recovery and fuel Confirm the recovery rate (gas recovers faster than electric) and that the gas or electrical service can support the unit before selecting.
What size water heater do I need for a family of 4?
A family of four typically has a peak-hour demand of about 70–80 gallons, so a 50-gallon gas tank (about 70–90 gallon first-hour rating) or a 50–66 gallon electric tank is the common choice. For tankless, plan on 7–10 GPM at a 70°F rise in cold climates so two showers can run at once. Always size from your actual peak-hour fixture use, not just bedroom count.
What size tankless water heater do I need?
Tankless units are sized by simultaneous flow (GPM) at your required temperature rise, not by gallons of storage. A single shower needs about 2.5 GPM; two showers plus a sink is roughly 6–7 GPM. In northern climates, incoming water near 40°F means a 70–80°F rise, which cuts a unit’s rated GPM roughly in half — so size from the cold-weather rating, not the spec-sheet maximum.
Is first-hour rating or recovery rate more important?
First-hour rating (FHR) matters most for households with concentrated peak demand, since it reflects storage plus one hour of heating. Recovery rate matters more for steady, all-day use. The DOE requires the FHR on the EnergyGuide label — match it to your peak-hour demand and you will rarely run out. Verify the final selection against the manufacturer data and local code.