How to use the water heater size calculator
- Choose tankless or tank.
- Tankless: list the hot water fixtures that may run at the same time and their flow rates, then enter the incoming and desired water temperatures and the fuel.
- Tank: enter how many showers, dishwasher and washer loads happen in your busiest hour, such as weekday mornings.
The formulas
Tankless: Flow (gpm) = Sum of fixtures running at once Temperature rise = Hot water temp − Incoming water temp Heat (BTU/h) = gpm × 500 × rise (°F) Gas input = Heat ÷ efficiency Electric kW = Heat ÷ 3,412 Tank: First-hour rating = Hot water used in your busiest hour
Worked example: tankless
The Department of Energy’s example: a faucet (0.75 gpm) and a shower (2.5 gpm) at the same time, with 50 °F water heated to 120 °F:
- Flow = 0.75 + 2.5 = 3.25 gpm (12.3 L/min)
- Rise = 120 − 50 = 70 °F
- Heat = 3.25 × 500 × 70 ≈ 113,800 BTU/h → about 120,000 BTU/h gas input at 95% efficiency
Worked example: tank
Busiest hour: three showers (60 gal), one dishwasher load (7 gal) and one top-loader load (25 gal) = 92 gallons. Look for a tank heater with a first-hour rating of about 92 gallons.
Typical flow rates
| Fixture | Typical flow |
|---|---|
| Bathroom faucet | 0.5–1.5 gpm |
| Kitchen faucet | 1.5–2.2 gpm |
| Shower (WaterSense) | 1.5–2.0 gpm |
| Shower (federal maximum) | 2.5 gpm |
| Bathtub filler | 4–7 gpm |
| Dishwasher / washer | 1–2 gpm |
To measure a fixture, run hot water into a bucket for one minute and measure the gallons.
Groundwater temperature matters
Incoming water is colder in northern states and in winter — often 40 °F or lower — so a tankless heater that easily supplies two showers in Florida may only manage one in Minnesota in January. Size for the coldest month. Raising the hot water setting above 120 °F increases the temperature rise and the scalding risk; 120 °F is enough for most homes.
Tips before you buy
- Read the manufacturer’s chart: tankless heaters list gpm at different temperature rises — use the one for your rise.
- Check venting and gas line size: high-output gas tankless units often need a larger gas line and stainless or PVC venting.
- Consider a heat pump water heater: these tank heaters use far less electricity than standard electric tanks and qualify for many utility rebates.
- Hire a licensed plumber for installation; permits are usually required.
How long do water heaters last?
Tank water heaters typically last around 10–15 years and tankless units 20 years or more with regular descaling, especially in hard-water areas. If your tank is over 10 years old, plan the replacement before it fails — sizing it correctly now avoids running out of hot water or paying to heat water you never use.