Tankless Water Heater Sizing Calculator
A tankless (on-demand) water heater heats water only as it is needed, eliminating standby heat loss from a storage tank. To size one correctly, you need two figures: the peak simultaneous flow rate in gallons per minute (GPM) and the required temperature rise from cold inlet to hot delivery. The product BTU/h = GPM x 500 x temperature rise gives the required output capacity. This calculator adds the flows from individual fixtures, computes the temperature rise, and returns both the required GPM and the minimum BTU/h output capacity you need.
Tankless heater sizing formula
Total GPM = (showers × 2.0) + (faucets × 0.75) + (dishwasher × 1.25)
Temperature rise = outlet temp − inlet temp (F)
Required output BTU/h = GPM × 500 × temperature rise
Shower flow of 2.0 GPM reflects the average WaterSense-compliant shower head flow rate. Faucets average 0.75 GPM (WaterSense standard). Dishwashers average about 1.25 GPM at inlet. The constant 500 = 8.33 lb/gal x 60 min/h x 1 BTU/lb-F. Choose a unit with a rated output BTU/h at your temperature rise that meets or exceeds this figure.
Sizing tips for tankless water heaters
- Check manufacturer flow charts: rated GPM decreases as temperature rise increases. A unit rated 5 GPM at 35 F rise may only deliver 3 GPM at 70 F rise.
- Cold climates with groundwater below 50 F often need two units in series or a large whole-house gas unit (150,000 BTU/h or more).
- Electric tankless units require dedicated high-amperage circuits. A 36 kW unit at 240 V draws 150 amps; verify your panel can support this.
- Set delivery temperature to 120 F to reduce scalding risk and meet most plumbing codes. Legionella control may require periodic flushing at 140 F.
- Annual descaling (flushing with dilute white vinegar) extends service life in hard water areas.
Tankless water heater sizing: frequently asked questions
What GPM do I need for a whole-house tankless water heater?
Add the flow rates of all fixtures you expect to run simultaneously. A shower typically uses 1.5 to 2.5 GPM, a bathroom faucet 0.5 to 1 GPM, a dishwasher 1 to 1.5 GPM. A household running two showers and a faucet simultaneously needs about 4 to 6 GPM. Choose a unit rated at or above that flow for your temperature rise.
What is temperature rise and why does it matter?
Temperature rise is the difference between your cold inlet water temperature and the desired hot water delivery temperature. In northern US states, groundwater in winter can be as cold as 37-45 F. To deliver 120 F hot water, you need a temperature rise of 75-83 F. Colder climates need more BTU capacity for the same GPM output.
How do I convert GPM and temperature rise to BTU input?
The formula is BTU/h = GPM x 500 x temperature rise (F). The constant 500 equals 8.33 lb/gal (water density) x 60 min/h x 1 BTU/lb-F (specific heat). For example, 3 GPM at 60 F rise = 3 x 500 x 60 = 90,000 BTU/h required output. Divide by energy factor to get input BTU/h.
What is the typical inlet water temperature in my region?
The DOE and water heater manufacturers use groundwater temperature data to estimate inlet temperature. Northern states (Minnesota, Maine, Montana) average 40-50 F. Mid-Atlantic and Pacific Northwest average 50-60 F. Southern states (Florida, Texas Gulf Coast) average 65-75 F. Lower inlet temperature means a larger required temperature rise.
Do I need gas or electric for a whole-house tankless heater?
Whole-house electric tankless heaters require very high amperage (100-200 amps at 240 V), which may require panel upgrades. Natural gas tankless units are more common for whole-house applications because they can deliver 150,000 to 200,000 BTU/h more economically. Point-of-use electric units work well for a single fixture.
Official sources
- U.S. DOE Energy Saver: Tankless or Demand-Type Water Heaters.
- EPA WaterSense: WaterSense Showerhead Specifications.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.