Tankless water heater size: GPM and BTUs
Add the fixtures that run at the same time and the incoming water temperature. Get the flow, the temperature rise and the gas or electric capacity it takes.
Defaults are the federal maximum flow rates: 2.5 gpm for showerheads (10 CFR 430.32(p)) and 2.2 gpm for lavatory and kitchen faucets (430.32(o)). Many fixtures flow less, so type the rate on the fixture or its box. DOE says to assume 50°F incoming water unless you know otherwise (or hold a thermometer under a cold tap), and that most uses need 120°F. Use the efficiency on the model's spec sheet.
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Why 500: a gallon of water weighs about 8.34 lb and it takes 1 BTU to warm a pound by 1°F, so each gpm warmed 1°F takes 8.34 × 60 min ≈ 500 BTU per hour. 1 kW = 3,412 BTU/h. Match the result to the model's rated flow at your rise, and check gas pipe sizing, venting and the electrical service. An estimate: local code and the AHJ govern.
Size a tankless water heater by flow and temperature rise. Add the flow of fixtures used at once (two 2.5 gpm showers are 5 gpm), then subtract incoming water temperature from the setting: 120°F − 50°F = 70°F. Heat output = gpm × 500 × rise, so 175,000 BTU/h; divide by efficiency for gas input, or by 3,412 for electric: 51 kW.
How to use the tankless water heater size calculator
List fixtures in use at once
How many showers, faucets and other fixtures run together, and each one's flow.
Set the temperatures
Incoming water (50°F unless you know otherwise) and the hot water setting (usually 120°F).
Read the size
Flow in gpm, the rise, heat output, gas input at the unit's efficiency, and electric kW.
What the tankless water heater size calculator does
Federal fixture defaults
Showerheads 2.5 gpm and faucets 2.2 gpm, the maximums in 10 CFR 430.32, all editable.
DOE method
Flow of fixtures used at once plus temperature rise, as DOE's sizing guide describes.
Gas and electric
Gas input in BTU/h at the efficiency you enter, and electric kW with amps at 240 V.
The math shown
Each step in a table, including why the factor is 500.
- Flow
- Sum of fixtures used at the same time, gpm
- Heat output
- gpm × 500 × temperature rise (8.34 lb/gal × 60 min ≈ 500), BTU/h
- Gas input
- Output ÷ thermal efficiency from the spec sheet
- Electric
- Output ÷ 3,412 BTU/h per kW
- Defaults
- 50°F incoming and 120°F setting (DOE); 2.5 gpm shower, 2.2 gpm faucet (10 CFR 430.32)
Tankless Water Heater Size Calculator FAQ
What size tankless water heater do I need?
One that delivers your peak flow at your temperature rise. Add the flow of everything that runs at once, find the rise from incoming water to your setting, then pick a model whose rated flow at that rise meets the total. Two showers at 2.5 gpm with a 70°F rise need 5 gpm.
How many BTUs does a tankless water heater need?
Heat output is gpm × 500 × the rise. For 5 gpm and a 70°F rise that's 175,000 BTU/h of output; a gas unit at 90% efficiency needs about 194,000 BTU/h of input. Check the model's spec sheet for its real efficiency and its flow at your rise.
What incoming water temperature should I use?
Measure it if you can: run a cold tap and hold a thermometer under it. DOE's guide says to assume 50°F unless you know otherwise. Colder water means a bigger rise and less flow from the same unit.
Why 500 in the formula?
A gallon of water weighs about 8.34 lb, and 1 BTU warms 1 lb of water by 1°F. One gallon per minute is 60 gallons an hour, so warming it 1°F takes 8.34 × 60 ≈ 500 BTU per hour.
How big is an electric tankless water heater for a whole house?
kW = gpm × rise × 500 ÷ 3,412. 5 gpm at a 70°F rise is about 51 kW, over 200 A at 240 V. DOE notes a 70°F rise is typically possible at about 2 gpm through electric units, so check the service and panel before quoting one.
Where the numbers come from
- U.S. DOE Energy Saver: Sizing a New Water Heater (archived July 2, 2026; the page has since left energy.gov)
- 10 CFR 430.32(o) and (p): maximum flow of faucets (2.2 gpm at 60 psi) and showerheads (2.5 gpm at 80 psi)
- NIST SP 811, Appendix B.9: Factors for units listed by kind of quantity (heat flow rate)
- NIST Special Publication 811, Guide for the Use of the International System of Units
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