Tankless Water Heater Size Calculator
Size a tankless water heater by figuring your peak hot-water flow and the temperature rise it has to deliver, so you buy a unit that won't run cold.
What your result means
Peak flow is how many gallons per minute you might draw at once. Temperature rise is the gap between your incoming water and your target output — the colder your groundwater, the bigger the rise and the harder the unit works. Manufacturers rate tankless units as GPM at a given rise, so match both. To estimate hot-water demand from pipe runs, see our pipe volume calculator.
How to use this calculator
- Add up the GPM of fixtures you'd run at the same time (shower ~2, sink ~1.5, tub ~4).
- Enter your incoming groundwater temp (colder up north, warmer down south).
- Set your desired output temp — 120 °F is typical.
- Read the temperature rise and the GPM-at-rise spec to shop for.
- Match a unit's rated GPM at your rise, not its best-case headline number.
The formula
A tankless heater's real capacity drops as the temperature rise climbs. That headline "up to 9 GPM" is at a small rise; at a 70 °F rise the same unit may only do 5 GPM. Always size by GPM at your actual rise, with a little headroom.
Worked example
Say you might run two showers at once for about 5 GPM, your groundwater is 50 °F, and you want 120 °F. The rise is 70 °F. With 10% headroom you want a unit that delivers about 5.5 GPM at a 70 °F rise — a larger gas unit, since electric models struggle at high flow and rise.
Typical fixture flow rates
| Fixture | Approx. GPM |
|---|---|
| Shower | 1.5 - 2.5 |
| Bathroom sink | 0.5 - 1.5 |
| Kitchen sink | 1.5 - 2.2 |
| Bathtub | 4.0 |
| Dishwasher / washer | 1.0 - 2.0 |
Tips & gotchas
- Cold-climate homes need bigger units — a 70 °F+ rise cuts usable flow a lot.
- Gas tankless units handle high flow and rise far better than electric.
- Don't size to one fixture; size to the fixtures you'd realistically run together.
- Hard water scales tankless heaters — plan on a softener or annual descaling.
- Add about 10% headroom so the unit isn't maxed out on the coldest days.
Frequently asked questions
What size tankless water heater do I need?
Size by peak GPM and temperature rise. A typical family wants roughly 5 to 7 GPM at their local rise; colder regions need the higher end and usually a gas unit.
What is temperature rise?
The difference between your incoming groundwater temperature and your target hot-water temperature. A bigger rise means the unit delivers fewer gallons per minute.
Why does my unit deliver less than its rating?
Headline GPM ratings are at a small temperature rise. At a real 60-70 °F rise, usable flow drops, sometimes by half. Always read the GPM-at-rise chart.
Gas or electric tankless?
Gas units handle high flow and high rise better and suit whole-home use. Electric works for warm climates, single fixtures, or point-of-use needs.
Do I need to add headroom?
Yes, about 10%. It keeps the unit from running flat out on the coldest days and covers an extra fixture turning on.
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Estimates only — see our full disclaimer.