What size tankless water heater do I need?

The question people ask is "what size tankless water heater do I need for a family of four", and the honest answer is that the family is barely relevant. What decides it is how many fixtures run at once, and how cold your water is when it arrives.

Here is the method, and then the arithmetic underneath it.

Step 1: add up simultaneous fixture flow#

Not total fixtures. Fixtures running at the same time, on the worst realistic morning.

Be honest and be specific. Most households peak at two showers plus a tap, which is 5.0 to 6.5 GPM. A household that fills a bath while someone showers peaks much higher. A single-bathroom house rarely exceeds 3.0.

Do not add every fixture in the house. Sizing for a scenario that never happens is how people end up buying a 36 kW electric unit and then a service upgrade.

Step 2: find your temperature rise#

Temperature rise = setpoint − incoming groundwater temperature.

Setpoint is almost always 120 °F. Incoming groundwater is regional and it is the number nobody looks up.

Two details matter here. Use the winter figure, not the annual average — groundwater bottoms out in late February, and a heater sized against a June measurement will disappoint in March. And if you are on a well rather than municipal supply, your water is generally a few degrees colder still.

Step 3: convert to capacity#

Now the arithmetic. Both forms come from the same physical relationship: one BTU raises one pound of water one degree Fahrenheit, and a gallon weighs 8.34 pounds.

For gas heaters:

BTU required = GPM × temperature rise × 500

(The 500 is 8.34 lb/gal × 60 min/hr, rounded. It already assumes roughly 83% efficiency, which is typical for a non-condensing unit. For a condensing unit at 94%, use 440 instead.)

For electric heaters:

kW required = (GPM × temperature rise) ÷ 6.83

Electric resistance heating is effectively 100% efficient at the appliance, which is why the electric equation has no efficiency fudge in it.

Worked example: two showers, Ohio#

Two showers at 2.5 GPM is 5.0 GPM. Ohio winter groundwater around 48 °F, setpoint 120 °F, so a 72 °F rise.

• Gas: 5.0 × 72 × 500 = 180,000 BTU
• Electric: (5.0 × 72) ÷ 6.83 = 52.7 kW

That electric figure is the moment most people abandon the electric option. 52.7 kW at 240 V is 220 amps — more than an entire 200 A residential service. The same household in Florida at a 48 °F rise needs 35 kW, which is still 146 amps and still usually a service upgrade. This is the arithmetic behind the electrical requirements for electric tankless units.

The gas figure, meanwhile, is comfortably within a mainstream residential unit.

The sizing chart, if you want the shortcut#

Flow required, for gas heaters at 120 °F setpoint:

The bottom two rows are where residential units run out. Most whole-home gas tankless heaters top out at 199,000 BTU, so a cold-climate house wanting three simultaneous showers is either buying two units or accepting less.

For context, the our APUS tankless water heater review unit is 190,000 BTU — comfortably a two-shower heater in a cold climate, and a genuine three-shower heater in the south.

The two mistakes that cause most complaints#

Sizing on the box number. The GPM printed on the carton is measured at a small rise. Every complaint that begins "it can't keep up in winter" is this mistake. Run the equation with your own rise before you buy.

Ignoring the minimum activation flow. Tankless heaters need a minimum flow before they fire — commonly 0.4 to 0.8 GPM. Below that, the burner will not light and you get cold water. This is why a trickling bathroom tap runs cold on some units, and why a heater with a 0.5 GPM threshold is meaningfully better than one at 0.8 for a house with older, low-pressure plumbing.

Both of these are handled automatically in our tankless sizing calculator, which sizes against your region's groundwater and reports the unit class you need rather than a raw number.