Tankless Water Heater Size Calculator

Understanding your result

The headline is the gas input in BTU per hour, or the electric power in kilowatts, with the flow, the rise and the heat output or the current in the sub-line. The stats give the flow, the rise and the gas input or current. The table lists each fixture’s flow, the totals, the heat output, the input, and the equivalent flow at the 77 °F rating rise, which is the number on product listings.

The notes flag the two limits that end most tankless plans: a gas input above the 199,000 BTU per hour residential maximum, which means two units in parallel, and an electric load beyond the largest units or a typical house’s electrical service.

For a tank instead, the water heater tank size calculator sizes by the busiest hour; the daily hot water use calculator gives the gallons that decide the running cost, which the running cost calculator turns into a bill; and the replacement cost calculator includes the gas line and venting for a tank-to-tankless conversion.

How we calculate this

Heating a gallon of water by one degree Fahrenheit takes 8.34 BTU, so heating one gallon per minute takes 500 BTU per hour per degree. The fixture flows are typical: WaterSense shower heads at 2.0 GPM (the federal maximum is 2.5), tub filling 4.0, kitchen faucets 1.5, bathroom faucets 1.0, dishwashers 1.5 and clothes washers 2.0 while filling. The 77 °F rating rise and the GPM rating convention are from the DOE test procedure for instantaneous water heaters. The conversion factors are on the unit conversions page and the UEF ratings on the efficiency ratings table.

The assumptions behind the numbers

Assumptions last reviewed October 8, 2026.

The calculator does not model the minimum flow a unit needs to fire (often 0.25 to 0.5 GPM), the warm-up delay, mixing valves at the tap, recirculation, or the maximum rise a particular model can achieve. The guide to water heater sizing compares tank and tankless sizing, and the guide to water heater running costs compares what each costs to run.

Two worked examples

Where to find your inputs

Fixtures at once. The realistic worst case: two showers in the morning, or a shower and the dishwasher. Not every fixture in the house.

Shower flow. Stamped on the shower head or in its packaging; time how long a bucket of known size takes to fill if unsure.

Groundwater. Run the cold tap for a minute in the coldest month and measure it, or use the regional figures in the hint.

Efficiency. The UEF on the model’s specification: about 0.95 for condensing gas, 0.82 for non-condensing, 0.98 for electric.

Common mistakes

• Using the rated GPM without the rise. Ratings are at 77 °F; in a cold winter your rise may be 80 °F or more.
• Sizing for summer groundwater. Winter is when the unit runs short.
• Adding every fixture. Size for what really runs at once.
• Forgetting the gas line. A 199,000 BTU unit needs a bigger line than a tank’s 40,000.
• Electric tankless in a cold climate. The kilowatts needed usually exceed the house’s service.
• Low-flow taps below the minimum flow. The unit may not fire for a trickle.

Questions people ask

What size tankless water heater do I need for two showers?

Two showers at 2.0 GPM each need 4 GPM of hot water. With 50 °F groundwater and a 120 °F setting the rise is 70 °F, so the unit must deliver 4 × 500 × 70 = 140,000 BTU per hour; at 95% efficiency that is a gas input of about 147,000 BTU per hour, a mid-size condensing unit. In the South, where groundwater is 70 °F, the same two showers need 100,000 BTU per hour of output.

Can an electric tankless heater run a whole house?

In warm climates, sometimes. A shower and a kitchen faucet running together at 3.5 GPM with 70 °F inlet water need about 26 kW, which a large whole-house electric unit can deliver on about 110 A of dedicated circuits. Two showers in a cold climate need about 41 kW, more than the largest units (27 to 36 kW) and more than most homes' electrical service can supply. Electric tankless units suit point-of-use and warm-climate homes best.

What does the GPM rating on a tankless heater mean?

It is the flow the unit can heat by 77 °F under the Department of Energy test procedure. At a smaller rise it delivers more, at a larger rise less, in proportion. A unit rated 5 GPM at a 77 °F rise delivers about 5.5 GPM at a 70 °F rise and about 3.9 GPM at a 100 °F rise. The calculator converts your flow and rise to the equivalent 77 °F flow so you can compare with listings.

How cold is groundwater where I live?

It roughly follows the average annual air temperature, and it falls in winter. In the northern United States, inlet water is about 40 to 45 °F in winter; in the middle of the country 50 to 55 °F; across the South 60 to 75 °F. Size for the coldest month, because that is when the rise is largest and the unit's flow at temperature is lowest. A thermometer in a glass of cold tap water after running it for a minute gives your figure.

Do tankless water heaters need a bigger gas line?

Usually. A whole-house gas tankless unit draws 150,000 to 199,000 BTU per hour at full fire, four or five times a tank heater's burner, which often needs a ¾-inch or 1-inch line from the meter and sometimes a larger meter. They also need dedicated venting, PVC for condensing units or stainless for non-condensing. This is a large part of why converting a tank to tankless costs more than a like-for-like replacement.

Why do I get cold water sandwiches with a tankless heater?

When a tap is turned off and on again within a minute or two, the hot water left in the pipes comes out first, then a slug of cold water while the burner relights, then hot again. It is normal for most tankless units; a small buffer tank or recirculation option reduces it. A unit that is too small for the flow causes a different problem, lukewarm water whenever too many fixtures are open.