Tankless Water Heater Sizing Calculator

A tankless water heater sizing calculator returns the BTU/hr output a unit needs. It multiplies the simultaneous flow rate in GPM by the temperature rise in °F and by 500.4. For 5 GPM, heating 50°F incoming water to 105°F, the required output is 137,610 BTU/hr. Size for your coldest expected incoming water temperature.

Last reviewed September 2026Method & sources Report an error

Required Output

137,610 BTU/hr

Breakdown
Required Output	137,610 BTU/hr
Temperature Rise	55°F

How to Calculate Tankless Water Heater Sizing

Unlike a storage tank, which is sized by capacity, a tankless water heater is sized by how much it can heat water on demand. The formula is BTU/hr = flow rate (GPM) × temperature rise (°F) × 500.4. The 500.4 constant comes from water's weight, 8.34 lb per gallon, times 60 minutes per hour. That's the standard formula for heating a flowing volume of water. Temperature rise is your desired output temperature minus your incoming (groundwater) temperature. Groundwater temperature varies by region and season.

Example: 5 GPM simultaneous flow requirement, 50°F incoming water

A 5 GPM simultaneous flow requirement, 50°F incoming water, 105°F desired output: 1. Temperature rise: 105 − 50 = 55°F 2. Required output: 5 × 55 × 500.4 = 137,610 BTU/hr

Tankless Water Heater Sizing: Compare Scenarios

Scenario	Required Output	Temperature Rise
Single fixture (2.5 GPM, shower only)	68,805 BTU/hr	55°F
Two fixtures at once (5 GPM)	137,610 BTU/hr	55°F
Whole-house heavy use (10 GPM)	275,220 BTU/hr	55°F

What Is Tankless Water Heater Sizing?

A tankless water heater sizing calculator converts two inputs into the BTU/hr output a tankless unit needs. The first is the flow rate you need to run simultaneously, such as a shower and a sink. The second is the temperature rise, from your cold incoming water to your desired hot water temperature. It is the correct sizing method for tankless and on-demand heaters. Storage tank gallon-capacity sizing works differently.

Tankless Water Heater Sizing: Pros and Cons

Pros

Uses the correct sizing method for tankless and on-demand water heaters: flow rate and temperature rise, not tank capacity, which is what the BTU/hr figure is built on
Directly incorporates incoming water temperature, which varies significantly by region and season and meaningfully changes the required output, so the result reflects your local conditions
Works for any combination of fixtures by letting you set the total simultaneous flow rate you need

Cons

Doesn't tell you what flow rate to plan for. Add up the GPM of each fixture. A shower is commonly 2-2.5 GPM, and a bathroom sink is around 1.5 GPM
Doesn't account for altitude or gas line sizing requirements for a gas tankless unit. Both can affect actual achievable output in specific installations
Incoming groundwater temperature varies by season as well as region. Sizing for your coldest expected incoming temperature, often in winter, avoids underperformance during the time of year you need it most

Tankless Water Heater Sizing: What a Pro Checks

Add up the GPM of every fixture you want to run at the same time, not just one. A shower alone is commonly 2-2.5 GPM, but running it alongside a sink pushes your target flow rate meaningfully higher.
Size for your coldest expected incoming groundwater temperature, usually winter. The same flow rate needs a bigger heater when incoming water is 40°F than when it's 70°F, since colder water needs a larger temperature rise.
Check altitude and gas line sizing requirements separately for a gas unit. Both can limit the actual output a tankless heater achieves beyond what this BTU/hr figure alone predicts.

Frequently Asked Questions

How do I size a tankless water heater?

Multiply your required simultaneous flow rate (GPM) by the temperature rise, which is desired output minus incoming water temperature, and then by 500.4. Running a shower and a sink at once (5 GPM combined) with a 55°F rise needs about 137,610 BTU/hr.

Why does incoming water temperature matter so much?

Because it directly sets the temperature rise the unit has to produce. The same flow rate needs a much bigger heater in a cold climate with 40°F winter groundwater than in a warm climate with 70°F groundwater. Colder water needs a larger temperature increase.

How is tankless sizing different from tank water heater sizing?

A storage tank is sized by capacity, meaning how many gallons it holds, since it heats water in advance and stores it. See the Water Heater Size calculator for that method. It heats water instantly as it flows through. So it's sized by its BTU/hr output relative to your flow rate and temperature rise.

What flow rate should I plan for?

Add up the GPM of every fixture you want to be able to run at the same time. A shower is commonly around 2-2.5 GPM, a bathroom sink about 1.5 GPM, and a dishwasher or washing machine varies by model. Two fixtures at once often means 4-5 GPM combined.

What incoming water temperature should I use?

Use your coldest expected groundwater temperature, typically in winter. Groundwater temperature varies significantly by region. Colder northern climates can see incoming water in the 35-45°F range in winter, while warmer southern climates might stay above 65-70°F year-round. Sizing for the coldest season avoids underperformance.

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