Tankless Water Heater Sizing Calculator

WATER HEATER CALCULATOR

Tankless Water Heater Sizing Calculator

Estimate the flow rate and heating capacity your tankless water heater may need based on simultaneous hot-water use and the required temperature rise.

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1. Select Fixtures Used at the Same Time

Enter the number of each hot-water fixture that could realistically operate simultaneously during peak demand.

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Shower Typical: 2.0 GPM

× GPM

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Bathroom Faucet Typical: 1.0 GPM

× GPM

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Kitchen Faucet Typical: 1.5 GPM

× GPM

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Dishwasher Estimated simultaneous draw: 1.5 GPM

× GPM

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Clothes Washer Estimated hot-water draw: 2.0 GPM

× GPM

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Other Hot-Water Fixture Enter your own flow rate

× GPM

Estimated Simultaneous Flow 5.5 GPM Only include fixtures that could realistically run at the same time.

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2. Determine the Temperature Rise

Tankless capacity falls as the required temperature rise increases.

Incoming Cold-Water Temperature

°F

Use the coldest realistic incoming-water temperature for conservative sizing.

Desired Hot-Water Temperature

°F

Required Temperature Rise 70°F 120°F − 50°F = 70°F

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3. Add a Sizing Margin

A modest margin can help avoid selecting equipment too close to its calculated requirement.

Capacity Margin The final equipment selection should still be based on the manufacturer’s performance chart at your design temperature rise.

YOUR TANKLESS SIZING ESTIMATE

Recommended Performance Target

Recommended Minimum Flow Capacity 6.1 GPM at approximately 70°F temperature rise

Peak Simultaneous Demand 5.5 GPM

Temperature Rise 70°F

Capacity Margin 10%

Approx. Heating Load 192,000 BTU/hr

Your Tankless Sizing Calculation

Peak Flow 5.5 GPM

Temperature Rise 70°F

Approx. Factor 500

Heating Load 192,500 BTU/hr

Peak Flow Breakdown

What to Look for When Shopping

Look for a unit capable of at least 6.1 GPM @ 70°F Rise

Approximate Useful Heating Requirement 192,500 BTU/hr

This is the approximate heat-transfer requirement to raise the calculated water flow through the entered temperature rise. It is not automatically the required gas burner input rating. A gas tankless heater’s input may need to be higher because equipment efficiency and operating conditions must also be considered.

What Does Your Result Mean?

⚠️ Always Verify GPM at Your Required Temperature Rise

Do not choose a tankless water heater based only on the maximum GPM printed in advertising. Maximum advertised flow may be measured at a relatively small temperature rise. As incoming water gets colder and temperature rise increases, the flow rate the heater can deliver generally decreases. Before purchasing a specific model, check the manufacturer’s performance chart and confirm that it can deliver your required GPM at your required temperature rise.

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How Tankless Water Heater Sizing Works

Tankless water heaters are normally sized around two fundamental requirements: how much hot water must flow at the same time and how many degrees that water must be heated. Flow is measured in gallons per minute (GPM), while the difference between incoming cold-water temperature and desired outlet temperature is the temperature rise.

For example, two 2.0-GPM showers operating together create approximately 4.0 GPM of demand. If a 1.5-GPM kitchen faucet operates at the same time, peak demand becomes approximately 5.5 GPM. If incoming water is 50°F and the desired outlet temperature is 120°F, the tankless heater must deliver that flow at approximately a 70°F temperature rise.

Tankless Water Heater BTU Formula

BTU/hr ≈ GPM × Temperature Rise × 500

The factor of approximately 500 combines the weight of water and minutes per hour. This formula is useful for understanding the approximate thermal load, but final equipment selection should be based on manufacturer performance data rather than the formula alone.

Example: Two Showers and a Kitchen Faucet

Suppose two showers rated at 2.0 GPM each and one kitchen faucet rated at 1.5 GPM may operate simultaneously. Peak demand is approximately 5.5 GPM.

If winter incoming water is 50°F and the desired hot-water temperature is 120°F, the required temperature rise is 70°F.

5.5 GPM × 70°F × 500 = 192,500 BTU/hr

You would then review manufacturer performance charts and look for a tankless water heater capable of supplying at least the required flow at approximately a 70°F temperature rise. Adding a reasonable sizing margin can provide additional flexibility.

Bigger Advertised GPM Does Not Always Mean Better Performance

A tankless heater advertised at 9 or 10 GPM may not provide that flow when incoming water is very cold. The number that matters is the unit’s available GPM at your actual design temperature rise. Climate and groundwater temperature can therefore have a major effect on tankless sizing.

Important: This calculator provides an estimated indication for educational purposes only. Actual tankless water-heater performance depends on incoming-water temperature, desired outlet temperature, fixture flow rates, simultaneous demand, equipment efficiency, fuel type, gas supply, electrical capacity, altitude, plumbing conditions, manufacturer controls and installation. For absolute accuracy, verify the exact model’s manufacturer performance chart at the required temperature rise and consult a qualified plumbing or HVAC professional before purchasing or installing equipment.

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