Tankless Water Heater Sizing Calculator (GPM and BTU)

How to do it

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   Step 1: Open Tankless Water Heater Sizing Calculator (GPM and BTU) and add your input

   Type your numbers or text, add your file, or pick the options you need. Nothing is uploaded: everything stays in your browser.

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   Step 2: What Tankless Water Heater Sizing Calculator (GPM and BTU) does

   Size a tankless water heater from the flow of the fixtures you use at once and the temperature rise.

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   Step 3: Read the result and compare

   The result appears at once. Change one value to see how the answer moves, then keep the figure that fits your case.

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   Step 4: Copy or save it

   Copy the result or download it. For an important decision, confirm it with the official source or an adviser.

About this tool

Enter how many showers, sinks, dishwashers and washers could run together and the incoming and the desired hot water temperature. The calculator adds the flow, works out the temperature rise and the BTU per hour and kilowatts needed.

Flow

Typical flows: shower 2.5 gpm, sink 1.5 gpm, dishwasher 1.5 gpm, washing machine 2 gpm. Add the fixtures that may run together.

Temperature rise

Temperature rise = desired outlet temperature − incoming water temperature. In a cold climate incoming water may be 40 to 50 °F; for most uses 120 °F is enough.

Heating power

BTU per hour = gallons per minute × 500 × temperature rise in °F. One kW is 3,412 BTU per hour. Compare the result with the rated output at your temperature rise, as gas and electric units lose capacity at high rises.

Worked example

Two showers, a sink and a dishwasher use 8 gpm. With water coming in at 50 °F and leaving at 120 °F (a 70 °F rise) the heater must deliver 280,000 BTU/h, or about 82 kW.

Tips for best results

Check every box before you rely on a result, because one wrong digit changes the whole answer. Use the exact figures from your payslip, bill, certificate or contract, and keep a note of the date you calculated. Rates, thresholds and rules change from time to time, so for tax, fees and legal matters treat the output as an estimate and confirm with the official source named on this page. You can run as many scenarios as you like; nothing is saved or sent, and refreshing the page clears the boxes.