Tankless Water Heater Size Calculator

Not sure what size tankless water heater your home needs? This tankless water heater size calculator helps you estimate the right unit size based on your household’s hot water usage and local water temperature. Simply enter the number of devices you expect to run at once (like showers or faucets), your incoming water temperature, and your desired hot water temperature. The tool will recommend a heater size in kilowatts (kW) or BTU/hr to ensure you always have enough hot water — without overspending on capacity.

Number of devices using hot water at once:

Incoming water temperature (°F):

Desired hot water temperature (°F):

How to Use the Tankless Water Heater Size Calculator?

Step 1: Enter the Number of Devices

Start by entering how many hot water devices you expect to use at the same time. This could include showers, dishwashers, washing machines, or faucets. The more devices running simultaneously, the higher the flow rate required. For example, two showers running at once would typically count as two devices.

Step 2: Input Incoming Water Temperature

Next, enter the temperature of the incoming cold water in your area. If you’re not sure, use a typical estimate:

Northern climates: 40–50°F
Southern climates: 60–70°F

This number is important because the colder your incoming water, the more heating your tankless system needs to do.

Step 3: Set Your Desired Hot Water Temperature

Most households prefer a hot water temperature between 110°F and 120°F. Enter your preferred value here. The calculator will determine how much of a “temperature rise” is needed to meet this target.

Step 4: Click “Calculate Size”

Once all fields are filled in, press the Calculate Size button. The tool will display the recommended heater size in kilowatts (kW) and BTU/hr, which are standard measurements used by manufacturers.

Step 5: Use the Result to Choose a Model

With the calculated value, you can compare models of tankless water heaters to find one that meets or exceeds the required output. Choosing the right size ensures your home has enough hot water during peak usage without wasting energy.

Frequently Asked Questions (FAQ) About the Tankless Water Heater Size Calculator

What happens if I choose a tankless water heater that’s too small?

If the unit is undersized, it may struggle to deliver enough hot water when multiple devices are in use. You could experience lukewarm water, reduced pressure, or even system overloads.

Can I oversize my tankless water heater just to be safe?

While a slightly oversized unit can offer peace of mind, too much capacity can be wasteful and more expensive up front. The goal is to match your home’s real demand — that’s what this calculator is designed to do.

What is a good average flow rate per device?

This calculator assumes about 2.0 GPM (gallons per minute) per device, which is standard for modern showers and faucets. If your fixtures are high-flow or low-flow, you can adjust the number of devices accordingly.

What are BTUs and kilowatts? Why are both shown?

BTU/hr (British Thermal Units per hour) and kilowatts (kW) are both used to measure heat output. U.S. heaters often list BTU/hr, while electric models and international units may list kW. This tool provides both for easier comparison.

Can I use this tool for commercial settings?

This calculator is designed for residential use. If you’re sizing a tankless heater for a commercial application (like a salon or gym), you’ll need to calculate a higher flow rate and possibly use a commercial-grade unit.

What if I’m using a well instead of municipal water?

Wells usually have colder water, especially in winter. You may want to assume a lower incoming temperature (e.g., 40°F) when using this calculator if your water comes from a deep well.

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