Free, Custom Tankless Water Heater Sizing Calculator

What Size Tankless Water Heater Do I Need?

When determining the right size for a tankless water heater, it’s crucial to match the unit’s capacity with your household’s hot water needs. The best way to do this is by simultaneously calculating the flow rate (in gallons per minute, or GPM) for the appliances you use and ensuring the heater can handle that demand. Our tankless water heater sizing calculator will factor in the GPM required and the temperature rises and recommend a unit that can deliver consistent, energy-efficient hot water on demand.

The calculator considers your peak hot water demand, required temperature rise based on incoming groundwater temperature, and the flow rates of your fixtures and appliances. It then provides a minimum recommended tankless heater size to meet your household’s needs comfortably. Check out our picks for the best tankless water heaters of 2024-2025! We rate electric tankless water heaters, gas tankless water heaters, propane tankless water heaters, and under-counter tankless water heaters!

Factors Affecting Tankless Water Heater Sizing

Several key factors influence the sizing of your tankless water heater:

• Peak demand: Determine the time of day when your home uses the most hot water and how many fixtures or appliances are running simultaneously.
• Flow rates: Calculate the total flow rate in gallons per minute by adding up the flow rates of the fixtures and appliances used during peak demand.
• Temperature rise: Consider the incoming water temperature in your area and the desired output temperature. A greater temperature rise requires a higher heating capacity.
• Household size: Larger households generally have higher peak demands and may require a higher-capacity tankless heater.

Gas vs. Electric Tankless Water Heaters

When selecting a tankless water heater, you must choose between gas and electric models. Gas tankless heaters typically have higher flow rates and can handle larger temperature rises, making them suitable for most whole-house applications. Electric tankless heaters are more compact and energy-efficient but have lower flow rates, which may be sufficient for smaller homes or point-of-use applications.

Frequently Asked Questions

What size tankless water heater do I need for a family of four?

A family of four with two bathrooms typically requires a tankless water heater with a minimum flow rate of 6-8 GPM, depending on the incoming water temperature and simultaneous hot water usage.

Can a tankless water heater provide enough hot water for multiple showers?

A properly sized tankless water heater can provide enough hot water for multiple simultaneous showers. Use our calculator to determine the size based on your peak demand and desired flow rates.

How does the groundwater temperature affect tankless water heater sizing?

Colder incoming water temperatures require a greater rise to reach the desired output temperature. This means homes in colder climates may need a higher capacity tankless heater to achieve the same flow rates as homes in warmer regions. Using our tankless water heater size calculator and understanding the factors influencing sizing, you can confidently select a model that will provide ample hot water for your household while optimizing energy efficiency and performance.