Tankless Water Heater Size Calculator: What Size Tankless Water Heater Do I Need?

How to Choose the Right Size Tankless Water Heater

Choosing the correct size tankless water heater is essential for ensuring a constant supply of hot water, maximizing energy efficiency, and maintaining overall comfort in your home. Whether you live alone or with a large family, the right unit size depends on three main factors: your household’s hot water demand (flow rate in gallons per minute), the desired temperature rise (difference between incoming and outgoing water temperature), and the energy source (gas, propane, or electric).

Understanding Flow Rate (GPM)

The flow rate, measured in gallons per minute (GPM), refers to how much hot water you use at a given time. Every fixture in your home that uses hot water contributes to your total flow rate:

• Shower: 1.5–2.5 GPM
• Kitchen sink: 1.0–1.5 GPM
• Bathroom sink: 0.5–1.0 GPM
• Washing machine: 2.0–2.5 GPM
• Dishwasher: 1.5–2.0 GPM

If you plan to run two showers and a dishwasher at the same time, your system needs to handle 2.5 + 2.5 + 2.0 = 7.0 GPM. This is your peak demand, and your tankless water heater must be rated to deliver that volume consistently.

Calculating Temperature Rise

Temperature rise is the difference between the temperature of incoming groundwater and the desired output temperature. If your groundwater is 50°F and you want hot water at 120°F, the temperature rise required is 70°F.

Groundwater temperatures vary by region. In northern states, it may be as low as 35–45°F, while southern states may enjoy groundwater temperatures of 60–70°F. Knowing this value is crucial for proper sizing.

Gas vs Electric Tankless Water Heaters

Gas and propane models generally have higher heating capacities, making them suitable for large households or colder climates. They often range from 140,000 to 199,000 BTU and can support 2–4 bathrooms simultaneously.

Electric models are simpler to install and ideal for smaller homes, apartments, or point-of-use installations. However, they typically max out at 8–10 GPM and are less efficient for whole-home use in cold regions. Sizing them requires calculating kW needed using this formula:

kW = (GPM × 8.34 × ΔT) / 3412

For example, to supply 4 GPM at a 60°F rise, you need about 5.86 kW of power.

Estimating Your Household Demand

Use the calculator above to estimate your needs. Start with the number of people in your home. Then consider typical peak hour usage. For a family of four, this might include two simultaneous showers, the dishwasher running, and someone washing hands. The calculator helps you estimate your GPM requirement based on this.

Other Considerations

• Number of bathrooms: Larger homes with more bathrooms will need a higher capacity unit.
• Simultaneous usage: Consider your lifestyle and whether multiple water outlets are used at once.
• Fuel availability: Check if natural gas or propane is accessible in your area before deciding between electric and gas models.
• Installation location: Cold climates require more powerful units, while warm regions may use lower capacity systems.

Energy Efficiency

Tankless water heaters are generally more efficient than tank models. They only heat water on demand, avoiding standby heat losses. Look for Energy Star-rated models, and consider models with condensing technology for additional energy savings.

A properly sized unit can reduce your energy usage by 25–35% compared to conventional systems.

Pros and Cons of Tankless Systems

Frequently Asked Questions

Can one tankless water heater support a whole house?

Yes, as long as it’s correctly sized. Larger units can provide 8–10+ GPM, enough for 3–4 bathrooms simultaneously.

What’s the average GPM needed for a family of four?

Typically between 6–8 GPM depending on usage patterns and fixture types.

How long do tankless heaters last?

With proper maintenance, up to 20 years. That’s 5–10 years longer than traditional tank water heaters.

Do tankless heaters work well in cold climates?

Yes, but you’ll need a higher BTU or kW rating to overcome the larger temperature rise.

Can I install a tankless heater myself?

Professional installation is highly recommended due to gas, plumbing, and electrical requirements. DIY installations may void the warranty.

Conclusion

Choosing the right size tankless water heater ensures comfort, efficiency, and long-term savings. Use our free calculator above to evaluate your needs based on household size, water usage, and fuel type. Whether you're upgrading an old system or installing a new one, proper sizing is the most important step.