Fast Plumbing Answers: How to Size a Tankless Water Heater

You're shopping for a tankless water heater. The efficiency and on-demand hot water benefits are intriguing—but you've always owned tank units, and you're not sure what size tankless is right for your home (or how many you need).

Don't worry: we'll answer all your questions in 2 minutes or less.

At Mother, we install 60+ tankless water heater systems each year for local homeowners. Our team of highly skilled licensed plumbers delivers the single best solution for your household based on daily demand, water usage habits, and appliances.

The tankless size you need depends on your household size and peak usage. A tankless system that delivers 8-10 gallons per minute (GPM) is best for 3-4 person households, while larger families may need 2 tankless units for optimal performance.

Going tankless in Dallas? Call Mother 24/7 for fast service—our priority scheduling means you always get the first available appointment!

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Why We’re Only Talking About Gas Tankless Systems

Important Note: The sizing metrics in this guide refer exclusively to gas tankless water heaters (natural gas or propane). Electric tankless units are far less preferable for whole-house use due to their exceptionally high amperage demands and a poor financial payback rate.

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Sizing Your Tankless: The Gallons Per Minute Rule

Forget the gallon capacity you're used to. When sizing a tankless water heater, you are concerned with delivery speed, not volume. This concept is called Gallons Per Minute (GPM).

Think of it this way: Your old tank was a swimming pool—it held all the water, and once empty, you had to wait. A tankless heater is a powerful firehose that heats the water instantly as it passes through the coil. The GPM rating tells you how much hot water that hose can deliver every single minute while maintaining the set temperature.

The basic math behind sizing is simple: calculate the total GPM needed by all fixtures that might run simultaneously in your home.

Flow Rate: How to Meet Your Home’s Water Demands

Gallons Per Minute (GPM) is the standard measure for all tankless systems. It’s the easiest way for us to understand whether a tankless can satisfy the instantaneous flow rate of all running fixtures. 

Flow rate is the maximum quantity of hot water that the unit can produce simultaneously to meet the demand of every running shower and appliance in your home. The flow rate of your tankless is measured in GPM.

Manufacturers size their products based on the maximum GPM the unit can produce while factoring in your local water temperature (the colder the incoming water, the lower the GPM output).

Here’s an example of the average flow rate of appliances and fixtures in your house:

• Shower: 2.5 GPM
• Dishwasher: 1.5 GPM
• Washing Machine: 2.0 GPM

If you run a shower (2.5) and the dishwasher (1.5) simultaneously, you need a unit rated for at least 4.0 GPM.

The Best Tankless Water Heater Size for Your Home

The correct tankless size hinges on your household's peak usage. This is the maximum number of fixtures (showers, faucets, etc.) that could run at the exact same time without running out of on-demand hot water. 

A single unit is generally sufficient for average-sized families. Larger, busier households may require two smaller units or a single high-capacity model to prevent temperature drops.

TABLE: How to size your tankless water heater by household size and water use

Switching from a water heater tank to a tankless? Learn the pros, cons, and costs in our updated guide- and see why only gas-powered homes should switch.

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Sizing Your Tankless Water Heater By Brand

The GPM rating of a tankless is your primary guide, actual system capacity varies slightly between manufacturers. Here are the tankless models you’ll need to meet your household’s water demands from Rinnai, Rheem, Navien and A.O. Smith:

Note: Some of the models in this section are condensing tankless units. A condensing unit uses a second heat exchanger to capture that waste heat before venting, making it up to 20% more energy efficient. This is common in high-efficiency models.

Rinnai

Rinnai is known for its reliability and wide range of high-efficiency models. Our Responsible Master Plumber Steven Smith says Rinnai is his preferred brand of gas tankless water heater.

• Small (Builder/Utility): V Series
  • Size: 6.5 GPM
• Medium (Standard Residential): RU Series
  • Size: 8.2 GPM
• Large (High Capacity): RUR Series
  • Size: 9.8 GPM
• High Efficiency (Condensing): RUR Series
  • Size: 10.0 GPM

Rheem

• Small (Builder/Utility): Performance Series
  • Size: 7.0 GPM
• Medium (Standard Residential): Performance Plus
  • Size: 8.4 GPM
• Large (High Capacity): ECO200DVELP-3
  • Size: 9.5 GPM
• High Efficiency (Condensing): Prestige Series
  • Size: 11.0 GPM

Navien

• Small (Standard): NPE-180A
  • Size: 6.0 GPM
• Medium (Residential): NPE-210A
  • Size: 8.4 GPM
• Large (High Capacity): NPE-240A
  • Size: 11.2 GPM
• High Efficiency (Premium Condensing): NPE-240A
  • Size: 11.2 GPM

AO Smith

• Small (Standard): AT-Series
  • Size: 6.6 GPM
• Medium (Residential): ATS-Series
  • Size: 8.0 GPM
• Large (High Capacity): AT-Series High-Capacity
  • Size: 9.2 GPM
• High Efficiency (Condensing): ATI-Series
  • Size: 10.0 GPM

Need More Help Sizing Your Tankless?

Choose the size of your gas tankless water heater based on two factors: your household size, and your home’s water demand at peak usage. 

Homes of 3-4 people should choose tankless units between 8-10 gallons per minute (GPM). Households of 5+ people should consider an ultra high-capacity unit (11+ GPM), or install two 7 GPM units to ensure optimal performance throughout the day.

If you live in a large home with multiple shower heads, large laundry and dishwashing demands, ask your licensed plumber for feedback. The best solution for you is likely installing two tankless water heaters.

Dallas: ready to install your tankless? Call Mother 24/7 for priority scheduling- we’re fully stocked with the best tankless systems for DFW homes.

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