How to Correctly Size a Tankless Water Heater: A GPM & Temperature Rise Guide

You know the moment. One person is enjoying a hot shower, someone else starts the dishwasher, and suddenly the shower turns into an unwelcome polar plunge. This frustrating “cold water sandwich” isn’t just a minor annoyance; it’s a classic sign of an undersized tankless water heater. Choosing the right size isn’t about picking the model with the biggest numbers or the best deal. It’s about doing a little bit of science to match the machine to your home’s unique needs.

Forget the confusing jargon. We’re going to break this down into a simple, three-step process. By the end of this guide, you’ll be able to confidently choose a tankless water heater that delivers on its promise of endless hot water, no matter who is using what, and where.

Step 1: Play Detective with Your Fixtures - Calculate Your Peak Flow Rate (GPM)

Before you can pick a heater, you need to understand your home’s “thirst” for hot water. This is measured in Gallons Per Minute (GPM). A higher GPM means more water is flowing. Your goal is to figure out the maximum amount of hot water you might use at the same time. This is your “Peak GPM.”

Think about your busiest morning. Is it possible someone is showering while the kitchen sink is running to rinse dishes? That’s a simultaneous use scenario.

Here’s a reference table to help you add things up. We’ve included both standard fixtures and modern, high-efficiency (HE) or WaterSense-certified ones, as they use significantly less water.

Typical Hot Water Flow Rates (GPM)

How to Calculate Your Peak GPM:

1. List the hot water fixtures you’re likely to use simultaneously.
2. Add their GPM values together.

Example Calculation for a Family of Four:
- One person taking a shower (HE): 2.0 GPM
- Another person washing hands at a bathroom sink (HE): 1.0 GPM
- Running the kitchen faucet to wash vegetables (Standard): 2.2 GPM

Peak Demand = 2.0 + 1.0 + 2.2 = 5.2 GPM

So, this household needs a water heater that can provide at least 5.2 GPM of hot water. Now, you know your home’s “appetite.” But this is only half of the equation.

Step 2: Location, Location, Location - Determine Your Temperature Rise

Here’s the secret that most sizing guides forget: a tankless water heater’s GPM rating is not absolute. Its actual output depends on how hard it has to work. The work it has to do is called the Temperature Rise (or Delta T).

It’s the difference between the temperature of the cold water entering your home and the hot water temperature you want.

Temperature Rise = Desired Hot Water Temp - Incoming Cold Water Temp

A homeowner in Minnesota, where the groundwater in winter can be a frigid 37°F, needs a heater with far more power to reach a comfortable 120°F shower temperature than a homeowner in Florida, where the groundwater might be 72°F.

• Minnesota Rise: 120°F - 37°F = 83°F Rise
• Florida Rise: 120°F - 72°F = 48°F Rise

The Minnesota heater has to work almost twice as hard! To figure out your required temperature rise, you need to know your incoming water temperature. The map below, based on data from the U.S. Geological Survey, shows the average groundwater temperatures across the United States. For the most accurate sizing, find your location and use the coldest temperature you’re likely to experience.

(Imagine a color-coded map of the USA showing groundwater temperatures here)

Once you’ve found your approximate incoming water temperature, just subtract it from your desired output temperature (120°F is a common and safe setting for most homes).

Step 3: The Final Match - Connect GPM and Temperature Rise

You now have the two critical pieces of the puzzle:
1. Your Peak GPM (from Step 1).
2. Your required Temperature Rise (from Step 2).

Now, you can look at the specifications for any tankless water heater and see if it’s the right fit. Manufacturers provide charts that show how a unit’s GPM output changes based on the temperature rise.

Let’s use our example product, the Camplux WA528, which is rated at a maximum of 5.28 GPM. Let’s see how it performs for our example family (requiring 5.2 GPM) in different climates.

A typical performance chart might look like this (this is an illustrative example):

Scenario 1: The Florida Family
- Peak Need: 5.2 GPM
- Temp Rise: 48°F
- Unit’s Performance: At a 45°F rise, the unit provides 4.1 GPM. This is less than their 5.2 GPM peak need. Even in a warm climate, this unit would be slightly undersized for their busiest moments, leading to a drop in temperature or pressure.

Scenario 2: The Minnesota Family
- Peak Need: 5.2 GPM
- Temp Rise: 83°F
- Unit’s Performance: At a 77°F rise, the unit only provides 2.5 GPM. This is drastically undersized. This heater would only be able to run a single shower and nothing else during the winter.

This is why simply looking at the maximum GPM on the box is a recipe for disappointment.

Conclusion: Size Smart, Live Comfortably

Sizing a tankless water heater is a critical step that pays dividends in comfort and satisfaction for years. By following these three steps—calculating peak flow, determining temperature rise, and matching the two—you move from guessing to knowing.

One final piece of advice: It’s often wise to choose a unit with about 10-15% more capacity than your calculated peak. This provides a buffer for extremely cold days, accounts for potential pressure drops, and ensures your system is ready if your family grows or you add a new appliance. This method works for any brand, any model, and any fuel type. Now you have the knowledge to make a truly informed decision.