Tankless Water Heater Sizing: GPM, Flow Rate & Temperature Rise Explained

Let’s be honest — figuring out the correct size for a tankless water heater isn’t as straightforward as it sounds. Unlike traditional models with big tanks that give you a visual cue, tank-free systems rely entirely on specs and numbers. So, how do you know what capacity is right for your home? What GPM rating do you really need?

If you go too BIG, you’re overspending on something you don’t need. Too SMALL, and you’ll be dealing with cold showers and frustrated family members during peak usage times.

I’ve helped countless homeowners get this right, and I’ve learned that choosing the proper capacity for your water heater isn’t just guesswork — it’s about understanding your household’s hot water needs and doing a bit of math. The GOOD NEWS? Once you know what to look for, it’s not that complicated.

In this article, I’ll break down everything you need to know (including calculators) to size your tankless water heater (both electric and gas models) correctly — no fluff, no confusion, just clear guidance. Let’s get started.

How To Determine What Size Tankless Water Heater You Need?

Back when we used conventional tank heaters, sizing was pretty straightforward — you just picked a 40-gallon or 50-gallon tank based on how much hot water your household typically used. Simple, right?

But with tankless water heaters, there’s no big tank to guide your decision. Instead, the entire sizing process shifts to something new: flow rate and temperature rise.

On-demand (tankless) heaters are rated by GPM (gallons per minute) — that’s the amount of hot water the unit can deliver at a given moment. But here’s the twist: the actual GPM you get depends on the temperature of your incoming cold water and how hot you want the outgoing water to be.

The bigger the temperature difference (or “temperature rise”), the lower the flow rate a unit can provide. So, to correctly size a tankless water heater for your home, you’ll need to factor in two key things (applicable for both gas and electric units):

Up next, we’ll break down how to figure out both — so you can confidently choose an instantaneous water heater that keeps up with your household’s hot water demands.

1. Calculate The Combined Flow Rate

The first step to sizing your tankless water heater is calculating the combined flow rate — that’s the total amount of hot water your household might use at one time. Unlike traditional tank heaters, tank-free models don’t store hot water; they heat it instantly as it flows through. So, to avoid running out of hot water during peak usage, your unit needs to meet this minimum flow demand on the spot.

This combined flow rate is measured in GPM (gallons per minute), just like the ratings you’ll see on tankless units. To figure it out, make a list of all the hot water fixtures or appliances you might use simultaneously — and how much hot water each one uses.

For example:

• Standard Shower: 2-2.5 GPM
• Bathtub: 2-3 GPM
• Kitchen Faucet: 2.5-3 GPM
• Washing Machine: 3-5 GPM
• Dishwasher: 2-4 GPM

If you’re running all three at the same time during peak hours, you’ll need a system that can handle at least 7 GPM. Sounds complicated? Well, no worries, I have developed a calculator to make this easier. Just select your requirements and click the CALCULATE button to find out the minimum GPM needed for your heater. Check it out here:

IMPORTANT: Only include hot water fixtures in this calculation — cold water taps don’t count as they have no connection to the on-demand heater. Also, your peak usage will naturally vary based on your household size and daily habits. Here are a few examples:

• Smaller Household: 1 shower + 1 kitchen faucet = 4-5.5 GPM
• Larger Household: 3 showers + 1 washing machine = 9-12.5 GPM

This number represents your home’s PEAK HOT WATER DEMAND — and it gives you the minimum flow rate capacity your tankless unit should provide. The following infographic lists typical GPM ratings required for various hot water outlets (shower, faucet, dishwasher, bathtub, cloth washer, etc.) that most homeowners prefer.

But don’t stop here — flow rate is just one part of the puzzle. Next, we’ll look at temperature rise and how it affects the performance of your heater.

2. Estimate The Maximum Temperature Difference Or Rise

Once you’ve figured out your household’s combined hot water flow rate, the next step in sizing your tankless water heater is calculating the maximum temperature rise — that’s the difference between the incoming cold water temperature and the desired hot water output temperature.

This temperature rise is highest during the coldest days of the year, when groundwater temperatures are at their lowest. Your heater will need to work hardest during this time, so that’s the number on which we want to base our sizing- no matter the type of tankless heater you have: gas and electric-powered.

Step 1: Set Your Desired Hot Water Temperature

First, choose the target temperature for your hot water. A safe and comfortable setting for most households is 120°F — hot enough for showers, laundry, and dishwashing, but not so hot that it risks scalding on contact.

Step 2: Determine Your Groundwater Temperature

Next, you’ll need to know how cold the incoming water is during winter in your area. The easiest way to estimate this is by using a U.S. groundwater temperature reference map, which lists average winter temperatures by region or state. Here is one we have created from useful references:

Understanding the map is simple. For example:

• In Florida, groundwater temperatures range from about 65°F to 72°F, depending on exact location.
• In Arizona, it varies widely from 40°F to 70°F, depending on the time of year and the specific area.
• But if you’re in Hartford, Connecticut, winter temperatures can be brutally cold — and the groundwater temperature averages just 47°F. That’s practically ice water.

Step 3: Calculate The Temperature Rise

Now, subtract the incoming groundwater temperature from your desired output temperature from the tankless water heater:

Temperature Rise = (Target Hot Water Temperature ) – (Groundwater Temperature )

For example, if you live in Hartford, the calculation would be:

120°F – 47°F= 73°F

This means your tank-free geyser must be capable of raising the water temperature by 73 degrees at your household’s peak flow rate. To make this easier, I have created this calculator-

The greater the temperature rise required, the lower the GPM your instant heater can deliver — and that’s where many people go wrong. A unit that works perfectly in a warm climate might underperform in colder areas if the sizing isn’t done right.

With this temperature rise in mind, you’re ready for the next step: matching your required flow rate and temperature rise to the right specs.

3. Finally, Choose The Right Size

Now that we’ve gathered all the necessary data, it’s time to choose the right gas or electric tankless water heater for your home. Here’s a quick recap of what we’ve calculated:

With this information in hand, the next step is to consult the performance chart that manufacturers typically provide in their product catalogs. These charts are designed to help homeowners match their hot water needs with the right model. You can often find them online as well — just search for the brand you’re considering. Here is a sample table (chart)-

However, please note that the table here is only for your understanding; you should always refer to the data in the manufacturer’s sizing chart while selecting your instant water heater. Here’s how to read the chart:

1. Find the temperature rise (left column) — Look for your region’s required temperature rise, which is based on the coldest time of year.
2. Check the GPM rating — The number where the row (temperature rise) and column (model) intersect is the maximum flow rate the unit can provide at that specific temperature rise.
3. Locate the model (top row) — Find the model number(s) based on temp. rise and GPM rating. These will vary between brands and may be listed under separate categories like condensing or non-condensing units.

Please note that the greater the temperature rise, the lower the GPM output. This is simply because the colder the incoming water, the harder your tankless heater has to work to bring it up to your desired temperature — which reduces the flow rate it can maintain.

What If One Unit Isn’t Enough?

If you have a larger household and don’t see a single model that meets your GPM requirement at the needed temperature rise — don’t worry. That doesn’t mean you’re out of luck. It just means you’ll need to install multiple tankless geysers to handle your peak demand, which is one of the biggest advantages of those water heaters. Think of it as building a reliable support system: when one unit can’t handle the pressure alone, the second one jumps in to help.

A Couple of Examples

For the larger peak demand (9-12.5 GPM), most single residential water heaters (except brands like Rinnai) won’t be able to deliver that high a flow at such a steep temperature rise. In this case, the solution is to install two units in parallel that can together provide at least 12 GPM.

For the smaller peak demand (4 GPM), many standard tankless models can meet or slightly exceed this at a 73°F rise — so you’ll have multiple options to choose from.

How Much Space Or Clearance Is Needed For A Tankless Water Heater?

Once we’ve selected an on-demand water heater with the right GPM rating, the next crucial step is to make sure we’ve got enough indoor space for proper installation. With tankless heaters, clearance and ventilation aren’t just formalities — they’re absolutely critical for safety, efficiency, and performance.

From what I’ve seen in the field over the years, one of the most common mistakes homeowners make is installing their tankless water heater in a tight, poorly ventilated space, thinking it’s okay because it’s compact. Sure, those geysers take up far less space than conventional tank-style units — but that doesn’t mean you can just tuck them anywhere.

I’ve come across units crammed into closets or corners with barely a few inches to spare — and believe me, it always leads to trouble. INSUFFICIENT AIRFLOW can mean poor energy efficiency at best, and in worst cases, overheating or even explosions. That’s not just theory — I’ve seen real damage caused by units that couldn’t “breathe.”

Why Does It Matter

Tankless water heaters — especially gas-powered ones — require a steady supply of fresh air for combustion, and they must be able to vent exhaust gases safely outside. If ventilation is BLOCKED or INSUFFICIENT, dangerous gases like carbon monoxide can build up, and the heater may malfunction or shut down. I’ve even encountered scenarios where units had to be completely reinstalled just because they were suffocating in the wrong spot.

So, even though they save space, give them space. Proper clearance also ensures:

• Unrestricted airflow
• Easier access for maintenance
• Safer operation over time

The Clearance I Usually Recommend

Based on my professional experience, here are the general clearance distances I advise when installing tankless water heaters indoors, especially when mounted on a wall:

These aren’t just random numbers. I’ve followed these guidelines across dozens of installs, and they’ve consistently helped prevent performance issues and safety risks. But remember — these are just general recommendations. The specific model you pick may have slightly different clearance/sizing requirements based on whether it’s a CONDENSING or NON-CONDENSING water heater (read comparison), and your local codes might have additional rules too.

So, if you’re unsure, here’s my honest advice:

• Check the manufacturer’s manual
• Consult your local plumbing or HVAC code
• When in doubt, call a licensed installer

Getting this part right from the beginning will save you from headaches down the road — and possibly much worse.

Wrapping It All Up: Getting the Size Just Right

Finding the right size tankless water heater is all about striking the perfect balance between its flow rate, physical dimensions, and your specific household needs. While the unit’s dimensions are crucial for ensuring proper clearance and ventilation, it’s really the flow rate (GPM rating) that makes or breaks the performance.

There’s no upside to oversizing or undersizing your water heater.

If it’s TOO SMALL, it simply won’t keep up with your demand — and over time, it could get damaged from overloading. On the flip side, going TOO BIG might seem safer, but it will likely never run at full capacity, wasting energy and increasing your installation costs unnecessarily.

But now that you’re armed with all the essential knowledge about sizing an instantaneous water heater — from calculating your combined HOT WATER FLOW RATE, to understanding the MAXIMUM TEMPERATURE RISE, to reading the manufacturer’s GPM charts — choosing the right unit should feel a lot less overwhelming.

Whether you’re living in a cozy apartment or managing a busy, high-demand household, I’m confident you’ll now be able to pick the perfect tankless heater that fits both your lifestyle and your budget.

So go ahead — enjoy that endless hot water on demand and keep those energy bills under control!