Tankless Water Heater Sizes Explained (Without the Tankful of Confusion)

By Brian on May 6, 2025

The Ultimate Guide to Tankless Water Heater Sizes

How big are tankless water heaters? Tankless water heaters typically measure 20-30 inches in height, 10-20 inches in width, and 5-10 inches in depth. They’re significantly smaller than traditional tank water heaters and can be mounted on walls to save valuable floor space.

If you’re looking to replace your bulky traditional water heater with something more space-efficient, you’re in the right place. Tankless water heaters are a game-changer for homeowners seeking to reclaim valuable space while enjoying endless hot water.

Unlike their tank-based cousins that can take up a small closet’s worth of floor space, tankless models mount directly on your wall and take up about the same space as a small suitcase. This compact design is one of the main reasons homeowners make the switch.

But here’s the thing: physical size doesn’t tell the whole story.

The real sizing question isn’t just about dimensions – it’s about capacity. A tankless water heater’s size is primarily measured by its flow rate (gallons per minute or GPM) rather than physical dimensions. Most residential tankless units deliver between 5-10 GPM, with the right size depending on your household’s peak hot water demand.

As the Department of Energy notes, tankless water heaters can be 24-34% more energy efficient than conventional storage tank water heaters for homes that use 41 gallons or less of hot water daily.

Choosing the wrong size can lead to that dreaded cold-water surprise mid-shower – something no homeowner wants to experience!

Learn more about how big are tankless water heaters:
– eco smart water heater
– natural gas hot water tank
– water heater energy savings

How Big Are Tankless Water Heaters? Quick Dimension Snapshot

When homeowners first consider switching to tankless water heaters, they’re often pleasantly surprised by just how compact these units really are. Unlike their bulky tank predecessors, tankless models take up a fraction of the space while delivering all the hot water you need.

How big are tankless water heaters exactly? They’re designed to be wall-mounted, completely freeing up your valuable floor space. This makes them ideal for tight utility closets, garages, basements, or even mounted outside in mild climates like we have here in Northern California.

Physical Dimensions by Type:

Most whole-house tankless units measure about 20-30 inches in height, 10-20 inches in width, and just 5-10 inches in depth—roughly the size of a small suitcase or medicine cabinet. Point-of-use models, designed to serve a single fixture, are even more compact at just 10-18 inches tall, 7-14 inches wide, and a mere 3-5 inches deep.

Gas-powered models tend to be slightly larger than electric ones due to their combustion components and venting requirements. That said, both types offer dramatic space savings compared to conventional tank heaters that dominate your utility area.

We’ve had countless customers amazed by the change. Just last month, a family in Folsom told us, “We replaced our massive 50-gallon tank that hogged a quarter of our garage with a tankless unit that’s barely noticeable on the wall. We’ve reclaimed so much storage space!”

Real-World Model Examples (How Big Are Tankless Water Heaters in Inches?)

Let’s look at some specific models to give you a concrete idea of the physical dimensions:

The Rheem Mid-Efficiency (8.4 GPM) measures 25.6″ H × 13.9″ W × 9.9″ D and weighs about 54 pounds. The Navien NPE Series comes in slightly larger at 23.6″ H × 17.3″ W × 12.8″ D, weighing around 60 pounds. For something more compact, the Camplux 2.64 GPM point-of-use model is just 20.3″ H × 12.8″ W × 5.9″ D and weighs a mere 25 pounds.

For perspective, a standard 50-gallon tank water heater typically towers at about 60 inches tall and 24 inches in diameter, consuming over 3 square feet of floor space. A tankless unit mounted on the wall takes up zero floor space and only projects about 10 inches from the wall—that’s a lot of reclaimed space for storage or other uses!

Small, Medium, Large: Does Physical Size Match GPM?

Here’s something that surprises many homeowners: the physical size of a tankless water heater doesn’t necessarily indicate its hot water output capacity (measured in gallons per minute or GPM).

A compact unit might deliver impressive flow rates thanks to advanced heat exchanger technology, while a physically larger unit might have more modest output. The magic happens inside, where efficiency and performance depend more on the internal components and engineering than exterior dimensions.

For example, condensing tankless water heaters—which include a second heat exchanger to capture additional heat from exhaust gases—tend to be slightly larger than non-condensing models. However, this small size increase delivers significant efficiency improvements that can save you money over time.

As we often remind our customers in Rancho Cordova and throughout Northern California: “Don’t judge a tankless water heater by its cover—it’s the performance inside that will keep your showers hot and your energy bills low!”

Key Factors That Drive Size (Flow Rate, Temperature Rise, Household & Fuel Type)

When my Sacramento customers ask about how big are tankless water heaters, I always remind them that the most important “size” isn’t what you see on the wall—it’s the unit’s capacity to deliver hot water when your family needs it most.

Think of sizing a tankless water heater like buying shoes for a growing teenager. You need to consider not just today’s needs, but what might be coming down the road. Four key factors determine the right size for your Northern California home:

Understanding Flow Rate (GPM) & Why It Matters

Flow rate is the star of the show when sizing your tankless system. Measured in gallons per minute (GPM), it tells you how much hot water your unit can deliver continuously—without ever running cold.

I was helping a family in Folsom last month who couldn’t figure out why they kept running out of hot water during their morning routine. The problem wasn’t their water heater’s physical size—it was that their 5 GPM unit simply couldn’t keep up with two simultaneous showers (2.5 GPM each).

Most households need to account for these typical fixture flow rates:

The math is simple but crucial: if your family routinely runs a shower (2.5 GPM) while someone washes dishes (1.5 GPM), you need at least 4 GPM of capacity. Miss this calculation, and you’ll be taking lukewarm showers before long!

Temperature Rise & Climate Impact

Here in Northern California, our groundwater is cooler than you might expect—typically 55-65°F depending on the season and your specific location. Your tankless water heater needs to boost that temperature to about 120°F for comfortable use.

This temperature difference (called ΔT or “delta T”) has a huge impact on your water heater’s performance. The colder your incoming water, the harder your unit must work, which actually reduces its effective GPM output.

I remember installing a tankless unit for a family in Auburn, where winter groundwater temperatures can dip to 50°F. We had to size up their unit because the same model that works perfectly in Sacramento would struggle with their colder incoming water.

The simple formula I share with customers is:
Desired Output Temperature (120°F) – Groundwater Temperature (55°F) = Required Temperature Rise (65°F)

Every tankless water heater comes with performance charts showing how many gallons per minute it can deliver at various temperature rises. As the temperature rise increases, the available GPM decreases—a critical detail many homeowners miss when shopping for units.

Gas vs. Electric Sizing Differences

The fuel type of your tankless water heater dramatically impacts both its physical size and its water-heating capacity. This is where I see many DIY-minded homeowners make costly mistakes.

Gas tankless water heaters are the workhorses of the industry. They can deliver impressive flow rates (often 8+ GPM) and handle whole-house demands even during our chillier Northern California winters. They’re measured in BTUs (typically 140,000-199,000) and pack serious heating power. However, they’re usually a bit larger physically due to their combustion components and require proper venting.

Electric tankless water heaters tend to be more compact and sleeker in appearance. The trade-off? They’re usually limited to lower flow rates (up to about 8 GPM) and work best in smaller households or for point-of-use applications. Instead of BTUs, they’re measured in kilowatts (typically 14-36 kW), and many homes need electrical upgrades to support them.

As I explained to a customer in Roseville last week: “For your family of five with three bathrooms, a gas tankless system will give you much better performance than electric, especially during those busy mornings when everyone’s getting ready at once.”

The Department of Energy’s research confirms this practical experience—tankless water heaters can be 24-34% more energy efficient than conventional storage tanks for homes using moderate amounts of hot water.

Need more specifics about installing a tankless system in your Sacramento home? Check out our detailed Sacramento tankless water heater installation guide for local considerations and expert advice.

Calculating Your Perfect Size—DIY Formula & Tools

Finding the right size tankless water heater doesn’t have to be complicated. Let’s break it down into a simple process that any homeowner can follow. With our three-step approach, you’ll be able to determine exactly what size unit will keep your showers hot and your energy bills low.

When we visit homes in Elk Grove or Folsom, homeowners often tell us they’re overwhelmed by all the options. Don’t worry—we’re going to make this easy!

Step 1: Calculate Your Maximum Flow Rate

First, think about your family’s morning routine. Who’s using hot water at the same time? Maybe you’re showering while someone else runs the dishwasher. Or perhaps two showers run simultaneously while the washing machine is going.

Make a quick list of all fixtures you might use at once, then add up their flow rates. A typical shower uses about 2.5 GPM, a bathroom faucet around 1 GPM, and a dishwasher approximately 1.5 GPM.

For example, if your household might run two showers (5 GPM total) and a dishwasher (1.5 GPM) simultaneously, you’d need at least 6.5 GPM of capacity.

Step 2: Determine Your Temperature Rise

Next, you’ll need to figure out how hard your tankless unit will have to work based on your local groundwater temperature.

Here in Northern California, groundwater typically ranges from 55-65°F depending on the season. To get comfortable 120°F hot water, you’ll need to raise the temperature by 55-65°F.

The calculation is simple:
Temperature Rise = Desired Hot Water Temperature – Incoming Water Temperature

So if you want 120°F water and your groundwater is 55°F, you need a 65°F rise. This number is crucial because how big your tankless water heater needs to be depends heavily on this temperature rise.

Step 3: Find a Unit That Meets Both Requirements

Now you’re ready to shop! Look for a tankless water heater that can deliver your required GPM at your specific temperature rise. Manufacturers provide performance charts that show exactly what each model can handle.

“Finding the right size made all the difference for us,” says Maria from Sacramento. “We calculated we needed 7 GPM with a 60°F rise. No more cold showers when the washing machine is running!”

Many manufacturers offer handy online calculators to simplify this process. Tools like NaviSizer let you input your specific requirements and get personalized recommendations. These tools take the guesswork out of sizing and help ensure you get the perfect unit for your home.

Example Calculation: 2 Showers + Dishwasher Morning Rush

Let’s walk through a real-life example for a typical Fair Oaks family:

Imagine it’s 7:30 AM on a busy weekday. Two showers are running while the dishwasher cleans last night’s dinner dishes. Here’s how we’d calculate the right size:

First, we add up the flow rates: Shower 1 (2.5 GPM) + Shower 2 (2.5 GPM) + Dishwasher (1.5 GPM) = 6.5 GPM total.

Next, we check the groundwater temperature. In Sacramento during winter, that’s about 55°F. For comfortable 120°F showers, we need a 65°F temperature rise.

Based on these numbers, this family would need a tankless water heater that can deliver at least 6.5 GPM at a 65°F rise. This typically points to a gas tankless model with at least 160,000 BTU input capacity.

We’d recommend an 8 GPM unit to provide a comfortable buffer and handle any unexpected hot water demands.

Replacing a 50-Gallon Tank with Tankless

“How does my 50-gallon tank compare to these GPM ratings?” This is one of the most common questions we hear from Roseville homeowners.

Traditional tank water heaters and tankless units speak different languages when it comes to capacity. Your 50-gallon tank is rated by its first-hour rating (FHR)—typically between 60-80 gallons depending on the recovery rate.

To match this performance with a tankless unit, we need to focus on peak demand rather than storage:

The big difference? Your tank stores a limited amount of hot water, while a tankless unit creates hot water on demand—forever. No more waiting for the tank to recover after everyone has showered!

For most Northern California homes replacing a 50-gallon tank, we typically recommend a tankless unit with 7-9 GPM capacity at your local temperature rise. This usually translates to a 160,000-190,000 BTU gas tankless water heater.

When sized correctly, your new tankless water heater will provide endless hot water while taking up a fraction of the space. And remember, our team at Go Pro Plumbing is always here to help you steer these calculations and find the perfect fit for your home’s unique needs.

From Blueprint to Wall: Dimensions, Clearance & Installation Options

Now that you’ve figured out what size tankless water heater you need in terms of capacity, let’s talk about the physical installation. Where will your new compact powerhouse actually go? This is where those physical dimensions we discussed earlier really come into play.

Clearance Requirements

Your tankless water heater might be compact, but it still needs some breathing room. Think of it like a person who doesn’t take up much space but still needs room to stretch their arms out!

Most tankless units require:
– At least 12 inches of clearance above the unit
– About 24 inches of space in front for service access (this is where your technician will stand during maintenance)
– Between 4-6 inches on each side
– Roughly 12 inches below the unit

These clearances aren’t just arbitrary numbers—they ensure proper airflow, allow for routine maintenance, and meet safety codes. One of our Sacramento customers learned this the hard way when they tried to squeeze their unit into a too-tight utility closet. We had to relocate it for safety reasons.

Indoor vs. Outdoor Installation

One of the cool things about how big tankless water heaters are is their flexibility in placement. You’ve got options!

Indoor Installation works great for many Northern California homes. Your unit will typically live in a utility room, garage, or basement. Just remember that indoor units need proper venting to the outside, which might require some additional wall space for vent pipes. The good news? Even with venting, you’re still saving tons of space compared to a traditional tank.

Outdoor Installation has become increasingly popular with our Sacramento customers. These units are designed with weather-resistant enclosures and eliminate the need for complex venting systems. While freezing is rarely an issue in our Northern California climate, these units have built-in freeze protection just in case.

“I never realized I could mount my water heater on the outside wall of my house,” a Folsom homeowner told us recently. “It freed up my entire utility closet for storage, and I barely notice it’s there!”

Mounting Height and Positioning

Most tankless water heaters look their best and function optimally when mounted about 4-5 feet from the floor. This height makes the controls easily accessible while maintaining those clearance requirements we mentioned.

The exact mounting height might vary based on several factors:
– What the manufacturer recommends
– Your local building codes
– Specific venting needs
– Whether everyone in the home can reach the controls
– How it looks in your space

We find that many homeowners are surprised by how unobtrusive a tankless unit can be when properly placed. Unlike that massive tank that dominated your utility room, a tankless unit can almost blend into the background.

For more detailed information about installation considerations, our tankless water heater installation guide walks you through the entire process.

Using Multiple Tankless Water Heaters in Large Homes

Sometimes, bigger homes need more hot water power. For sprawling Northern California properties or homes with extraordinary hot water demands, we sometimes recommend installing multiple tankless units.

This approach, called “cascading” or “zoning,” offers several benefits:

First, you get zoned hot water delivery. By placing units closer to where hot water is used, you reduce wait times significantly. Imagine having hot water instantly in your master bath, even though it’s on the opposite side of the house from the kitchen!

Second, there’s built-in redundancy. If one unit needs maintenance, the others keep working. No cold showers while repairs are happening!

Third, the system can balance loads intelligently. Multiple units work together, activating only when needed, which can actually save energy compared to running one large unit all the time.

We recently installed a dual-unit system for a family in El Dorado Hills with a 5-bathroom home. The mom told us, “Before, the kids would always complain about cold showers if they weren’t first in line. Now everyone gets hot water simultaneously, even on busy mornings.”

Multiple units make the most sense when:
– Your home has more than 3-4 bathrooms
– Your hot water fixtures are spread far apart
– Your family’s peak demand exceeds what a single unit can handle (typically 10-12 GPM)
– You have specialty fixtures like jetted tubs or rainfall showers

The beauty of tankless technology is its flexibility—whether you need a single compact unit or a sophisticated multi-unit system, there’s a configuration that will work perfectly for your home.

Sizing Mistakes, FAQs & Pro Tips (Keep the Hot Water Flowing)

After installing hundreds of tankless water heaters throughout Northern California, we’ve seen just about every sizing mistake imaginable. Let me share some real-world wisdom to help you avoid the common pitfalls that can leave you shivering in the shower.

The most frequent mistake we see? Underestimating peak demand. Many Sacramento homeowners focus on average usage rather than those busy mornings when everyone’s getting ready simultaneously. Just last month, we helped a family who couldn’t understand why their new tankless unit left them with lukewarm showers—they’d forgotten to account for their teenagers’ marathon shower sessions overlapping with the dishwasher’s morning cycle!

Another sizing trap is ignoring temperature rise. Northern California’s groundwater temperatures fluctuate seasonally, and that 8 GPM unit that works perfectly in summer might struggle to deliver 6 GPM during those chilly winter months when your incoming water is 10 degrees colder.

We also see folks overlooking future needs—that perfect-sized unit becomes woefully inadequate when baby number three arrives or you add that dream primary bathroom. Think ahead when sizing your system.

DIY enthusiasts often make electrical and gas supply errors, not realizing that tankless units have specific requirements that typically exceed what’s installed for conventional tank heaters. A common scenario: a homeowner installs a new tankless unit but keeps the existing ½-inch gas line, then wonders why the unit can’t reach full capacity.

Finally, many people forget about maintenance accessibility. That snug installation in the crawlspace might save room, but it makes annual maintenance (crucial for performance) nearly impossible.

As one Sacramento customer told us after an undersized installation: “I tried saving $300 by choosing the smaller unit. Three months and one cold shower too many later, I spent $1,200 replacing it with what my plumber originally recommended. Lesson learned!”

How Big Are Tankless Water Heaters for a Family of Four?

For a typical family of four in Northern California with 2-3 bathrooms, we generally recommend units with 8-10 GPM flow capacity. In gas models, this translates to about 160,000-190,000 BTU. If you’re considering electric (though we rarely recommend this for whole-house applications in larger homes), you’d need a 27-36 kW unit.

These specifications usually mean a medium-to-large tankless water heater with physical dimensions around 24-28 inches in height, 16-20 inches in width, and 9-12 inches in depth—roughly the size of a carry-on suitcase.

Your family’s specific habits matter tremendously, though. A family that naturally staggers shower times might do fine with 7 GPM, while those who all shower simultaneously before work and school might need 10 GPM or more. This is where professional sizing really pays dividends.

Can You Oversize on Purpose? Pros & Cons

“Better safe than sorry” might work for umbrellas, but it’s not always the best approach for tankless water heaters.

Some homeowners consider intentionally oversizing their unit for future-proofing against family growth or home additions. Others want extra capacity for unusually high-demand situations, like when holiday guests arrive. A slight oversizing can also reduce stress on the unit during peak usage.

However, there are real drawbacks to significant oversizing. The most obvious is the higher upfront cost—both for the equipment and potentially for installation if larger gas lines or venting are required. Surprisingly, you won’t see significant energy savings from oversizing since tankless units only heat water on demand anyway.

Our professional recommendation: Size for your current needs plus a small buffer (about 10-15% extra capacity). Don’t substantially oversize unless you have specific future plans that warrant it. As one of our Elk Grove customers put it: “I’m glad we went with the slightly larger unit—it handled everything perfectly when my in-laws visited for two weeks!”

FAQ Blitz

Q: How many fixtures can run simultaneously with an 8 GPM tankless water heater?

In Northern California’s climate, an 8 GPM unit typically supports 2-3 showers OR 1 shower plus multiple smaller fixtures running simultaneously. It’s about your total simultaneous demand, not the number of bathrooms in your home.

Q: What electrical requirements do tankless water heaters have?

Electric tankless water heaters are power-hungry beasts. They typically require 2-4 separate 40-60 amp 240V circuits, with total amperage often exceeding 100 amps. This frequently necessitates electrical panel upgrades, which is why many homeowners opt for gas models when available. Gas tankless units, by contrast, only need a standard 120V outlet for their electronic controls.

Q: How does climate change affect tankless water heater sizing?

This is becoming a more common question! As climate patterns shift, groundwater temperatures may change. In areas experiencing colder winters or more temperature variability, selecting a unit with slightly higher capacity provides a buffer against these changes. We’re already seeing this in parts of Northern California where winter ground temperatures seem to be dropping.

Q: Can I install a tankless water heater myself?

While technically possible, we strongly recommend professional installation. Proper sizing, gas line modifications, venting, and electrical work require specialized knowledge and tools. We’ve rescued many DIY installations gone wrong, and unfortunately, most manufacturers void warranties on improperly installed units. Plus, there’s the small matter of building codes and permits—something homeowners often overlook until inspection time.

For more information about the efficiency advantages of properly sized tankless systems, visit our efficiency guide.

Conclusion

Understanding how big tankless water heaters are—both in physical dimensions and capacity—is crucial when upgrading your home’s hot water system. After exploring all the details, let’s bring everything together with the key takeaways.

When it comes to physical size, tankless water heaters are remarkably compact, typically measuring just 20-30 inches tall, 10-20 inches wide, and only 5-10 inches deep. This sleek profile is a game-changer for homeowners tired of that bulky tank eating up valuable space in their garage or utility room.

But as we’ve finded throughout this guide, the true “size” that matters goes beyond mere dimensions. The capacity of your tankless unit—measured in gallons per minute—is what determines whether your morning routine flows smoothly or ends with an unexpected cold shower.

Your perfect tankless water heater size depends on four critical factors: your household’s simultaneous hot water demands, Northern California’s groundwater temperatures, your choice between gas or electric models, and the number of bathrooms and fixtures in your home. Getting this calculation right means enjoying endless hot water without paying for capacity you don’t need.

A properly sized tankless water heater delivers benefits beyond just saving space. You’ll experience consistent hot water without frustrating temperature fluctuations, optimal energy efficiency that keeps utility bills in check, and a system that performs reliably for years to come. The right size means maximum comfort and convenience for your entire household.

At Go Pro Plumbing, we’ve guided countless Sacramento and Rancho Cordova homeowners through the tankless sizing process. Our experienced technicians understand the unique challenges of Northern California homes and can help you steer the options to find your perfect match.

Ready to reclaim your space and enjoy the luxury of endless hot water? We’re here to help with same-day service throughout Northern California. Our team will walk you through every step—from choosing the right size to professional installation and ongoing maintenance.

For more information about our comprehensive tankless water heater services, visit our tankless water heater services page. Let Go Pro Plumbing transform your hot water experience with the perfect tankless solution for your home.