What Water Heater Size Do I Need?

Last Revised: 4/21/25

Do you ever run out of hot water during your morning shower? Have you ever wondered if your current water heater is really the right size for your household? Choosing the correct water heater size isn’t just about convenience – it directly impacts your energy bills, system lifespan, and daily comfort. 

At Mattioni, we’ve been helping homeowners through this process for over 75 years. We’re a team of trusted local experts who understand how to match your unique home and specific needs with the right water heater system. 

In this article, we’ll walk you through how water heater sizing works, how to estimate your home’s peak hot water demand, and the key differences between tank and tankless systems – so you can make the right choice with confidence. 

Here’s what we’ll cover: 

• Why proper sizing matters 
• How to calculate your household’s hot water demands 
• Sizing guidance for tank vs. tankless systems 
• What happens if your water heater is too small, or too big 
• When to talk to a professional 

Let’s jump in. 

Why Choosing the Right Water Heater Size Matters 

Getting the size right isn’t just about comfort – it’s about performance and efficiency. A water heater that’s too small will constantly run out of hot water and may wear out faster due to overuse of the heating elements. On the other hand, a system that’s too large wastes energy by heating (and keeping hot) more water than you need.  

The goal is to find a system that consistently meets your household’s hot water needs without overworking or wasting energy. That’s why sizing is a critical first step when replacing or upgrading your water heater – it’s necessary to limit your utility bills, prolong system lifespan, and protect your home’s comfort. 

How to Estimate Your Hot Water Needs & Water Heater Size 

Water heater sizing is based on your household’s peak hot water usage, which almost always occurs during the morning or evening when multiple appliances or people need hot water at the same time. 

Here’s a rough breakdown of typical hot water usage per activity: 

• An 8-minute Shower: 20 gallons (roughly 2.5 gallons per minute) 
• 1 Dishwasher Cycle: 6 gallons 
• 1 Load of Clothes in the Washing Machine: 20 gallons 
• Handwashing and Shaving: 1-2 gallons 

To estimate your household’s peak hot water demand, tally the hot water activities that might happen during your busiest hour. For example, a household of four might need 40-50 gallons during peak use. If needed, a professional plumber can help you pinpoint your exact usage profile. 

For traditional tank water heaters, the key measurement is First Hour Rating (FHR) – how many gallons of hot water the unit can supply in an hour when fully heated. Pro tip: A water heater can provide about 70% of its total volume of hot water before the temperature begins to drop as cold water is mixed in. Selecting a tank size in which your household’s peak usage is less than 70% of the total volume will ensure you never run out of hot water. 

For tankless systems, the focus is on flow rate, measured in gallons per minute (GPM), which tells you how much hot water the unit can produce on demand. While a tankless water heater can provide endless hot water, they don’t produce unlimited hot water. In other words, you’ll need to line up your household’s peak hot water usage with the tankless water heater’s flow rate (GPM).  

Curious whether a tankless water heater is right for you? Read our guide on Tank vs. Tankless Water Heaters. 

Water Heater Size Guidelines for Tank and Tankless Units 

Looking to get a quick gauge on how much water your household uses? Here’s a more direct breakdown of hot water demands based on your household. 

Tank Water Heater Size Guidelines:

• 1-2 people: 30-40 gallon water heater 
• 2-3 people: 40-50 gallon water heater 
• 3-4 people: 50-60 gallon water heater 
• 5+ people: 60-80 gallon water heater 

50-gallon tank water heaters are a popular choice as they can often sufficiently facilitate hot water for a family of four, and only occasionally struggle to keep up with a family of five. 80-gallon tank water heaters are a good option if you have five or more people in your household, and a 30-gallon tank water heater is well-suited for a single-occupancy home. 

Tankless Water Heater Size Guidelines:

These systems are sized based on flow rate (GPM) and the temperature rise required in your region.  

• Temperature rise: the difference in temperature between the incoming water supply and the desired temperature of your water heater. It’s based on (1) weather conditions, and (2) your water heater’s thermostat setting. 

To determine what flow rate you’ll need out of a tankless water heater, simply add together the gallons per minute (GPM) required of each fixture during your peak water usage. For example, if you need to run a shower (2.5 GPM) and a dishwasher (1.5 GPM) at the same time, you’ll need a unit with a GPM of roughly 4, depending on the temperature rise. 

Tankless systems are ideal for households with fluctuating hot water demands, and they’re more energy-efficient (other than hybrid/heat pump water heaters) since they only heat water as needed. If your home often uses multiple hot water fixtures at once, choose a system with a higher flow rate or talk to a professional about how installing multiple tankless units can avoid underperformance.  

Consequences of Installing the Wrong Water Heater Size 

Undersized units will struggle to keep up with demand, leading to cold showers, reduced efficiency, and faster wear on the system. You may find yourself constantly adjusting your schedule around hot water availability. 

Oversized units may be less disruptive to your shower schedule, but they’ll drive up your energy bills. This is especially true for tank systems, as they’ll have to continually keep more hot water stored than you’ll ever actually use. 

Regardless of whether you’re installing a tank or a tankless system, an improperly sized water heater can also void warranties or lead to premature failure if it’s regularly overworked. Choosing the right size from the start ensures better long-term performance and fewer headaches down the road. 

When to Call a Professional for Water Heating Sizing 

While in some situations you can simply replace your old water heater with a new one of the same volume, sizing a water heater isn’t always straightforward. Your household’s hot water habits and the number of occupants are the two most consequential factors in play. 

A professional plumber will consider your specific needs, inspect your current system, and provide recommendations based on real usage – not just point to general charts. 

At Mattioni, we take the time to understand your home and lifestyle before making a recommendation. Whether you’re switching to a tankless or upgrading an aging tank unit, we’ll help you make a smart, efficient choice. 

Take the Guesswork out of Sizing Your Water Heater 

Now you know how choosing the correct size water heater ensures your comfort, energy efficiency, and system longevity. You know that your peak hot water demand determines what size is right for your home – regardless of whether you’re installing a tank or tankless water heater.  

Choosing the wrong size could lead to cold showers, inflated energy bills, and a system that wears out faster than it should. 

At Mattioni, we’ve been helping homeowners make the smart, comfortable choice for over 75 years. We take the guesswork out of the equation so you can enjoy reliable, long-lasting, and energy-efficient hot water – day in and day out. 

Ready to upgrade or replace your water heater? Call Mattioni today at (610) 400-8510 to schedule your consultation or book an appointment now.