How to Size a Water Heater

Before you buy, learn how to size a water heater and avoid frequent homeowners' mistakes. The sizing guide will help you choose the right gas- or electric-powered tank-type model so you can always have enough hot water.

Either you are looking to buy a gas or electric water heater from Rheem, AO Smith, Bradford White, State, GE, Kenmore or any other manufacturer; you will have a wide range of models available; from small point-of-use to high-recovery and high-efficient types. The worst thing you can do is to end up with the undersized tank which will bring only problems, such as uncomfortable showers, not enough hot water, heavy condensation, rusting and eventually premature failure.


Why sizing a tank-type water heater is important

A water heater is, after the home space heating, the second largest energy consumer, and with the rising cost of energy, water heating should not be a money waster. Before you buy a new unit, you have to think of the operating costs, not only the price.

One of the most frequent questions I received and also found in many reviews are "the heater is not producing enough hot water" or "no hot water after 15 minutes of shower."

The above two concerns should be taken into account (I mean seriously) when buying a new unit. And this is the reason why this article exists... to show you, a homeowner, how to size a water heater properly so you won’t spend more or have uncomfortable showers.

What to consider when sizing a water heater?

Before you learn how to size a water heater you should do this small project and get the answers on the paper: 
  • What is the number of people who live in the household? How many kids and teenagers? Teenagers usually have a habit of using lots of hot water. It is also important to factor in any frequent guests you have and plan to have, and their overnight stays (grandparents, for example).

  • How many plumbing fixtures there are in a home, including dishwashers, clothes washer...

  • If you have a whirlpool tub, the tank capacity should be the same or higher.

  • What is the pattern of hot water usage? When it is used the most; early morning before work and school, the evening before bed or any other time?

  • Location is also important as the ground or incoming (inlet) water, for example, it is colder in northern regions than southern.

As seen from the above, it is important to investigate what is the hot water usage pattern and habits of the family members in a home. It is vital to know what part of the day is when the high demand for hot water occurs.

For example, most of the family members are taking a shower between 9 and 11 PM, at the washing machine and dishwasher are running at the same time.

This is the time of the high use demand.

How to select based on the product specifications

When checking the heaters, you will notice two important elements in the specifications provided by the manufacturer: Peak Hour Demand or First Hour Delivery and Recovery Rate.

Peak Hour Delivery shows how much of hot water one heater can provide during the busiest one-hour period.

Recovery rate is the amount of water (in gallons per hour) one heater can raise when the temperature increases by 100 F.

When you compare electric vs. gas units, gas models have the higher First Hour Delivery so its tank can be smaller than the electric ones. Keep in mind that only 70% of hot water of the heater's tank size is actually available for use (50-gallon tank size has 35 gallons of hot water).

Table of the average consumption and peak hour demand

This calculation is based on the family size of four:
Average consumption
Peak time usage
(in one hour period)
Used Hot Water
Shower 20 4 80
Clothes washer 32
Hand dishwash. 4 1 4
Shave 2 1 2
Food prep. 4 1 4
House cleaning 6
Hand washing 2
Hot water demand in the peak time (gal.) 90

Manufacturer recommendation

Family size (# of people) Peak demand (gallons)
regular/high demand
2 45-55
3 55-65
4 65-75
5 75-85
6 85-100
7 100 or more

Every water heater comes with the Energy Guide label found on the product, and it shows the peak hour demand capacity, also known as the First Hour Rating (FHR). If you are in the store and didn't have time to learn how to size a water heater you can use the FHR value instead.

All the tips for sizing the water heater from the above are very useful for those who are buying a new or replacing the old heater. Always consider your future family needs for hot water; so buying a larger tank size is a good idea if running out of hot water often, your family grows or planning to add some new fixtures.

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