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What Size Water Heater Do I Need Calculator? – Answered

Fact checked by Stephen Conklin

what size water heater do i need calculator

Many homeowners contemplating buying a water heater have this question, what size water heater do I need calculator? Unfortunately, there’s no such thing as a definitive answer, as everything depends on your peak hot water consumption.

As a rule, the heater should deliver hot water in amounts sufficient to satisfy the needs of everyone in the household at peak hours. Based on standard consumption, the average size water heater is 40 to 50 gallons for a family of 3 people.

Keep reading to determine what size gas or electric water heater is best for your family.

Table of Contents

How Do I Determine What Size Water Heater I Need?


Like everything else, water heaters come in various sizes because they reflect the diverse needs of the modern period.

Family or household size is another crucial parameter. For example, a five-member household will need a larger-sized water heater than a two- or three-member family.

So, how do you answer the question, how big of a water heater do I need?

Ways to Determine Water Heater Size

You have two ways to determine the most appropriate water heater size for your family.

1. Manual Computation


This is a method that guarantees accurate estimations of the water heater size you need. However, it requires doing the math for two variables – the appliance’s first-hour rating (FHR) and the household’s peak hot water consumption.

  • Calculation formula and Rule

As a rule, the water heater’s first-hour rating (FHR) should be equal to or higher than a household’s peak hot water consumption. We can write the formula as follows.

FHR (first-hour rating) ≥  PHWC (peak hot water consumption)

A heater’s first-hour rating estimates its efficiency in raising the water temperature to your preferred levels. And when we talk about efficiency, we always consider speed and energy use.

Storage tank water heaters use gas or electricity. Water heater BTU (British Thermal Unit) describes the unit’s ability to heat a pound of water by one degree Fahrenheit.

A higher BTU rating translates to faster water heating, relating to the equation’s FHR by impacting the water heater’s recovery rate. After all, FHR = (water heater capacity x 70%) + recovery rate.

To determine what FHR you need, multiply the number of residents in your home by 12. For example, if there are six household members, multiplying six by 12 results in 72 gallons. This number is your required FHR.

  • Estimate Peak Hour Demand

The next part of our equation relates to how many gallons of hot water are consumed during the busiest times of the day.

Most families have their peak hot water usage in the morning when everyone is taking a shower and getting ready for work or school.

Try to observe what time of day your family usually uses more hot water than usual. You can create a table or worksheet to estimate your household’s peak hot water consumption. You can have a look at the sample below.

Gallons of hot water per usage x No. of times used in 1 hour = Gallons used in 1 hour
One shower 20 x =
Washing the face/hands at 2 GPM 3 x =
Shaving at 0.05 GPM 2 x =
Washing the dishes by hand at 2 GPM 4 x =
Food preparation at 2 GPM 2 x =
Dishwasher 7 x =
Laundry washing machine (top-loader) 25 x =
Laundry washing machine (H-axis) 15
Total Peak Hour Hot Water Demand =
For example, suppose three family members shower successively, one household member shaves, and another only washes their face. In that case, the total hot water consumption is 65 gallons (60 + 3 + 2 = 65).

Let us now use our formula to calculate water heater size.

FHR (first-hour rating) ≥ PHWC (peak hot water consumption)

In our example above, our peak hot water consumption is 65 gallons. Hence, supplanting our equation with this number, we will get the following.

FHR ≥ 65 gallons

Our equation shows that we need to pick a water heater with an FHR of at least 65 gallons per hour. So, you might want to look at individual water heaters and filter them using this metric.

2. Family Size


The third method for sizing a water heater for your family is to use a family size-based size chart.

Here is an example of what we mean.

Family Size (Number of Member/s) Water Heater Size in Gallons
1 – 2 30 to 40
2 – 3 40 to 50
3 – 4 50 to 60
5 or more 60 to 80

This chart is one of the most straightforward methods for determining what size water heater to buy without a water heater sizing calculator. However, it’s not as accurate as the manual computation method above. Still, this approach is useful for folks in a hurry to buy a water heater.

The Importance of Choosing the Right Size Water Heater


Choosing the right size water heater is crucial to prevent unnecessary problems. For example, a heater too small for the family can produce a shortage in the hot water tank, forcing you to wait.

Sadly, pushing the appliance beyond its recovery capabilities and first-hour rating can stress the heating elements and other components. It can lead to a premature breakdown, requiring costly repairs or even more expensive replacement.

On the other hand, a water heater too large for the household is a waste. You will spend more money on energy to heat the water, a proportion of which you do not use.

For example, why install an 80-gallon water heater when only two individuals use hot water regularly? A 30-gallon unit should be sufficient for a two-person household.

Frequently Asked Questions


Is a 40-gallon water heater enough for a 3-bedroom house?

Yes, a 40-gallon water heater is sufficient for a 3-bedroom house, provided the number of occupants does not exceed 3. As pointed out above, water heater sizing calculations do not account for the number of bedrooms but the average hot water consumption per household member.

What is a good size water heater for a family of 5?

Generally, 50 to 60 gallons is a good water heater size for a family of five, given this household consumes about 46 to 56 gallons of hot water per hour in total.

You will want to consider the appliance’s first-hour rating (FHR) to determine how much hot water the unit can deliver per hour during peak hot water consumption.

For example, if the family consumes 60 gallons at peak hours, the water heater must have an FHR of at least 60 gallons.

Should I oversize my water heater?

No, it is unwise to oversize a heater because you will waste energy and increase your electric/gas bill as a result. This is because, unlike tankless water heaters, storage-tank units require standby energy to keep the water at the required temperatures constantly.

For example, a 40-gallon water heater is sufficient for a family of 4. If this household installs a 60-gallon unit, the appliance will need more fuel (electricity or gas) to heat the extra 20 gallons.


You have two options for determining what size water heater do I need calculator for. A family size-water heater chart is the most straightforward method. Unfortunately, it does not account for hot water consumption.

The better method for determining what size water heater is most appropriate for your household involves comparing the water heater’s FHR and your family’s peak hot water consumption. They should be evenly matched.

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