Hot Water Heater Size Calculator: Complete Guide [2023]

Last updated on August 28th, 2023

When shopping for a gas or electric water heater, you may choose from a broad variety of types, from compact point-of-use heaters to larger high-recovery and ultra-efficiency versions from brands like Rheem, AO Smith, Bradford White, State, GE, Kenmore, and many more. An inadequately sized tank can only cause headaches in the form of cold showers, insufficient hot water, condensation, corrosion, and, worst of all, an untimely tank breakdown. The water heater size calculator in this post will help you to decide what is the best size of water heater for your family.

It is good to learn about common pitfalls that homeowners face when sizing a water heater before making a purchase. Choose the correct gas or electric tank-type model, including tankless, with the aid of the sizing guide, suggestions, calculators, and chart so you can have a hot shower whenever you want. If you buy a water heater that is too tiny, you will not have enough hot water, but if you buy a water heater that is too huge, you will pay for the water you never use.What to consider before purchasing the right size of water heater?

It is important to know the answers to the following questions before learning how to choose the right water heater for your family of 3, 4, 5, or more, your little house with 1 or 2 bathrooms, or your large house with 3 or more bathrooms.

• How many individuals do we have in this house? How many young people are we talking about? The use of excessive amounts of hot water is a common practice among teenagers. Take into account the number of guests you have or expect to have to stay overnight regularly (grandparents, for example).
• The number of appliances and bathtubs/whirlpools/etc. which are all connected to the home’s plumbing system.
• Can you imagine how many people would all be taking showers at once?
• Which bathtub is the largest, and why?
• Another helpful hint is to make sure the tank capacity is equal to or greater than the whirlpool tubs.
• How does one typically make use of one’s hot water supply? The most common times of day for its use are either the early morning hours before work and school or the late evening hours before bed.
• How hot is the water pumping out of the ground? The temperature of the groundwater below the surface varies with latitude, altitude, and season (climate). For instance, ground or incoming (inlet) water is colder in the north than in the south.

As can be seen, looking into the household’s hot water usage pattern and behaviors is crucial. Understanding when the peak hours of hot water use are throughout the day is crucial.

The most enormous demand for hot water typically comes between 9 and 11 p.m., when many members of the family are taking two or more simultaneous showers and when the washing machine and dishwasher are both operating at the same time.

Here are just a few of the metrics you should look for when browsing heater specifications:

• Power
• Efficiency
• Peak hour demand
• First-hour delivery
• Recovery rate

The technical data analysis process will educate you on the various water heater models and their appropriate sizing.

When talking about water heating, the input or power is the quantity of energy (in Watts) or gas (in BTU) consumed in one hour. A higher input water heater facilitates a quicker recovery. An indicator of a water heater’s efficiency is its Peak Hour Delivery, which displays the maximum volume of hot water (in GPH) produced during the busiest period.

In order to determine the First Hour Rate, we take the total amount of hot water that was produced in the first hour and multiply it by the hourly rate.

Also Read: Water Softener Size Calculator

When the temperature of the water is increased by 60 degrees Fahrenheit, the Recovery Rate is the volume of water (in gallons per hour) that a single heater can bring up to that new temperature. Since it’s included in the FHR, it may also be thought of as the heater’s ability to heat a given quantity of water to a desirable temperature within a certain amount of time.

When comparing gas and electric models, the higher First Hour Delivery of the gas model allows the tank to be smaller while still generating the same amount of hot water as the electric type. Remember that only around 70 percent of the hot water stored in the tank is truly usable (a 50-gallon tank size has 35 gallons of hot water for use).

How Can You Estimate Your Peak Hour Demand?

There are two approaches (one straightforward, one more difficult) to calculating the Peak Hour Demand for your home. Here is the straightforward approach for estimating peak hour demand.

Calculation:

The first thing you should do is take a headcount of everyone currently residing in your house. You should also include stepchildren or other members of the family who live with you regularly but do not live there permanently.

In Step 2, you should increase your household’s headcount by 12 times. For instance, if five people are living in your home, your FHR would be 60.

Note:

The Peak Hour Demand capacity, also known as the First Hour Rating, may be found on the Energy Guide label found on every water heater (FHR). Forget about learning how to properly size a water heater; instead, use the FHR value while shopping (a higher number is better).

Table of Average consumption (gallons) vs Peak time use (per hour)

This calculation is based on a family size of 4:

Application	Average consumption (gallons)	Peak time usage  (in one hour period)	Used Hot Water (gal.)
Clothes washer	32
Automatic Dishwasher	14
Shower	20	4	80
Shave	2	1	4
Hand dishwashing	4	1	4
Food preparation	2	1	2
House cleaning	6
Hand washing	2

Pro Tip:

It may be more cost-effective in some cases, to install multiple smaller water heaters with better recovery rates rather than a single large one. In order to meet the high demand for hot water, it is possible to install numerous units and connect them in parallel.

How to measure the size of a tankless water heater?

The tank of tankless water heater is not used to store heated water. On-demand, they turn to the heat exchanger, which warms the water as it flows through it.

The question, “how big of a water heater do I need?” needs the following points to be considered:

1. The flow rate (in GPM): For example: if you wish to run two fixtures at once, one with a flow rate of 0.5 GPM and the other with a flow rate of 1 GPM, the tankless water heater should supply at least 1.5 GPM.
2. How many lights do you plan on turning on at once?
3. Where can I find the overall volume and the individual device flow rates? Determine the total flow rate by adding the individual rates of each device you plan to use in parallel.
4. How much hotter is it getting? Calculate the increase in temperature by subtracting the inbound and outbound water temperatures. Remember that the incoming water temperature can change depending on the season and location. (For instance, if the desired output temperature is 120 degrees Fahrenheit but the entering water temperature is only 50 degrees Fahrenheit, the required temperature increase is 70 degrees. Most manufacturers will include a chart or table like the following detailing the range of water flows experienced at a given increase in temperature.

Bathroom (GPM)	0.5- 1.5
Shower (GPM)	1- 2
Kitchen (GPM)	1.5
Dishwasher (GPM)	1.5
Bathtub (GPM)	4
Clothes washer (GPM)	2

FAQ: Hot Water Heater Size Calculator

Which formula should I use to get the optimal water heater capacity?

The following recommendations might be used as a starting point for a rough estimation of the required size.

• 30-40 gallons of water available for 1-2 people.
• 40-50 gallons for 2-3 people.
• 50-60 gallons for 3-4 people.
• 60-80 gallons for 5 people or more.

For a household of five, what size tankless water heater would you recommend?

If you live in the northern area of the United States, where the input water temperature is lower, you will need a 10 GPM gas tankless heater or a 27 kW electric tankless heater to provide adequate hot water for a household of five. The tankless water heater must work harder to heat the water to the desired 110 or 120 degrees Fahrenheit.

Should I install a single huge water heater or two smaller ones?

Multiple, smaller tankless water heaters installed at different points in the home can improve energy efficiency. As a result, there is no need to bring hot water up from the basement, as it can simply be collected from tankless appliances in close proximity.

Conclusion: Hot Water Heater Size Calculator

When it comes time to replace an old water heater or invest in a whole new one, the advice provided above should be invaluable. If you frequently run out of hot water, your family is growing, or you want to install more fixtures, it is a smart idea to purchase a larger tank capacity to accommodate your hot water needs in the future.

The tank capacity and recovery rate (in gallons per hour – GPH) are the primary criteria for sizing tank-style water heaters, while the hot water flow rate is the primary consideration for tankless models (in gallons per minute—GPM). I hope that this hot water heater size calculator would be helpful for you to estimate the right size of your heater.