Short answer
The US Department of Energy sizes a storage water heater by its first hour rating, not by tank gallons alone. You estimate the most hot water your household uses in one busy hour, then look for a model whose first hour rating at least matches it. The calculator below uses the department's own worksheet numbers.
If you are asking what size water heater you need, the honest answer is that it depends on how your household uses hot water in its busiest hour, not on a number printed on a box. This water heater size calculator follows a published federal method, then explains what the result means.
It covers storage tank water heaters, including heat pump water heaters with a tank. Tankless heaters are sized differently, so they get a short section below. If your heater is failing, see how long a water heater lasts and why a water heater is not heating.
Water heater size calculator
Enter how many times each hot water use happens in your busiest hour of the day. The calculator multiplies each count by the gallons per use that the worksheet lists and adds them up.
Method from a published guide, shown as an estimate. Installers and manufacturer labels give the final sizing. Nothing you enter leaves your browser.
The calculator runs in your browser. Nothing you enter is sent anywhere, stored or tracked by the calculator.
Where these numbers come from
The figures are from the US Department of Energy's Energy Saver page titled Sizing a New Water Heater, under the heading for storage and heat pump (with tank) water heaters. Energy Saver's list of related pages dates that page March 26, 2025, and the page itself shows no date. The page includes a worksheet for estimating peak hour demand, and the table below copies its average gallons of hot water per use.
| Use | Average gallons per use |
|---|---|
| Shower | 20 |
| Shaving | 2 |
| Hand dishwashing or food prep | 3 |
| Automatic dishwasher | 7 |
| Clothes washer, top-loader | 25 |
| Clothes washer, horizontal-axis | 15 |
The worksheet's footnote says the estimates are based on averages from a variety of information published on websites, and that some manufacturers also offer calculators based on how long each use lasts. Treat the numbers as averages, not measurements of your own house.
The page also states that the worksheet does not estimate total daily hot water use. It estimates one hour only, the busiest one.
What first hour rating means
The Department of Energy defines the first hour rating as the number of gallons of hot water the heater can supply per hour, starting with a tank full of hot water. It says the rating depends on the tank capacity, the source of heat (burner or element) and the size of the burner or element.
That last point explains why tank gallons alone mislead. Natural Resources Canada's Water Heater Guide (revised October 2012) says the capacity of a gas-fired water heater should be judged by its first hour rating, not its tank size, and notes that because of larger burners some gas water heaters with smaller tanks have a higher first hour rating than models with larger tanks. The same guide says the rating should match or exceed the maximum estimated hot water use in your house in one hour.
Where to find the first hour rating on the EnergyGuide label
The Department of Energy says the EnergyGuide label lists the first hour rating in the top left corner as Capacity (first hour rating). Three limits are worth knowing:
- The Federal Trade Commission requires an EnergyGuide label on all new conventional storage water heaters, but not on heat pump water heaters, according to the same page.
- The same page says product literature from the manufacturer may also give the first hour rating, which is where to look for a heat pump water heater.
A worked example
The Department of Energy page shows its own example: three showers at 20 gallons each, one shave at 2 gallons and one round of hand dishwashing at 3 gallons. The page prints a total peak hour demand of 66 gallons and says that household needs a model with a first hour rating of 66 gallons or more. One thing to know: those three rows add up to 65, not 66. We copy what the page prints, and the one-gallon difference is a reminder that this is an estimate, not an exact figure.
Here is a second example with round, illustrative numbers. Suppose a household has a busy weekday morning in which two people shower, someone runs the automatic dishwasher and someone washes a few dishes by hand:
- 2 showers at 20 gallons: 40 gallons
- 1 automatic dishwasher at 7 gallons: 7 gallons
- 1 round of hand dishwashing at 3 gallons: 3 gallons
The total is 50 gallons. Following the Department of Energy method, that household would look for a first hour rating of at least 50 gallons. The numbers are for practice only. Your own busiest hour may include more showers or a laundry load.
What size water heater do I need? Tank size for a family
Many people want a quick answer such as a tank size for a family of four. The sources used here do not give a gallons-per-person chart for storage tanks. The Department of Energy uses the first hour rating and the peak hour worksheet instead. The only household-size chart on its sizing page is for solar water heating tanks, which is a different system.
What the sources do say is useful:
- A single-family storage water heater holds from 20 to 80 gallons, according to the Department of Energy's storage water heater page.
- ENERGY STAR says that if your current water heater meets your needs most of the time, you can probably replace it with one of the same size.
- ENERGY STAR also says to check the first hour rating, which shows how much hot water the unit can deliver in a busy hour under standard test conditions, and that a higher first hour rating can improve hot water performance without buying a larger tank.
So the practical method is: note how your current heater performs, run the calculator for your busiest hour, and compare the result with the first hour rating of each model you are considering.
How household peaks change the answer
Your peak hour is the hour you use the most hot water, and it often follows a pattern. The Department of Energy says to work out what time of day you use the highest amount, morning, noon or evening, and to keep in mind how many people live in your home. Run the calculator for that hour, not for an average one.
- More people, more overlap. Several showers back to back in the morning is a common peak.
- Guests and big events. The Department of Energy's selection page notes that when more than one use of hot water is occurring, or when there are guests in the home, the requirement for hot water goes up.
- Appliances. A top-loader uses 25 gallons per load in the worksheet, against 15 for a horizontal-axis washer.
The worksheet averages are not the only way to count. The same Natural Resources Canada guide estimates about 30 litres (8 gallons) per shower with energy-saving shower heads, against 20 gallons in the Department of Energy worksheet. Treat any result as a range.
Bigger is not always better
It is tempting to buy the biggest tank you can fit. Both federal sources push back on that.
The Department of Energy's storage water heater page says an oversized unit carries a higher purchase price and increased energy costs due to higher standby energy losses, which is the heat a full tank loses to the air when no tap is running. Natural Resources Canada's guide says the same, and advises against oversizing a tank for an infrequent high hot water need such as a large soaker tub.
The opposite mistake costs you too. An undersized heater runs out of hot water in your busiest hour, which is the problem the calculator is meant to help you avoid. The goal is a first hour rating close to your estimate, not far above it and not below it.
A short note on tankless water heater sizing
A tankless heater has no tank, so it is not sized by first hour rating. The Department of Energy says tankless or demand-type water heaters are rated by the maximum temperature rise possible at a given flow rate. To size one, you work out two numbers:
- Flow rate. List the hot water devices you expect to use at the same time and add their flow rates in gallons per minute. The department's example adds a faucet at 0.75 gallons per minute and a shower head at 2.5 gallons per minute, a total of 3.25. If you do not know a flow rate, it suggests holding a pan or bucket under the tap and measuring a minute.
- Temperature rise. Subtract the incoming water temperature from the temperature you want. Unless you know otherwise, the department assumes incoming water at 50 degrees Fahrenheit. For most uses it suggests water at 120 degrees, a rise of 70 degrees.
ENERGY STAR's page for gas tankless heaters gives average demand of 2.5 gallons per minute for a shower or bathtub, 3.3 for a clothes washer, 2.2 for a sink and 1.3 for a dishwasher. Natural Resources Canada adds that colder winter inlet water lowers what a tankless unit can deliver. Our guide to tankless versus tank water heaters compares the two types in more detail.
When to ask an installer
This calculator gives you a number to bring to the conversation. It does not replace a site visit. Ask a qualified plumbing or heating contractor to confirm the size when:
- You are changing fuel type, for example from electric to gas. The Department of Energy notes that switching fuels can mean adding a breaker, running a gas line or venting the heater outside.
- You have a combination water and space heating system. The department says to consult a qualified contractor for these, including some heat pump systems and tankless coil and indirect water heaters.
- Your hot water use is unusual: a large household, a big tub or frequent guests.
The Department of Energy and ENERGY STAR both advise getting cost estimates in writing, asking for references and asking whether the company will get a local permit if one is needed. See also how to flush a water heater and how to shut off your water.
What to look up where you live
Permit and code rules for water heater replacement are local. Check the current rules for your area with:
- City or county building department. Ask whether a permit is required to replace a water heater and who may pull it.
- State or provincial plumbing or contractor licensing board. Use it to confirm a plumber's licence and check for complaints.
Note for Canadian readers: Natural Resources Canada publishes a Water Heater Guide with the same first hour rating idea, and provinces and municipalities set licensing and permit rules. The worksheet above uses US gallons. Energy costs and regulations differ from the US, so confirm sizing with a licensed contractor in your province.
For plumbing company owners
What size water heater do I need is a search your customers make before they call anyone, and a useful page earns trust early. If you want your company to be the one they find, see our guides to near me searches for plumbers and septic companies and your Google Business Profile, or book a call with LiveLeadsLane. LiveLeadsLane is a marketing practice for contractors, not a directory, and it does not take jobs or refer homeowners.
