Water heater size calculator

How the demand and the rating to look for are worked

A storage water heater is sized by its first hour rating, the gallons of hot water it can supply in an hour that starts with a full tank of hot water. DOE's worksheet gives the demand to match it against. Count the hot water uses in the busiest single hour of the home, multiply each by DOE's average gallons for that use, and add. The rating of the heater should be at least that many gallons.

Gallons per use

gallons = uses x average gallons per useDOE's worksheet lists 20 gallons for a shower, 2 for a shave, 3 for hand dishwashing or food prep and 7 for an automatic dishwasher load. A clothes washer load is 25 gallons in a top-loader and 15 in a front-loader (H-axis). DOE calls these estimates based on averages from a variety of published information. Enter your own gallons per shower if you know them.

Peak hour demand

demand = sum of (uses x gallons per use)DOE's example has 3 showers, 1 shave and 1 round of hand dishwashing in the busiest hour. That is 60 + 2 + 3 = 65 gal. DOE's page prints 66 gallons for this example. Its own line items add to 65 (3 x 20 + 2 + 3), and this page adds the line items.

Minimum first hour rating

first hour rating >= demandDOE says to look for a model with a first hour rating of the demand or more. For the example the line items give 65 gallons or more, where DOE's page prints 66. A rating above the demand leaves room, and a rating below it is short.

First hour rating

gallons per hour, starting with a full tankDOE defines it as the gallons of hot water the heater can supply per hour, starting with a tank full of hot water. It depends on the tank capacity, the source of heat (burner or element) and the size of the burner or element. 10 CFR 430 Appendix E calls it an estimate of the maximum volume of hot water a non-flow activated water heater can supply within an hour that begins fully heated.

Where the rating is printed

EnergyGuide label, top left cornerThe label lists it as "Capacity (first hour rating)". DOE says the Federal Trade Commission requires that label on new conventional storage water heaters but not on heat pump water heaters. Product literature from the maker may also give the rating. Look there first.

Test conditions

supply 58°F, delivery 125°F10 CFR 430 Appendix E holds the supply water at 58°F ± 2°F (section 2.3) and sets delivery at 125°F ± 5°F (section 2.4). A home with colder supply water, or one that needs water hotter than 125°F at the tap, gets less hot water in the hour than the label rating. Your house may differ.

Test draw pattern

bin from Appendix E, Table I, by first hour ratingSection 5.4.1, Table I picks the draw pattern for the 24-hour test from the rating. In that table, under 18 gallons is Very-Small-Usage, 18 up to under 51 gallons is Low-Usage, 51 up to under 75 gallons is Medium-Usage, 75 gallons or more is High-Usage. The example's 65 gallons falls in Medium-Usage. That bin describes the test and is not a size recommendation. It is a label for the test.

Model check

covers it if rating >= demand, otherwise short by demand - ratingWith a model's rating entered, 66 gal against the example's 65 gal covers it with 1 gal to spare, and 63 gal is short by 2 gal. UF/IFAS EDIS suggests a rating that matches within 1 to 2 gallons of the peak-hour demand. This page flags a model that close and still reports it short, since DOE's rule is the demand or more. Close is not enough.

Seven cases through the same formula

The first row is DOE's worksheet example, and the page gives 65 gal, where DOE prints 66. Rows four and five are the same busy morning with a front-loader and with a top-loader, 69 gal and 79 gal. The last row applies a shower of 15 gal to the DOE example and drops it to 50 gal. That shower figure is an example chosen for this page, and the other rows use DOE's gallons.

The demand is also the lowest rating to look for. A model rated at the demand or above covers it, and one below is short, even by a gallon. UF/IFAS suggests a rating within 1 to 2 gallons of the demand, so the page notes a model that close and still reports it short.

DOE worksheet used by the form

Average gallons of hot water per use as DOE lists them in the archived copy of its page from July 2, 2026. DOE says they are estimates based on averages from a variety of published information. Treat them as typical. Change them if yours differ.

Draw pattern by first hour rating

Table I in section 5.4.1 of 10 CFR 430 Subpart B Appendix E picks the draw pattern for the 24-hour test from the first-hour rating. The bin describes the test, and it is not a size recommendation.

Standards and data sources

• U.S. Department of Energy, Energy Saver, Sizing a New Water Heater (archived copy of July 2, 2026) - the first hour rating definition, the peak hour demand worksheet with its average gallons per use, the worked example, and the advice to choose a model with a rating of the demand or more. On October 26, 2026 the live page returned a not-found error, so this is the archived copy of DOE's page from July 2, 2026. Its example prints 66 gallons, and its own lines add to 65.
• 10 CFR Part 430, Subpart B, Appendix E (eCFR) - the definition of first-hour rating (section 1.5), the 58°F supply and 125°F delivery test conditions (sections 2.3 and 2.4), and the draw pattern bins by first-hour rating (section 5.4.1, Table I).
• University of Florida IFAS Extension, EDIS FY1025, Energy Efficient Homes: Water Heaters - the advice to look for a tank-style water heater with a first hour rating that matches within 1 to 2 gallons of the peak-hour demand. This page shows it as a note on a model that is that close and does not use it as the pass rule.

Limitations of this calculator

• This page gives a preliminary rating to shop for. The installer, the water heater maker's literature and the local authority having jurisdiction decide the final selection and the installation.
• The gallons per use are DOE's. DOE says they are estimates based on averages from a variety of information published on websites. A home with longer showers or a different dishwasher will differ, so enter your own gallons per shower when you know them. The other uses use DOE's figures as they are.
• The count is yours. This page cannot know how many showers start in your busiest hour, and DOE leaves that to you. Pick the time of day with the highest use, and keep the number of people in the home in mind. A count that is too low gives a rating that is too low.
• The worksheet covers one hour only. DOE notes that it does not estimate total daily hot water use, and this page does not either.
• A first hour rating is measured at 58°F supply water and 125°F delivered water (10 CFR 430 Appendix E, 2.3 and 2.4). Colder supply water, or water needed hotter than 125°F at the tap, gives less hot water in the hour than the label rating. The page does not adjust the rating for either.
• The page does not turn a rating into a tank size. DOE says the rating depends on the tank capacity, the source of heat and the size of the burner or element, so two models with the same tank can carry different ratings. Compare the rating on the label.
• Tank-style heaters only. A tankless or demand-type heater has no first hour rating and is sized by flow and temperature rise, which the tankless water heater size calculator works.
• Combination space and water heating systems and solar water heating systems are not covered. Their makers or installers size them. DOE notes that solar contractors use worksheets and computer programs for that job.
• A heat pump water heater with a tank is sized by the same rating, as DOE's page says. The EnergyGuide label is not required on heat pump water heaters, so take the rating from the maker's literature.
• Counts are accepted from 0 to 20 for each use. A demand over 200 gal gets a warning, because the sources here work household examples of tens of gallons and do not cover a load that large.

Common mistakes when sizing a water heater

1. Shopping by tank gallons. A bigger tank does not always mean a higher first hour rating. DOE says the rating depends on the tank capacity, the source of heat and the size of the burner or element. Read the rating on the EnergyGuide label, and compare it with the demand, which is 65 gal by the line items of DOE's example.
2. Counting the whole day. The demand is for the busiest single hour. DOE's example is 3 showers, 1 shave and 1 round of hand dishwashing, 65 gal by its line items. Adding every use from morning to night would give a larger number that no hour needs, and the worksheet does not estimate daily use.
3. Swapping the washer type. DOE counts 25 gal for a top-loader load and 15 gal for a front-loader. In a busy morning of 2 showers, 2 shaves, 1 hand dishwashing and 1 dishwasher load, the front-loader gives 69 gal and the top-loader gives 79 gal. The second one falls in High-Usage at 75 gallons and up.
4. Taking a rating a few gallons short as a match. DOE's wording is a rating of the demand or more. UF/IFAS puts it as matching within 1 to 2 gallons. With a demand of 65 gal, a 63 gal model is short by 2 gal and a 62 gal model by 3 gal. This page reports both as short. Ask the installer whether the counts are generous enough to accept it.
5. Keeping 20 gallons for a different shower. In DOE's example the showers are 60 gal of the 65 gal, at DOE's average of 20 gal a shower. At 15 gal a shower, the DOE example drops from 65 gal to 50 gal. Change it only for a figure you know, since a guess low gives a rating that is too low.
6. Looking for an EnergyGuide label on a heat pump water heater. DOE says the Federal Trade Commission requires the EnergyGuide label on new conventional storage water heaters but not on heat pump water heaters. The first hour rating of a heat pump model with a tank is in the maker's product literature, and the same rule applies to it.

Common questions about water heater size

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Used on the same job

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