What size water heater do I need for a family of four?

A. O. Smith's guide works a family of four with two full baths, a dishwasher and a clothes washer: 90 gallons over a two hour peak, met by a 50 gallon tank with at least 27,225 Btu/h of gas input at a 90°F rise. In Georgia a three bedroom, two bath house needs a first hour rating of at least 60 gallons.

What is a first hour rating?

The federal test procedure's estimate of the most hot water a storage heater can supply in an hour starting fully heated. It is printed on the EnergyGuide label and decides which daily draw pattern the heater is rated on: under 18 gallons very small, 18 to 51 low, 51 to 75 medium and 75 and up high.

Does gas or electric change the size I need?

Not in Georgia's table, where every gas minimum equals its electric minimum. In A. O. Smith's method the hot water needed is the same, but an electric element recovers far more slowly than a gas burner, so an electric heater usually needs more storage to meet the same peak.

How much hot water does a shower use?

A. O. Smith's guide does not count showers; it counts 20 gallons of 140°F water for each of the first two people and 5 for each after that over a two hour peak. For flow, federal law limits showerheads to 2.5 gallons a minute at 80 psi.

How do I size a tankless water heater?

Add the flow of everything that will run at once and find the temperature rise from the coldest incoming water. Two showers and a faucet at federal maximum flows are 7.2 gpm. Then choose a model whose chart delivers that flow at that rise. Georgia's table sends tankless heaters to the manufacturer.

Is the A. O. Smith sizing guide wrong?

Its electric worked example subtracts 41.0 from 55.0 and prints 11.0 gallons short, where the answer is 14.0. Its electric recovery table is also built on about 8.33 lb a gallon while its gas table and formula use 8.25. The method itself is sound.

How much does a new water heater cost?

HomeAdvisor's June 2026 guide gives $600 to $2,500 for a tank unit, $150 to $450 for replacement labor, and $1,400 to $3,900 for a tankless heater including installation, with a permit $25 to $300.

How much does a tankless water heater cost?

HomeAdvisor's June 2026 guide gives $1,400 to $3,900 for a tankless water heater with installation, of which labor alone is $600 to $1,900. A tank unit is $600 to $2,500 before $150 to $450 of labor. The guide does not price brands separately.

How much does a gas or electric tankless hot water heater cost?

HomeAdvisor's June 2026 guide gives $1,400 to $3,900 for a tankless hot water heater with installation, of which labor is $600 to $1,900. It does not split gas from electric units. HomeAdvisor's June 2026 guide gives $1,400 to $3,900 per job, the same range.

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Water Heater Size Calculator

Georgia Table 506, the federal first hour rating rule and A. O. Smith's sizing guide

Georgia is the rare state whose plumbing code sizes a house's water heater, by first hour rating, and its table prints a gas column and an electric column that are identical on every row. A. O. Smith's sizing guide works the same question over a two hour peak, and in its own worked example subtracts 41 gallons from 55 and gets 11. This calculator runs both, shows where each lands, and checks a tank and burner against the peak.

Size a water heater three ways at once

Bedrooms

Baths

People in the house

people

Full baths

baths

Dishwasher

Clothes washer

Fuel

Tank size (0 for tankless)

gal

Gas input (gas only)

Btu/h

Largest element (electric only)

Temperature rise

°F

Showers running at once (for tankless)

no.

Faucets running at once (for tankless)

no.

Heaters in the job (for the price table below)

no.

First hour rating is printed on the heater's EnergyGuide label and specification sheet. Temperature rise is the heater's outlet temperature less the coldest incoming water; Rheem's winter rules of thumb for incoming water are 40°F in the northern states, 50°F in most of the South and 60°F in Southern California, the Southwest and Gulf states.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

Two hour peak against storage and recovery

Water heater prices

As each row states
Item	Low	High	Unit
Tank water heater, the unit	$600	$2,500	unit only
Tank replacement labor	$150	$450	labor only
Tankless water heater	$1,400	$3,900	unit and installation
Tankless labor	$600	$1,900	labor only
Permit	$25	$300	added

Source: HomeAdvisor, how much does a water heater replacement cost. Last price check: September 13, 2026.

The source's own words set what each row covers: tank units range $600 to $2,500 by size, fuel and brand; labor alone is $150 to $450 for a tank and $600 to $1,900 for a tankless heater; tankless is $1,400 to $3,900 for the unit and installation. It also prints a table by tank size reaching $7,500 for 80 gallons without saying what that covers, which does not sit inside its own $600 to $2,500 unit range, so that table is left out. Its headline reads 2025 on a page dated June 2026.

Worked example

A. O. Smith's own design case: a family of four in a house with two full baths, a dishwasher and a clothes washer, on a 50 gallon tank at a 90°F rise. In Georgia it would be three bedrooms and two baths.

Georgia. Table 506, three bedrooms, 2 to 2 1/2 baths: at least 60 gal first hour rating, gas or electric.
Two hour peak. 20 + 20 for two people, 5 + 5 for two more, 10 for the second bath, 10 dishwasher, 20 washer: 90 gal.
From the tank. 70 percent of 50: 35 gal, leaving 55 gal for recovery, 27.5 an hour.
Gas. 27.5 × 8.25 × 90 ÷ 0.75 = 27,225 Btu/h at least. A 40,000 Btu/h burner makes 40.4 gal an hour: enough.
Electric, 4,500 W. The guide's table gives 20.5 gal an hour, 41.0 in two hours. 55.0 − 41.0 = 14.0 gal short, not the 11.0 the guide prints.
Fix the shortfall. 14.0 ÷ 0.7 = 20 gal more tank, where the guide's figure gives 15.7.

The calculator uses the guide's printed formula for recovery rather than its electric table, so its electric figure is 20.7 gallons an hour and the shortfall 13.6. Either way the electric example is short by more than the guide says.

The formula

Storage and recovery together have to cover the peak. A. O. Smith's formulas:

usable = tank × 0.7 gas recovery (gal/h) = Btu/h × 0.75 ÷ (8.25 × rise) electric recovery = W × 3.413 ÷ (8.25 × rise)

0.7: draw efficiency: the share of a tank usable before incoming cold water cools it too far
0.75: recovery efficiency assumed for a gas heater; 1.0 for electric
8.25: pounds per gallon in the guide's formula; its electric table uses about 8.33
rise: outlet temperature less the coldest incoming water, °F
first hour rating: the federal test's hot water in the first hour from a full tank; the figure Georgia's table sets

First hour rating and A. O. Smith's two hour peak answer different questions. The rating is a tested property of a heater. The peak is an estimate of a household. A heater can meet one and not the other.

Why this page uses three documents

Water heater calculators usually count people and suggest a tank size. Nothing in that step is written down anywhere a reader can check.

Georgia writes a minimum into its plumbing code, the federal test procedure defines the rating that minimum is set in, and A. O. Smith publishes the peak demand method with worked examples. All three are primary and checkable, and they measure different things.

Reading them closely turned up an identical pair of columns in the state table, a wrong subtraction in the manufacturer's electric example and two recovery tables built on different gallons. The Department of Energy's own consumer sizing pages returned 404 and were not used. The site publishes no price of its own.

Georgia Table 506 minimum first hour ratings
Baths	Bedrooms	Gas, FHR gal	Electric, FHR gal	Federal draw pattern it is rated on
1 to 1 1/2	1	40	40	low usage, 38 gal a day
1 to 1 1/2	2	45	45	low usage, 38 gal a day
1 to 1 1/2	3	48	48	low usage, 38 gal a day
2 to 2 1/2	2	47	47	low usage, 38 gal a day
2 to 2 1/2	3	60	60	medium usage, 55 gal a day
2 to 2 1/2	4	62	62	medium usage, 55 gal a day
2 to 2 1/2	5	70	70	medium usage, 55 gal a day
3 to 3 1/2	3	60	60	medium usage, 55 gal a day
3 to 3 1/2	4	67	67	medium usage, 55 gal a day
3 to 3 1/2	5	70	70	medium usage, 55 gal a day
3 to 3 1/2	6	72	72	medium usage, 55 gal a day

Read 2026-09-13. Georgia State Amendments to the IPC, revised January 1, 2026. Draw pattern column from 10 CFR 430 Appendix E Table I.

Download this table as CSV. Same cells as printed above, nothing added.

The one state table that sizes a water heater, and its identical columns

The International Plumbing Code does not size a house's water heater. Georgia adds a section that does:

Water heaters installed in residential occupancies shall be sized in accordance with Table 506 or the manufacturer's recommendations. The water heater must at a minimum meet the First Hour Rating (FHR) requirements of Table 506.

Georgia Table 506, as amended for 2026, with the federal draw pattern each rating falls in
Baths	Bedrooms	Gas, FHR gal	Electric, FHR gal	Federal draw pattern it is rated on
1 to 1 1/2	1	40	40	low usage, 38 gal a day
1 to 1 1/2	2	45	45	low usage, 38 gal a day
1 to 1 1/2	3	48	48	low usage, 38 gal a day
2 to 2 1/2	2	47	47	low usage, 38 gal a day
2 to 2 1/2	3	60	60	medium usage, 55 gal a day
2 to 2 1/2	4	62	62	medium usage, 55 gal a day
2 to 2 1/2	5	70	70	medium usage, 55 gal a day
3 to 3 1/2	3	60	60	medium usage, 55 gal a day
3 to 3 1/2	4	67	67	medium usage, 55 gal a day
3 to 3 1/2	5	70	70	medium usage, 55 gal a day
3 to 3 1/2	6	72	72	medium usage, 55 gal a day

Every gas figure equals its electric figure. The table is laid out in fuel pairs and carries no difference between them anywhere, so the fuel you pick does not change the minimum.

The third bath adds nothing at three or five bedrooms. Three bedrooms is 60 gallons with two baths or three; five bedrooms is 70 either way. At four bedrooms the third bath adds 5. A one bath house with four or more bedrooms has no figure at all, and neither does anything beyond six bedrooms or three and a half baths, which go to the manufacturer.

The rating the table asks for is a federal test number. First hour rating is defined in the Department of Energy's test procedure as “the maximum volume of "hot" water that a non-flow activated water heater can supply within an hour that begins with the water heater fully heated”. The same procedure decides which daily draw pattern a heater is rated on, and every Georgia minimum from 51 gallons up puts a heater on the medium usage pattern of 55 gallons a day.

The manufacturer's worked example, and the subtraction in it

A. O. Smith's residential sizing guide works in a two hour peak rather than one hour:

Residential peak usage, based on accepted practice, is the two hour period during the day when the heaviest draw of hot water will occur.

It counts 20 gallons of 140°F water for each of the first two people, 5 for each person after that, 10 for each full bath after the first, 10 for a dishwasher and 20 for a clothes washer. A family of four with two baths and both machines comes to 90 gallons. It takes 70 percent of the tank as usable, so a 50 gallon tank gives 35, and the burner or elements must make the other 55 in two hours.

For the electric heater the guide gets the shortfall wrong. A 4,500 W element recovers 20.5 gallons an hour on its table, 41.0 gallons in two hours. The guide prints:

11.0 gallons of hot water "short" two hour peak

55.0 less 41.0 is 14.0. The next line tells you how to fix a shortfall:

To allow for draw efficiency, divide the "shortage" by .7 when increasing tank size.

On the printed 11.0 that is 15.7 gallons more tank. On the correct 14.0 it is 20.0. The worked example undersizes its own correction by about four gallons.

The two recovery tables also use different water. The guide's glossary formula divides by 8.25 lb a gallon. Put every cell through it: the gas table matches on 55 of 55, the electric table on 6 of 70. At 8.33 lb, the usual weight of water, the electric table matches on 65 of 70. It is a one percent difference and it is the reason the electric example reads 20.5 gallons an hour where the formula in the same guide gives 20.7.

Tank size is not first hour rating

A tank's gallons and its first hour rating are different numbers. The rating counts what the heater delivers in an hour starting full, so it includes what the burner or elements make during that hour.

Georgia sets the rating, not the tank. A heater with a smaller tank and a larger burner can meet Table 506 where a bigger, slower tank does not.

The label shows both. The EnergyGuide label and the specification sheet print the first hour rating; the tank size is on the model number and the data plate.

Tankless: flow at once and temperature rise

A tankless heater has no storage to draw on, so it is sized on the flow running at the same moment and how far it has to heat it. Georgia's Table 506 says so in its first footnote and sends tankless heaters to the manufacturer.

The flows are federal maximums. A showerhead may use no more than 2.5 gallons a minute at 80 psi and a lavatory or kitchen faucet no more than 2.2 at 60 psi. Georgia's own fixture table is lower for faucets, 1.5 gpm for a private lavatory and 2.0 for a sink, so a Georgia house built to its code draws less than the federal ceiling.

The rise is set by the incoming water in winter. Rheem's rules of thumb are 40°F for the northern tier of states, 50°F for most of the South and 60°F year round in Southern California, the Southwest and Gulf states. The calculator turns flow and rise into heat output with A. O. Smith's formula; the manufacturer's chart of flow at a given rise is what decides the model.

The sources, and the one that disappeared

Read 2026-09-13.

Georgia State Amendments to the International Plumbing Code (2024 Edition), Georgia Department of Community Affairs, revised January 1, 2026. Section 506 and Table 506, minimum first hour rating by bedrooms and baths, and amended Table 604.4 fixture flow rates. The 2018-edition amendments, revised January 1, 2020, print Table 506 identically.

10 CFR Appendix E to Subpart B of Part 430, Uniform Test Method for Measuring the Energy Consumption of Water Heaters, US Department of Energy test procedure, via the Legal Information Institute. Section 1 definition of first-hour rating, Table I draw pattern by first-hour rating, and the daily volumes of Tables III.1 to III.4.

10 CFR 430.32, Energy and water conservation standards, US Department of Energy, via the Legal Information Institute. Paragraph (p), showerheads 2.5 gpm at 80 psi; paragraph (o), lavatory and kitchen faucets 2.2 gpm at 60 psi.

Residential Sizing, A. O. Smith, AOSSG88150, revised June 2010, on the manufacturer's own document store. Design factors for two hour peak usage, tank draw efficiency, gas and electric recovery tables, worked examples and glossary formulas.

How to size a water heater, Rheem Manufacturing Company. Winter incoming water temperature rules of thumb by region, and the flow rate method for tankless heaters.

The Department of Energy's consumer sizing page was not used. Its Energy Saver pages on sizing a new water heater and on tankless heaters, cited by other publications including a University of Florida extension paper, returned HTTP 404 from energy.gov when this page was written. The federal test procedure itself is still published and is quoted instead.

Outside Georgia, no code table may apply, and the manufacturer's sizing governs. A. O. Smith states that its factors may be adjusted to suit individual needs.

Frequently asked questions

What size water heater do I need for a family of four?: A. O. Smith's guide works a family of four with two full baths, a dishwasher and a clothes washer: 90 gallons over a two hour peak, met by a 50 gallon tank with at least 27,225 Btu/h of gas input at a 90°F rise. In Georgia a three bedroom, two bath house needs a first hour rating of at least 60 gallons.
What is a first hour rating?: The federal test procedure's estimate of the most hot water a storage heater can supply in an hour starting fully heated. It is printed on the EnergyGuide label and decides which daily draw pattern the heater is rated on: under 18 gallons very small, 18 to 51 low, 51 to 75 medium and 75 and up high.
Does gas or electric change the size I need?: Not in Georgia's table, where every gas minimum equals its electric minimum. In A. O. Smith's method the hot water needed is the same, but an electric element recovers far more slowly than a gas burner, so an electric heater usually needs more storage to meet the same peak.
How much hot water does a shower use?: A. O. Smith's guide does not count showers; it counts 20 gallons of 140°F water for each of the first two people and 5 for each after that over a two hour peak. For flow, federal law limits showerheads to 2.5 gallons a minute at 80 psi.
How do I size a tankless water heater?: Add the flow of everything that will run at once and find the temperature rise from the coldest incoming water. Two showers and a faucet at federal maximum flows are 7.2 gpm. Then choose a model whose chart delivers that flow at that rise. Georgia's table sends tankless heaters to the manufacturer.
Is the A. O. Smith sizing guide wrong?: Its electric worked example subtracts 41.0 from 55.0 and prints 11.0 gallons short, where the answer is 14.0. Its electric recovery table is also built on about 8.33 lb a gallon while its gas table and formula use 8.25. The method itself is sound.
What temperature rise should I use?: The outlet temperature less the coldest incoming water. Rheem's rules of thumb for winter incoming water are 40°F in the northern states, 50°F in most of the South and 60°F year round in Southern California, the Southwest and Gulf states. A. O. Smith's example uses a 90°F rise.
How much does a new water heater cost?: HomeAdvisor's June 2026 guide gives $600 to $2,500 for a tank unit, $150 to $450 for replacement labor, and $1,400 to $3,900 for a tankless heater including installation, with a permit $25 to $300.
How much does a tankless water heater cost?: HomeAdvisor's June 2026 guide gives $1,400 to $3,900 for a tankless water heater with installation, of which labor alone is $600 to $1,900. A tank unit is $600 to $2,500 before $150 to $450 of labor. The guide does not price brands separately.
How much does a gas or electric tankless hot water heater cost?: HomeAdvisor's June 2026 guide gives $1,400 to $3,900 for a tankless hot water heater with installation, of which labor is $600 to $1,900. It does not split gas from electric units. HomeAdvisor's June 2026 guide gives $1,400 to $3,900 per job, the same range.

Check these numbers yourself

Georgia State Amendments to the International Plumbing Code (2024 Edition)Georgia Department of Community Affairs, revised January 1, 2026, read 2026-09-13. Section 506 and Table 506, and amended Table 604.4.
10 CFR Appendix E to Subpart B of Part 430US Department of Energy uniform test method for water heaters, via LII, read 2026-09-13. First-hour rating definition, Table I and daily draw volumes.
10 CFR 430.32US Department of Energy conservation standards, via LII, read 2026-09-13. Showerhead and faucet maximum flow.
Residential Sizing, AOSSG88150A. O. Smith, revised June 2010, read 2026-09-13. Design factors, recovery tables, worked examples and glossary.
How to size a water heaterRheem Manufacturing Company, read 2026-09-13. Winter incoming water temperatures by region.
How Much Does a Water Heater Replacement Cost?HomeAdvisor staff writer, reviewed by Jeff Botelho, updated 2026-06-17, read 2026-09-13.

How every figure here is verified: Sources & Method. Who builds this: Lucas Pradella, and the method is on About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-13. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Georgia Table 506 is quoted from the state's own amendments, read on 13 September 2026, and applies only where Georgia's code does. A. O. Smith's factors are a manufacturer's design guide and, in its own words, may be adjusted to suit individual needs. Gas and electrical work on water heaters is commonly permit work. Prices are HomeAdvisor's, not this site's.