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

A four person household typically uses around 54 gallons of hot water in its busiest hour, counting four showers, four hand and face washes, two shampoos and a dishwasher run. That asks for a first hour rating near 54 gallons, which a 50 gallon tank with an ordinary recovery rate delivers. The gallons on the tank matter less than the first hour rating on the label.

Is a bigger water heater always better?

No. A tank larger than the household needs pays standby loss every hour of every day to keep water hot that nobody draws, and on a heat pump water heater a larger tank also needs more clearance and more air volume around it. Size to the peak hour demand and then check that the space and the recovery rate suit the household, rather than buying headroom you never use.

What is the difference between tank size and first hour rating?

Tank size is how much water the vessel holds. First hour rating is how many gallons of hot water the heater actually delivers in one hour starting from full, which is the usable part of the storage plus everything the burner or element can reheat during that hour. First hour rating is the number that decides whether you run out, and it is the one printed on the EnergyGuide label.

Does the number of bathrooms change the size I need?

It changes the peak, not the total. Two bathrooms mean two showers can run at once, which concentrates the same daily use into a shorter window, and a concentrated draw is exactly what a first hour rating measures. A household that showers one after another all morning needs less first hour rating than the same household showering simultaneously.

Sizing calculator

Water Heater Size Calculator

Quick answer

Size a water heater on its first hour rating, not its gallons. A four person home with two bathrooms uses about 54 gallons of hot water in its busiest hour, which asks for a first hour rating near 54 and lands on a 50 gallon tank with a normal recovery rate.

Result

Peak hour demand 70 gal Published standard

First hour rating to shop for 70 gal Published standard

Tank size that usually delivers it 75 gal Convention

Shop on the first hour rating printed on the yellow EnergyGuide label, not on the gallons. Two 50 gallon tanks can differ by 20 gallons of first hour rating because the burner or element rating differs.

The formula

Peak hour demand = sum of (activity gallons x how many times in the busiest hour). Target first hour rating = peak hour demand.

The gallons per activity come from the DOE and Energy Star first hour rating worksheet and are gallons of HOT water at the tap, not mixed water. Matching the first hour rating to the peak hour demand is the published method. The tank size suggested underneath it is a convention, because a 50 gallon tank with a fast burner can out-deliver a 65 gallon tank with a slow one.

The figures above are computed with these exact inputs: People in the home 4, Bathrooms 2.

What this calculator assumes

Some of these are published test procedures or code tables and some are planning conventions. They are not the same kind of number, so each one is labeled.

Assumption	Kind	Where it comes from
Gallons per activity from the DOE and Energy Star first hour rating worksheet	Published standard	Gallons per activity from the DOE and Energy Star first hour rating worksheet
Usable Fraction: 0.7	Convention	Planning figure for the fraction of a tank delivered at the stored temperature before the outlet cools. The DOE first hour rating is a measured test value and governs where one is published
Water Weight: 8.33	Published standard	Published density of water at 60 F, 8.33 lb per gallon

Gear that matches this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Intermediate

Rheem

Rheem PROE50 M2 50 Gallon

$945.99

The professional line 50 gallon electric, which is the single most common residential water heater size in the country.

Best for: A straight 50 gallon electric replacement

Check price on Amazon

Beginner

GE 40 Gallon Lowboy

$522.00

A lowboy is short and wide, which is the answer when the tank has to fit under a stair or in a crawlspace and the tall version physically will not go in.

Best for: Height restricted closets and crawlspaces

Check price on Amazon

Beginner

GE 30 Gallon Electric

$576.00

Thirty gallons suits one or two people. Going smaller than the household needs is the cheapest mistake to make and the most annoying to live with.

Best for: One or two people, or an apartment

Check price on Amazon

Beginner

GE Smart 40 Gallon Short

$621.00

A ten year warranty usually means a thicker anode rod rather than a better tank, and the anode is a part you can replace yourself for about thirty dollars.

Best for: A short-height 40 gallon slot

Check price on Amazon

Intermediate

GE Smart 40 Gallon Short, 12 Year

$675.00

The twelve year version of the same tank. Compare the price difference against the cost of one anode rod before paying for it.

Best for: Hard water areas where the anode does real work

Check price on Amazon

Expert

Bradford White

Bradford White RE340T6 40 Gallon

$1,003.13

Bradford White sells through plumbing supply rather than retail, so this is the tank a plumber is most likely to already stock parts for.

Best for: Anyone who wants the plumber to have parts on the truck

Check price on Amazon

Frequently asked questions

What size water heater do I need for a family of four?: A four person household typically uses around 54 gallons of hot water in its busiest hour, counting four showers, four hand and face washes, two shampoos and a dishwasher run. That asks for a first hour rating near 54 gallons, which a 50 gallon tank with an ordinary recovery rate delivers. The gallons on the tank matter less than the first hour rating on the label.
Is a bigger water heater always better?: No. A tank larger than the household needs pays standby loss every hour of every day to keep water hot that nobody draws, and on a heat pump water heater a larger tank also needs more clearance and more air volume around it. Size to the peak hour demand and then check that the space and the recovery rate suit the household, rather than buying headroom you never use.
What is the difference between tank size and first hour rating?: Tank size is how much water the vessel holds. First hour rating is how many gallons of hot water the heater actually delivers in one hour starting from full, which is the usable part of the storage plus everything the burner or element can reheat during that hour. First hour rating is the number that decides whether you run out, and it is the one printed on the EnergyGuide label.
Does the number of bathrooms change the size I need?: It changes the peak, not the total. Two bathrooms mean two showers can run at once, which concentrates the same daily use into a shorter window, and a concentrated draw is exactly what a first hour rating measures. A household that showers one after another all morning needs less first hour rating than the same household showering simultaneously.

Researched guidance, not professional advice. These results are planning figures from published test procedures, code tables and manufacturer specifications, and they do not replace a sizing done by the person who will sign the permit. A water heater is a pressure vessel: never cap, plug or remove the temperature and pressure relief valve, and replace one that will not open when the test lever is lifted. Gas line, flue and vent work belongs to a licensed plumber or gas fitter, because improper venting produces carbon monoxide. Water at 140 F scalds a child in seconds. Turn off the breaker before touching an element, and never energise an element that is not submerged.