What Size Water Heater Do I Need? Size by the Busiest Hour

By Ray Bennett · Updated 2 October 2026 · 6 min read · Checked against the sources at the end

What Size Water Heater Do I Need? Size by the Busiest Hour

Short answer

Size a tank water heater by your busiest hour of hot water, not by the number of bedrooms. A.O. Smith counts about 20 gallons a shower, 7 for a dishwasher and 20 to 25 for a load of laundry. Add up yours and pick a heater whose first hour rating matches it or a little more. A tankless heater is sized by gallons a minute instead.

What size water heater do I need? A.O. Smith, Rheem and Bradford White all answer it the same way: count the hot water your household uses in its busiest hour, then choose a heater that can deliver that much.

Their shortcuts by household size are another matter. They disagree with each other, and Rheem’s two guides disagree with each other too. This page works through the count, the rating to match it to, and what changes the answer.

Before you start

What to carry: a torch, a pen and a tape measure, for the label on the old heater and the space it sits in. Twenty minutes.

Gas or electric? Either. The counting is the same, but compare first hour ratings, not tank sizes. Rheem says two gas heaters with the same tank can deliver different amounts of hot water, depending on the burner.

Measure the space. A.O. Smith says most tanks stand 50 to 70 inches tall and 20 to 30 inches across, and need a drip tray under them and room for the pipes.

Three sizing mistakes the makers warn about. Sizing by bedrooms. A.O. Smith says bedroom count is not the best way to size a water heater, because a three bedroom home could have two people or five.

Trusting the rating to the gallon. Bradford White says to choose a first hour rating above your busiest hour, because the rating comes from a simulated test and is not exactly what the heater will deliver.

Going smaller with a heat pump. Rheem says not to go smaller when switching to a heat pump water heater, and recommends going up a tank size, because it heats more slowly.

Add up your busiest hour

Two showers and the dishes, counted with A.O. Smith's figures. Count only what happens in that one hour.

Check 1Free, ten minutes

Count the gallons

The figures. A.O. Smith counts about 20 gallons for a shower, more for a long one, half the tub for a bath, 3 gallons for washing dishes by hand, 7 for a dishwasher and 20 to 25 for a load of laundry.

Only that hour. A.O. Smith says to count only the water used during the peak hour. For a big family that is usually bath time, and for a small one the dishwasher and the washing machine can matter more.

A worked example. Rheem’s own: two showers at 30 gallons between them, a load of laundry at 20 and 5 in the kitchen make 55 gallons, so it says to look for a first hour rating of at least 55.

Match it to the first hour rating

Check 2

Read the first hour rating, not the tank size

What it means. A.O. Smith says the first hour rating is how much hot water a tank can deliver in the first hour of heavy use, starting full. It adds that the heater keeps heating as you use it, which is why the rating is higher than the tank size.

How close to go. A.O. Smith says the same as your busiest hour or slightly larger. Bradford White says above it.

Too small. A.O. Smith says the water runs cold at times. Rheem lists the signs: hot water running out before everyone has showered, the temperature swinging at busy times, and waiting for the heater to recover: how long a water heater takes to heat up.

Too big. A.O. Smith and Bradford White both say an oversized heater costs more to run. Rheem says a heater far too big costs more to buy and runs less efficiently, but it tells people it is better to size slightly over than under.

The rules of thumb, and why they disagree

The maker’s guide	Small household	Middle	Large
A.O. Smith sizing guide	Up to 2 people: 30 to 40 gallons	3 people: 40 to 50; 4 people: 50 to 75	5 people: 75 to 100
Rheem, 2025	1 or 2 people: 50 to 60	3 or 4 people: 50 to 65	5 or more: up to 80
Rheem gas guide, 2026	1 or 2 people: 30 to 40	2 to 4 people: 40 to 50; 4 or 5: 50 to 55	5 or more: 75, or tankless
Bradford White	Not given	A typical 3 person home: 40 may do	Not given

For one or two people the answers run from 30 to 60 gallons. That is why every one of these guides comes back to the busiest hour. A.O. Smith’s own examples: 40 gallons suits one or two people with average use, and 50 is a comfortable fit for many families of four, depending on habits.

What changes the answer

Check 3

Adjust for the house, not just the people

Where you live. Rheem says the water coming into the house is colder in some places than others. A home in Florida can get by with a smaller tank, and a colder place needs more.

Big tubs and big showers. Rheem and Bradford White both say soaking tubs and showers with several heads use far more hot water, and may need a larger tank or a tankless heater.

The years ahead. Rheem says to think about how long you will stay and size for the household you expect, not only the one you have.

Waiting for hot water is not size. A.O. Smith says no water heater on its own delivers instant hot water. The distance to the tap, cold pipes and a recirculation system matter instead.

A tank that used to be enough. If yours suddenly runs out sooner than it did, look for a fault before buying bigger: a shower that goes cold.

Sizing a tankless heater

A tankless heater has no tank to run out of. It is sized by the flow of everything that runs at the same time.

Check 4

Add up the flow, in gallons a minute

What runs at once. A.O. Smith says to list every tap and appliance that uses hot water at the same time. As a rough guide it gives 0.75 gallons a minute for a tap and 2.5 for a normal shower head. A bucket and a watch measure your own.

The heater’s range. A.O. Smith says tankless heaters are rated from about 2.5 to 11 gallons a minute. Bradford White says the size depends on the flow and on the temperature rise, the gap between the water coming in and the water you want.

Too small. Rheem says a tankless heater does not run out, but it can slow to a trickle if it is too small for the demand. Whether to go tankless at all: tankless vs tank.

The supply. A.O. Smith gives gas tankless burners of 140,000 to 199,000 BTU an hour and electric ones of 14 to 32 kilowatts, so ask the installer whether the gas line or the electrical supply can feed one: tankless water heater problems.

If the heater you have is getting old, its age is in the serial number: how old is my water heater.

Questions people ask

What size water heater do I need for a family of 4?

A.O. Smith says 50 gallons is a comfortable fit for many families of four, depending on habits. Its sizing table gives 50 to 75. Count your busiest hour to be sure.

Is a 40 gallon water heater big enough?

A.O. Smith says 40 gallons works well for one or two people with average use. Bradford White says it might satisfy a typical household of three.

How many showers will a 50 gallon water heater give?

A.O. Smith says a standard shower uses about 20 gallons, so a 50 gallon tank can usually manage two to three in a row before running low.

What is a first hour rating?

How much hot water a tank heater can deliver in the first hour of use, starting full. It counts the stored water and what the burner or elements heat in that hour.

Is a bigger water heater better?

Not by much. A.O. Smith says an oversized one costs more to run. Rheem says slightly oversized is fine, and far oversized costs more to buy and to run.

What size tankless water heater do I need?

Add up the gallons a minute of everything that runs at once, then allow for how cold your incoming water gets. A.O. Smith gives 2.5 for a shower and 0.75 for a tap.

Sources: A.O. Smith "What Size Water Heater Is Right for You?" and "What Size Water Heater Do You Need For Your Home?", Rheem "Understanding Water Tank Size: How to Choose What You Really Need" (2025) and "What Size Gas Water Heater Do I Need for My Home?" (2026), and Bradford White "What Size Water Heater Do You Need?"