What size water heater do you need?

Q: What size water heater do you need?

Fixr (January 2026) puts a 40-gallon tank at 1 to 4 people; This Old House (June 2026) says 2 to 3 people need 40 to 50 gallons and 3 to 4 need 50 to 60. For the same family of four, one rule says 40 gallons and the other says 60. Headcount is a poor guide, because what empties a tank is simultaneous use. The number that settles it is the first-hour rating.

The two published rules, side by side

Tank size	Fixr, household size	This Old House, household size
30 gallons	1 to 2 people	part of the 1 to 2 person range
40 gallons	1 to 4 people	2 to 3 people (40 to 50)
50 gallons	3 to 5 people	3 to 4 people (50 to 60)
60 gallons	not listed	3 to 4 people
75 gallons	4 to 6 people	5 or more people
80 gallons	6 or more people	5 or more people

For a household of four, Fixr says a 40-gallon tank is enough and This Old House points at 50 to 60. That is a full size apart, and the price difference between them is real: $550 to $2,300 against $700 to $2,500 on Fixr's own figures.

Fixr adds a simple increment rule: start at 30 gallons for one to two people and add about 10 gallons for every additional one to two people.

Why headcount is the wrong measure

Both rules count people. What actually empties a tank is how much hot water is drawn at once.

Two people who shower one after the other at 7am put a harder load on a tank than four people spread across the morning. A large soaking tub can take most of a 50-gallon tank on its own. Fixr names exactly these cases as the reason to go bigger at the 50-gallon size: a large soaking tub, multiple high-flow showerheads, or running hot-water appliances at the same time.

Fixr gives a useful reference point: a 50-gallon tank delivers roughly 50 to 60 minutes of continuous warm water at a standard shower flow, and a 40-gallon tank 40 to 50 minutes. That is more than 50 gallons, because what comes out of the showerhead is tank water mixed with cold.

The number that settles it: first-hour rating

Fixr's second sizing method is the one to use, and it is the one almost no quote mentions.

First-hour rating (FHR) is how many gallons of hot water a unit can deliver in one busy hour, starting from a full tank. It combines the tank's capacity with how fast the burner or element can reheat. Fixr's guidance: work out your peak hour of hot water use, then choose a unit whose FHR meets or slightly exceeds it.

Why it beats gallons. Two 50-gallon tanks with different burner ratings have different first-hour ratings. A high-recovery 50-gallon gas unit can out-deliver a 65-gallon electric one in a busy hour, because gas reheats faster: Fixr puts the heat-up time at 30 to 40 minutes for gas and up to 60 minutes for electric.

Our practical reading, stated as such: if you run out of hot water at a predictable time each day rather than during unusual demand, the problem is recovery rate, not capacity, and a bigger tank of the same type may not fix it. Neither source puts it this way; the inputs above are theirs.

Sizing a tankless unit instead

Tankless units are not sized in gallons at all. They are sized in flow rate, gallons per minute.

Home	Flow rate	Unit cost	Source
Single faucet or appliance	1 to 2 GPM	$150 to $500	This Old House
Small apartment or condo	2 to 4 GPM	$500 to $1,000	This Old House
2 to 3 bedroom home	5 to 7 GPM	$1,000 to $2,000	This Old House
4+ bedroom home	8 to 10 GPM	$1,500 to $3,000	This Old House

This Old House also frames it by bathrooms: 1 to 2 bathrooms need 2 to 3 GPM, medium homes 4 to 6 GPM, large homes with multiple bathrooms at least 7 GPM. Fixr's rule is different again: start at about 5 GPM and add roughly 1 GPM per household member.

Note that the two framings conflict for a small household in a large house. We report both rather than picking.

Getting it wrong in either direction costs money

Too small. Fixr states that an undersized unit runs constantly to keep up, which shortens its life as well as producing cold showers.

Too big. Fixr states that an oversized unit keeps an unnecessarily large volume of water hot at all times, which wastes energy. On its own operating figures, a 75-gallon electric tank costs $700 to $1,100 a year to run against $600 to $750 for a 50-gallon one. Over a 10-year life that is $1,000 to $3,500 of standing heat loss you bought by accident.

The upgrade itself is cheap by comparison. Going from 40 to 50 gallons costs roughly $150 to $200 more on Fixr's size table. Going from 50 to 75 costs $400 to $900 more, plus higher running costs for the life of the unit.

What to ask the contractor

Ask for the first-hour rating of the unit being quoted, not just the gallons. Then describe your worst hour honestly: how many showers, back to back or spread out, plus the dishwasher or the washing machine if they run then. A contractor who can answer that question with a number is sizing the unit; one who only asks how many people live there is applying a rule of thumb that, as the table at the top shows, disagrees with itself.

Sources

Price figures are national US ranges from the sources above as of the date shown. Your local price depends on labor rates, permits and site conditions. Read how we get our numbers.