Does household size or bathroom count matter more for water heater sizing?

Bathroom count sets the ceiling on how many showers can run simultaneously, since a household cannot use more showers at once than it has bathrooms, while household size determines whether that ceiling is actually reached. A large household with one bathroom has the same peak simultaneous shower demand as a small household with one bathroom; extra people beyond the bathroom count mainly add to a smaller baseline load.

What is the DOE first hour rating worksheet?

It is a standardized method for estimating a household's peak hour hot water demand by summing gallons used per activity, such as 10 gallons per shower or 6 gallons per dishwasher cycle, for whichever activities are likely to overlap during the household's busiest hour. The resulting sum is then compared against a water heater's DOE-tested first hour rating to size a tank correctly.

Should I size a water heater for average daily use or peak hour use?

Peak hour use, not average daily use, since a tank only fails a household during its single busiest, most concentrated hour of hot water draws. Average daily consumption is typically much larger than peak hour demand but is spread across 24 hours of recovery time, which is why sizing against a daily total tends to badly oversize a tank.

How much does one extra bathroom change the recommended tank size?

In this page's worked estimates, each additional bathroom that a household can actually use simultaneously, up to the number of people in the home, adds about 10 gallons to peak hour demand, one additional shower's worth. That 10 gallon jump is often enough to move a household from a 30 gallon tank into a 40 or 50 gallon recommendation.

Does a bathtub change peak hour demand differently than a shower does?

Yes. A bath uses about 20 gallons per the DOE per-activity table, double the 10 gallons a shower uses, so a household that regularly fills a bathtub during its peak hour should substitute that 20 gallon figure for the 10 gallon shower figure used in this page's worked examples, which meaningfully raises the recommended first hour rating.

Sizing

Water Heater Size Chart by Household Size

Researched from the DOE test procedures, NFPA 54 and NEC code tables, and manufacturer specifications. Updated 2026-09-17.

Quick answer

A household of 4 people with 2 bathrooms has an estimated peak hour hot water demand of about 33 gallons, calculated as 13 gallons of baseline hand washing, hand dishwashing and food preparation plus two simultaneous 10 gallon showers. DOE sizing guidance recommends choosing a water heater with a first hour rating at or slightly above that peak hour demand, which points to a labeled first hour rating of at least about 34 gallons for this household.

Sizing a water heater correctly starts with peak hour demand, the total gallons of hot water a household is likely to use during its single busiest hour of the day, not an average daily usage figure. The Department of Energy's first hour rating worksheet builds that peak hour figure from a table of gallons used per activity, then recommends matching a tank's first hour rating to it closely.

This page works that method by hand for household sizes from 1 to 6 people across 1 to 4 bathrooms, so the arithmetic is visible rather than a black box. The peak hour demand calculator and water heater size calculator run the same method for a household's specific mix of fixtures and habits.

What is peak hour demand, and why is it different from daily usage?

Peak hour demand is the total gallons of hot water a household draws during its single most concentrated hour, typically a weekday morning when showers, hand washing and sometimes a dishwasher or clothes washer overlap. A water heater only needs to survive that one busy hour without running cold; total daily hot water use across 24 hours is a much larger number but is almost never the limiting factor, since recovery between busy periods covers the rest of the day.

How many gallons does each activity actually use?

Typical hot water use per activity, from the DOE first hour rating worksheet
Activity	Gallons used
Shower	10 gal
Bath	20 gal
Shampoo hair (at sink)	4 gal
Hands and face washing	4 gal
Shaving	2 gal
Hand dishwashing	4 gal
Automatic dishwasher	6 gal
Food preparation	5 gal
Clothes washer	7 gal

Published standard Source: DOE first hour rating worksheet, the standard per-activity hot water use figures used to build a household peak hour demand estimate.. These are per-use figures. A household's peak hour demand is the sum of whichever activities plausibly overlap during its single busiest hour, not every activity on this list at once.

What is the estimated peak hour demand by household size and bathroom count?

This page assumes up to as many simultaneous showers as there are bathrooms, capped by the number of people in the household, plus a fixed 13 gallon baseline for hands and face washing, hand dishwashing and food preparation that commonly overlaps morning shower time. That baseline is the sum of three rows from the table above: 4 plus 4 plus 5 gallons.

Estimated peak hour demand in gallons, by household size and bathroom count
People	1 bathroom	2 bathrooms	3 bathrooms	4 bathrooms
1	23 gal	23 gal	23 gal	23 gal
2	23 gal	33 gal	33 gal	33 gal
3	23 gal	33 gal	43 gal	43 gal
4	23 gal	33 gal	43 gal	53 gal
5	23 gal	33 gal	43 gal	53 gal
6	23 gal	33 gal	43 gal	53 gal

Convention Source: Calculated as 13 gallons baseline plus 10 gallons per simultaneous shower, where simultaneous showers equal the smaller of bathroom count and household size. This is a worked planning estimate, not a published DOE lookup table.. A household with an unusually heavy morning routine, a bathtub instead of a shower, or a dishwasher and clothes washer both running during the same hour will exceed these figures; add the relevant rows from the per-activity table above.

What tank size actually clears each demand level?

Cross-referencing these demand figures against the first hour rating chart on this site shows how much headroom common tank sizes carry at a typical 90 F rise.

Minimum tank size clearing each demand level, electric 4,500 W element at a 90 F rise
Peak hour demand	Minimum tank clearing it	That tank's estimated FHR
23 gal	30 gal electric	41 gal
33 gal	30 gal electric	41 gal
43 gal	40 gal electric	48 gal
53 gal	50 gal electric	55 gal

Convention Source: Cross-referenced against the electric FHR figures on the first hour rating chart on this site, at a 90 F rise.. A gas tank of smaller capacity often clears the same demand level, since gas burners recover faster; see the gas rows on the first hour rating chart.

Why does adding a bathroom raise recommended tank size more than adding a person does?

Adding a person without adding a bathroom does not increase how many showers can physically run at once, since the number of showerheads is what limits simultaneous draws, not the number of people waiting to use them. Adding a bathroom, on the other hand, directly enables one more simultaneous shower, which is why the table above shows demand rising with bathroom count up to the point where bathroom count meets or exceeds household size, and flattening out after that.

What does the DOE actually recommend once peak hour demand is known?

DOE sizing guidance recommends selecting a storage water heater whose first hour rating is close to, and not below, the calculated peak hour demand, commonly framed as choosing an FHR within about 1 gallon of that figure. Undersizing below peak hour demand means the household runs into cold water before the busy hour ends; oversizing well beyond it mainly adds standby heat loss without adding useful capacity for that busy hour.

Tanks matched to common household sizes

These correspond to the peak hour demand levels worked out below; the first hour rating chart on this site shows exactly which tank clears which demand at a given rise.

Electric, 2 to 4 people

Rheem

Rheem PROE50 M2 50 Gallon

$945.99

A 50 gallon electric tank sized for the mid-range peak hour demand figures common to a 2 to 4 person household.

Best for: A straight 50 gallon electric replacement

Check price on Amazon

Gas, 2 to 4 people

GE RealMAX Choice 40 Gallon Natural Gas

$1,051.49

A 40 gallon gas tank's stronger burner clears the same demand range at a smaller gallon capacity than an electric tank needs.

Best for: A like-for-like swap into an existing masonry or B-vent flue

Check price on Amazon

Heat pump, larger household

Senville

Senville Hybrid Heat Pump 50 Gallon

$1,099.99

A 50 gallon heat pump water heater for a larger household prioritizing lower operating cost over raw recovery speed.

Best for: A first heat pump water heater in a warm garage or basement

Check price on Amazon

Frequently asked questions

What size water heater does a family of 4 need?: A family of four with two bathrooms has an estimated peak hour demand of about 33 gallons, built from a 13 gallon baseline of hand washing, dishwashing and food preparation plus two simultaneous 10 gallon showers. A tank with a first hour rating at or slightly above 33 gallons, such as a 30 gallon electric tank rated near 41 gallons FHR at a 90 F rise, comfortably clears that demand.
Does household size or bathroom count matter more for water heater sizing?: Bathroom count sets the ceiling on how many showers can run simultaneously, since a household cannot use more showers at once than it has bathrooms, while household size determines whether that ceiling is actually reached. A large household with one bathroom has the same peak simultaneous shower demand as a small household with one bathroom; extra people beyond the bathroom count mainly add to a smaller baseline load.
What is the DOE first hour rating worksheet?: It is a standardized method for estimating a household's peak hour hot water demand by summing gallons used per activity, such as 10 gallons per shower or 6 gallons per dishwasher cycle, for whichever activities are likely to overlap during the household's busiest hour. The resulting sum is then compared against a water heater's DOE-tested first hour rating to size a tank correctly.
Should I size a water heater for average daily use or peak hour use?: Peak hour use, not average daily use, since a tank only fails a household during its single busiest, most concentrated hour of hot water draws. Average daily consumption is typically much larger than peak hour demand but is spread across 24 hours of recovery time, which is why sizing against a daily total tends to badly oversize a tank.
How much does one extra bathroom change the recommended tank size?: In this page's worked estimates, each additional bathroom that a household can actually use simultaneously, up to the number of people in the home, adds about 10 gallons to peak hour demand, one additional shower's worth. That 10 gallon jump is often enough to move a household from a 30 gallon tank into a 40 or 50 gallon recommendation.
Does a bathtub change peak hour demand differently than a shower does?: Yes. A bath uses about 20 gallons per the DOE per-activity table, double the 10 gallons a shower uses, so a household that regularly fills a bathtub during its peak hour should substitute that 20 gallon figure for the 10 gallon shower figure used in this page's worked examples, which meaningfully raises the recommended first hour rating.

Work from your own household's actual morning routine, not a generic household-size lookup. A household of four that never runs two showers back to back has a much lower real peak hour demand than the worst-case bathroom-count estimate above, while a household with a heavy laundry and dishwashing overlap can exceed it.