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Water Heater Sizing Calculator

Find the tank capacity or tankless flow rate a household actually needs from peak-hour demand.

What are you sizing?

Storage tankSized by first-hour rating against peak-hour demandTanklessSized by simultaneous flow at your temperature rise

Peak-hour usage

Count what happens in your busiest hour, not your whole day. Four back-to-back showers before work is the demand that sizes the heater.

Showers

20 gal each — 10 minutes at 2.0 gpm

Tub baths

25 gal each — Filled tub, tub filler flow

Lavatory or hand washing

4 gal each — Shaving, washing up

Kitchen sink use

4 gal each — Food prep, hand dishwashing

Dishwasher cycles

6 gal each — Modern unit, hot fill

Warm laundry loads

20 gal each — Front loader, warm wash

Water temperature

Region

Sets a typical incoming temperature. Override it below if you have measured yours.

Incoming water temperature

°F

Measure at an outside hose bibb in the coldest month.

Target delivery temperature

°F

120°F is the standard setting. Above 120 needs a mixing valve to prevent scalding.

Fuel and technology

Fastest recovery, so a smaller tank delivers the same first-hour rating.

Recommended tank

40 galNatural gas or propane

Delivers a first-hour rating of about 68 gal against a peak-hour demand of 58 gal.

Peak-hour demandHot water used in your busiest hour: 58 gal
First-hour rating to look forPrinted on the EnergyGuide label: 65 gal
Recovery at this fuelThrough a 90°F rise, the rating basis: 40 gal/hr
Energy to heat one tank70°F rise from 50°F to 120°F: 23 kBTU

Compare first-hour rating, not tank size

Two 40 gallon heaters can have very different first-hour ratings depending on burner or element capacity. You need a unit whose label shows at least 65 gallons in the first hour — that figure, not the gallon capacity on the side of the tank, is what determines whether you run out.

Peak-hour demand, draw by draw
Draw	Count	Gallons each	Gallons
Showers	2	20	40
Lavatory or hand washing	2	4	8
Kitchen sink use	1	4	4
Dishwasher cycles	1	6	6
Peak-hour demand			58 gal

Gas, electric and heat pump at a 65 gallon first-hour rating
Technology	Recovery	Tank needed	Stocked size	First-hour rating
Natural gas or propane — selected	40 gal/hr	36 gal	40 gal	68 gal
Electric resistance	20 gal/hr	64 gal	65 gal	66 gal
Heat pump (hybrid)	20 gal/hr	64 gal	65 gal	66 gal

What this assumes

Tank sizing is based on first-hour rating against peak-hour demand, the method used by the U.S. Department of Energy.
Tankless sizing uses simultaneous fixture flow and the required temperature rise from incoming groundwater temperature.
Mixed-use fixture flows assume modern low-flow fixtures unless you state otherwise.

Important

Switching fuel types, going tankless, or moving to a heat pump water heater frequently requires new gas, venting or electrical work that costs more than the appliance. Price the whole installation, not the unit.

Understanding the result

Peak hour, not daily total

A water heater is sized for the busiest hour of the day, not for daily consumption. Four back-to-back showers before work is the demand that matters, and it is measured against the unit's first-hour rating — how much hot water it can deliver in one hour starting fully heated.

Tankless is sized by temperature rise

A tankless unit's advertised flow rate assumes a specific temperature rise. In Minnesota, incoming water near 40°F requires a 65–80°F rise, which can cut a unit's usable output nearly in half compared with the same unit in Florida. Always size against your local groundwater temperature.

Heat pump water heaters change the math

A heat pump water heater uses roughly a third of the electricity of a resistance unit, but needs about 700–1,000 cubic feet of surrounding air space, produces cool exhaust air, and recovers more slowly. They work well in garages and basements and poorly in small closets.

Common questions

Is a 40 or 50 gallon water heater better?

It depends on peak-hour demand, not household size alone. A two-bathroom home where showers overlap often needs 50 gallons; the same household showering at staggered times is well served by 40. Compare first-hour rating against your calculated peak hour.

Is tankless worth it?

Tankless units last longer and never run out, but installation frequently requires a larger gas line, new venting, or a significant electrical upgrade. The payback depends far more on installation cost than on energy savings.

You may also need

The next decision usually follows directly from this one.

Related trades

PlumberInstall and repair the water supply, drainage, gas and fixture systems that keep buildings habitable.