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.