Water Heater Size Calculator

Size a water heater by first hour rating from the showers, baths, dishwasher and laundry loads in your busiest hour, with gas, electric and tankless recommendations.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To size a storage water heater, add up the hot water drawn in your busiest hour (about 20 gal per shower, 14 per dishwasher load, 32 per laundry load) to get the first hour rating you need, then choose a tank whose rating meets it. Two showers, a dishwasher load, a laundry load, two hand washes and a shave in one hour need a 96 gal first hour rating, which calls for a 75 gal gas tank, and a tankless heater would have to deliver 6.5 gpm at a 70 F rise.

Results

First hour rating needed: 96 gal; Hot water drawn in the busiest hour; look for at least this first hour rating on the yellow EnergyGuide label
Gas tank: 75 gal gas; A 75 gal gas tank typically has a first hour rating of about 125 gal
Electric tank: Two tanks or tankless; Above an 80 gal electric tank (about 90 gal first hour rating); consider two tanks, a tankless unit or spreading the loads out
Tankless flow needed: 6.5 gpm; At a 70 F temperature rise (50 F incoming to 120 F)
Daily hot water use: 80 gal; About 20 gal per person per day for 4 people
Planning estimate only: Verify with a plumber; A licensed plumber sizes the heater to local code, fuel supply, venting and the recovery rate of the model you buy

Water Heater Size formula

First hour rating = Showers x 20 + Baths x 20 + Dishwasher loads x 14 + Laundry loads x 32 + Hand washes x 4 + Shaves x 2
Tank size = smallest tank whose typical first hour rating meets the need (40 gal gas about 70, 50 gal gas about 85, 50 gal electric about 60, 80 gal electric about 90)
Tankless flow (gpm) = Showers x 2.5 + 1.5 for one faucet;  Temperature rise = 120 F - incoming water temperature
Daily use = People x 20 gal

Worked example

A three-person home in a cold climate where the peak hour has 1 shower, 1 bath, 1 dishwasher load, no laundry, 2 hand washes and 1 shave: 20 + 20 + 14 + 0 + 8 + 2 = 64 gal first hour rating. A 40 gal gas tank (about 70 gal FHR) or a 65 gal electric tank (about 75 gal FHR) covers it; a 50 gal electric tank at about 60 gal would run short.

For a tankless unit, 1 shower x 2.5 gpm + 1.5 gpm for a faucet = 4.0 gpm at a 120 - 40 = 80 F rise, which is a large gas tankless in a cold climate. Daily use is about 3 x 20 = 60 gal of hot water.

Quick reference

First hour rating by peak hour use, with the tank that meets it
Peak hour use	First hour rating	Gas tank	Electric tank
1 shower, 2 hand washes, 1 shave	30 gal	40 gal	50 gal
2 showers, 1 dishwasher load	54 gal	40 gal	50 gal
2 showers, 1 laundry load	72 gal	50 gal	65 gal
2 showers, dishwasher, laundry	86 gal	75 gal	80 gal
2 showers, dishwasher, laundry, 2 hand washes, 1 shave	96 gal	75 gal	Two tanks or tankless
3 showers, 1 bath, 1 laundry load	112 gal	75 gal	Two tanks or tankless

Sizing a storage tank by first hour rating

Tank capacity in gallons is not the number that matters; first hour rating is. It combines the stored hot water with how fast the burner or element reheats, which is why a 40 gal gas tank can deliver about 70 gal in an hour while a 50 gal electric tank delivers about 60. Write down everything that draws hot water in your busiest hour, usually the morning shower rush or the evening with dishes and laundry running, add the gallons, and buy a heater whose label meets that figure.

If the total lands above what one tank can supply, the cheapest fix is often to move laundry or the dishwasher to a different hour, which drops the demand by 32 or 14 gal at no cost. Two tanks in series or a tankless unit are the hardware answers for a large household that cannot spread its use out.

Tankless sizing and the plumber's role

Tankless heaters are sized by flow at a temperature rise, not by gallons. Two showers running together need about 5 gpm, plus 1.5 gpm for a faucet. In a moderate climate the incoming water is around 50 F, so reaching 120 F is a 70 F rise; in the north it is 80 F and the same unit delivers less flow. A large gas tankless delivers about 5 to 7 gpm at a 70 F rise, and electric units rarely exceed 3 to 4 gpm, so cold-climate homes often need two units or a gas model.

This calculator is a planning estimate. A licensed plumber checks the gas line capacity, venting, electrical service (a 4,500 W element or a 27 kW electric tankless), expansion tank and local code requirements, and confirms the size against the manufacturer's rating for the model you are buying.

Key facts

The first hour rating (FHR) on the yellow EnergyGuide label is how many gallons of hot water a full tank delivers in an hour of steady use; the DOE recommends a heater whose FHR is within 1 or 2 gallons of your peak hour demand.
The DOE peak hour worksheet counts 20 gal per shower or bath, 14 gal per dishwasher load, 32 gal per clothes washer load, 4 gal per hand and face washing and 2 gal per shave.
A 50 gal gas tank recovers roughly twice as fast as a 50 gal electric tank, because a 40,000 BTU burner puts in about 11.7 kW of heat against 4.5 kW for a standard electric element, so its first hour rating is higher.
Federal law limits showerheads to 2.5 gpm, and the DOE recommends a 120 F thermostat setting to limit scalding and standby heat loss.
Tankless heaters are rated by gpm at a temperature rise; a unit that delivers 7 gpm at a 40 F rise may deliver only about 4 gpm at a 70 F rise.

Frequently asked questions

What size water heater does a family of four need?

It depends on the peak hour. Two back-to-back showers and a dishwasher load need a 54 gal first hour rating, which a 40 gal gas or 50 gal electric tank meets. Add a laundry load in the same hour and the need is 86 gal, so a 75 gal gas or 80 gal electric; moving the laundry to another hour lets the smaller tank work.

What is first hour rating?

The gallons of hot water a fully heated tank can deliver in one hour of continuous use, printed on the yellow EnergyGuide label. It counts both the stored water and the recovery from the burner or element, so match it to your peak hour demand rather than the tank size alone.

How many gpm tankless water heater do I need?

Count 2.5 gpm per shower running at once plus 1.5 gpm for a faucet, and note the temperature rise. Two showers need 6.5 gpm, at a 70 F rise in a moderate climate or an 80 F rise in a cold one; a unit's gpm rating drops as the rise increases.

Is a 40 gallon water heater enough?

A 40 gal gas tank has a first hour rating around 70 gal, enough for two showers, a dishwasher load and a couple of hand washes (62 gal) in one hour. A 40 gal electric tank has a lower rating, so it suits one or two people or a home that spreads its hot water use across the day.

Sources and references

Worksheet for Estimating Peak Hour Demand / First Hour Rating (gallons per shower, bath, dishwasher and clothes washer) (AHRI)
DHW First Hour Rating sizing tool, Building America Solution Center (Pacific Northwest National Laboratory, U.S. Department of Energy)
How to size a water heater (peak demand, showerhead flow and incoming water temperature) (Rheem)
WaterSense Showerheads (2.5 gpm federal standard) (EPA)

Next steps for this project

The usual order for a insulation & hvac project. See all insulation & hvac calculators.

Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.