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 11, 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.

Your measurements

A column or tube: volume is the circular area (from the diameter) times the height.

Showers in the peak hourAbout 20 gal of hot water each, roughly 8 minutes at a 2.5 gpm showerhead.Baths in the peak hourAbout 20 gal each.Dishwasher loads14 gal per load.Laundry loads (warm or hot)32 gal per load.Hand and face washing4 gal each.Shaving2 gal each.People in the householdUsed for the daily use estimate at about 20 gal of hot water per person.Incoming water temperatureGroundwater, not tap temperature: about 40 F across the northern tier in winter, 50 F mid-country, 60 F and up in the Gulf South. A tankless loses about a third of its flow going from a 60 F rise to an 80 F rise.

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First hour rating needed96 galAll results

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; the 2015 DOE standard makes a residential tank over 55 gal a condensing power-vent unit, so most 75 gal tanks sold are light-duty commercial atmospheric models at 76,000 BTU
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). Warning: that takes about 227,500 BTU/h, more than one 199,000 BTU gas tankless delivers; plan two units or drop a shower from the peak
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, gas line capacity, venting and the first hour rating printed on 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. Know what the sizes above 55 gal really are. Since the 2015 DOE standard an electric tank over 55 gal is a heat pump unit, and a residential gas tank over 55 gal must be condensing. The 75 gal atmospheric tank at the supply house is therefore a light-duty commercial model rated at 76,000 BTU.

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 199,000 BTU gas tankless delivers about 5 gpm at a 70 F rise and 4.7 gpm at 80 F. A 27 kW electric tankless gives about 2.6 gpm at 70 F and needs three 40 A circuits; 36 kW gives 3.5 gpm on four 40 A circuits and usually a 200 A service.

The fuel supply is where tankless jobs go wrong. A 199,000 BTU unit needs a 3/4 in gas line sized for the whole house at that input (IRC Table G2413.4(1)) and often a larger meter. The 40,000 BTU tank it replaces ran on 1/2 in pipe. Venting is Category III stainless or the maker's PVC, with a condensate drain on condensing models. A gas tank in a closet needs two combustion air openings of 1 sq in per 1,000 BTU (IRC G2407.6.1) or a direct-vent model. Tanks last 10 to 12 years; the anode rod is gone by year 5 in soft water and a new one buys time.

What this calculator leaves out

The result is a first hour rating. Not included: the 2 gal expansion tank, a drain pan with a 3/4 in drain where a leak would damage the house (IRC P2801.6), the T&P discharge line and seismic straps. Also a chimney liner or new vent, gas line and meter upsizing, and the 10 AWG 30 A circuit for a 4,500 W element. Also the plumbing permit, typically $50 to $150, and hauling the old tank, which most suppliers do for a delivery fee.

Code limits that apply

IRC Table G2413.4(1)
Gas piping sized for every appliance on the line; a 199,000 BTU tankless needs 3/4 in or larger
If you miss it: A tankless starved on the old 1/2 in line throws a flame-failure code every time the furnace fires with it
IRC G2407.6.1
Gas tank in a closet gets two combustion air openings of 1 sq in per 1,000 BTU, high and low
If you miss it: A starved burner soots and backdrafts CO into the closet; the CO alarm or the final inspection catches it
IRC P2903.4
Thermal expansion tank on a closed system with a PRV, check valve or backflow preventer
If you miss it: The T&P valve weeps on every heating cycle and the relief line drips until the plumber comes back
IRC P2804.6.1
T&P discharge line full 3/4 in, no valve or threaded end, ending under 6 in above the floor
If you miss it: A capped or reduced relief line turns the tank into a pressure vessel; a plugged outlet is the classic inspection fail
IRC P2801.6
Drain pan with a 3/4 in drain under any heater where a leak would damage the house
If you miss it: A tank in an attic or hall closet fails at year 12 and the leak comes through the ceiling below
IRC P2801.8
Two seismic straps, upper and lower third of the tank, in Seismic Design Category D
If you miss it: A single-strap kit fails the plumbing final in California, and an unstrapped tank tears its gas line loose in a quake

Provision	Limit	If you miss it
IRC Table G2413.4(1)	Gas piping sized for every appliance on the line; a 199,000 BTU tankless needs 3/4 in or larger	A tankless starved on the old 1/2 in line throws a flame-failure code every time the furnace fires with it
IRC G2407.6.1	Gas tank in a closet gets two combustion air openings of 1 sq in per 1,000 BTU, high and low	A starved burner soots and backdrafts CO into the closet; the CO alarm or the final inspection catches it
IRC P2903.4	Thermal expansion tank on a closed system with a PRV, check valve or backflow preventer	The T&P valve weeps on every heating cycle and the relief line drips until the plumber comes back
IRC P2804.6.1	T&P discharge line full 3/4 in, no valve or threaded end, ending under 6 in above the floor	A capped or reduced relief line turns the tank into a pressure vessel; a plugged outlet is the classic inspection fail
IRC P2801.6	Drain pan with a 3/4 in drain under any heater where a leak would damage the house	A tank in an attic or hall closet fails at year 12 and the leak comes through the ceiling below
IRC P2801.8	Two seismic straps, upper and lower third of the tank, in Seismic Design Category D	A single-strap kit fails the plumbing final in California, and an unstrapped tank tears its gas line loose in a quake

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

A closed system (PRV or check valve at the meter) needs a 2 gal expansion tank, IRC P2903.4; without it the T&P valve weeps.
The T&P discharge line is 3/4 in, no valve, no threaded end, ending under 6 in above the floor or pan (IRC P2804.6.1).
In Seismic Design Category D (California, Pacific Northwest) the tank needs two straps, upper and lower third, per IRC P2801.8; inspectors fail single-strap kits.
A gas heater left alone on a masonry chimney after a furnace upgrade needs a liner; the oversized flue condenses and backdrafts CO.
Post-2015 tanks are about 2 in wider and 2 to 4 in taller than the one they replace; measure the closet and the door first.
Replacement needs a plumbing permit in most jurisdictions, typically $50 to $150; the inspector checks the T&P line, expansion tank, straps and venting.

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.

Do I need an expansion tank when I replace my water heater?

Yes if the house is a closed system, which means a pressure reducing valve, backflow preventer or check valve at the meter; most utilities install one now. IRC P2903.4 requires it. A 2 gal tank goes on the cold inlet, pre-charged to house pressure (typically 50 to 60 psi) and supported, not hanging on the pipe. Without it the T&P valve weeps on every heating cycle.

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)

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