Water Heater Size Calculator

How water heater sizing works

Most people buy a water heater by tank size, and that is the wrong number. The number that matters is the first hour rating, or FHR, which is how many gallons of hot water the unit can deliver in the busiest hour of the day when the tank starts full. The US Department of Energy's guide to supplying hot water in the home puts it plainly: pick a model by its first hour rating, not by how many gallons the tank holds.

Two things build that rating. The first is how much hot water your household draws in its peak hour. The second is how fast the unit can reheat water while it is being drawn, because a tank with fast recovery delivers more hot water in an hour than the same tank with a slow heating element.

To estimate your peak hour, count what runs at the same time. A shower draws roughly 10 to 20 gallons, a dishwasher cycle about 6 gallons, and a load of laundry about 7 gallons. Add the ones that overlap in your busiest hour and you have the demand the heater has to cover.

This calculator starts from a rule contractors use every day: about 12 gallons of hot water per person during the busiest hour. It adds the dishwasher and the laundry when you say those run in that hour, applies a safety margin, and turns the result into a first hour rating to shop for. Everything runs in your browser, so nothing you type is uploaded.

Worked example: a family of four

Four people, two full bathrooms, and the dishwasher running during the morning rush.

• Four people x 12 gallons: 48 gallons
• Dishwasher in the same hour: +6 gallons
• Peak hour demand: 54 gallons
• Safety margin of 30 percent: 70.2 gallons
• First hour rating to shop for: 75 gallons

That rating points to a 50-gallon gas tank, which recovers about 40 gallons an hour and so carries an FHR near 75. On electricity the same household needs a bigger tank, around 80 gallons, because an electric element reheats at roughly half the rate of a gas burner. If you would rather go tankless, the household needs a unit rated near 7.5 gallons per minute.

Daily use for that household works out at about 112 gallons of hot water, which is the figure to use when you compare running costs between models.

Change one input and the answer moves. Take away the dishwasher and the peak hour drops to 48 gallons, the rating to 65, and the gas tank to 40 gallons. Add a load of laundry to the same hour and the peak climbs to 61 gallons, the rating to 80, and the gas tank to 60 gallons.

Tank, tankless or heat pump

A storage tank heats a body of water and keeps it ready. Its first hour rating comes from two parts: the hot water already sitting in the tank, and the water the burner or element can reheat while you draw. A gas burner typically reheats about 40 gallons an hour and a standard electric element about 20, which is why the same household needs a larger electric tank for the same job.

A tankless unit skips the tank and heats water as it flows. Size it by flow rate instead of volume, because it can only heat so many gallons per minute. The calculator reports the flow rate your household needs, using a shower at 2.5 gallons a minute, a dishwasher at 1.5, a washer at 2.0, plus about a gallon for the sinks. A unit has to hold that flow at your incoming water temperature, and cold ground water in a northern winter can cut the rated flow by a third or more.

A heat pump water heater is an electric storage tank with a compressor instead of a large element. It uses far less electricity, but it reheats slowly, so it usually needs a larger tank than a plain electric model. The electric column in the results is the right starting point for both, and the ENERGY STAR water heater pages list the models that meet the efficiency bar.

Water heater size chart by household

These are the figures this calculator returns with no dishwasher and no laundry in the peak hour, so one shower runs at a time. Add the extras in the calculator for a busier morning.

Read the table across, not down. The gas and electric columns differ because the two fuels reheat at different speeds. A 30-gallon gas tank and a 30-gallon electric tank deliver different amounts of hot water in an hour, and for a household of 4 that gap widens to a 50-gallon gas tank against a 66-gallon electric one.

Why the first hour rating matters more than the tank size

Two 50-gallon tanks can deliver different amounts of hot water in an hour. A 50-gallon gas tank that recovers 40 gallons an hour carries a first hour rating near 75 gallons. A 50-gallon electric tank that recovers 20 gallons an hour carries a rating closer to 55. Same tank, same price bracket, 20 gallons an hour of difference, and that difference is what a household feels when the third person steps into the shower.

So when you compare models, read the yellow EnergyGuide label and find the first hour rating line. Match it to your peak hour demand with a small margin. The tank size printed in the product name tells you how much water is stored, not how much you can use.

There is a second reason to shop by rating. Recovery rate depends on the burner or element size, and manufacturers sell the same tank shell with different burners. Two products that both say 50 gallons can sit $150 apart in price and 15 gallons apart in hourly delivery. The label settles it.

What this estimate leaves out

Four things can move the answer. The temperature of your incoming water matters a great deal. Ground water runs near 75 F in south Florida and near 40 F in Minnesota in winter, and cold incoming water slows every gallon the heater produces, so a northern home often needs the larger end of the range.

Your habits matter as much as your headcount. Two people who take 20-minute showers use more hot water than four people who take 5-minute showers. A soaking tub can drain a tank by itself. A recirculation loop, which keeps hot water at the tap, raises the load. A household that spreads its hot water across the day needs less peak capacity than one that puts everything into a single morning hour.

Climate also decides which technology fits. A heat pump water heater pulls heat from the air around it, so it works best in a warm basement or garage, and it cools the space it sits in. In a cold climate that becomes a drawback in winter, and the unit may need a backup element on the coldest days.

Finally, the calculator does not price anything. It gives you a first hour rating, a tank size and a flow rate. With those in hand, compare models on the EnergyGuide label and check the ENERGY STAR list for efficiency. A licensed plumber should confirm the final choice, the venting, and the gas or electrical supply before you buy.

This is an estimate, not plumbing advice. Product ratings and code requirements change without notice. Confirm the final size with the manufacturer or a qualified installer.