Water Heater Size Calculator
What size water heater do you need? Count the hot water your home uses in its busiest hour. We turn that into the first-hour rating and the tank size to look for, for gas, electric or heat pump.
How to use it
- Enter how many people live in the home. We fill in a typical busy hour.
- Change the numbers to match your real busiest hour, usually a weekday morning.
- Pick gas, electric or heat pump.
- Look for a heater whose first-hour rating is at or just above your total.
Why tank size starts with your busiest hour
Tank water heaters are not sized by the number of bathrooms or the size of the house. They are sized by how much hot water you use in the one hour of the day when everyone needs it at once. In most homes that is the morning rush: two or three showers, someone washing up at the sink, and maybe the dishwasher running.
The number that tells you whether a heater can keep up is its first-hour rating. It is the number of gallons of hot water the heater can supply in one hour, starting full. The U.S. Department of Energy suggests choosing a heater with a first-hour rating within 1 or 2 gallons of your peak-hour use. Tank size in gallons matters less than most people think, because a heater refills with hot water while you use it.
How much hot water each use takes
The calculator uses these average amounts of hot water for each activity, the same figures the Department of Energy uses in its sizing worksheet. Low-flow showerheads and newer appliances may use less.
| Use | Hot water per use |
|---|---|
| Shower | About 10 gallons |
| Bath (tub filled) | About 20 gallons |
| Shaving | About 2 gallons |
| Hand and face washing | About 4 gallons |
| Hair shampoo at the sink | About 4 gallons |
| Washing dishes by hand | About 4 gallons |
| Dishwasher load | About 6 gallons |
| Food prep | About 5 gallons |
| Clothes washer load (warm or hot) | About 7 gallons |
Example: a family of four with three showers, one hand wash and the dishwasher in the same morning hour uses about 30 + 4 + 6 = 40 gallons. They should look for a first-hour rating of at least 40 gallons.
Gas, electric or heat pump
The fuel changes how big the tank needs to be for the same first-hour rating. A gas burner heats water faster than electric elements, so a gas tank refills faster. That is why a 40-gallon gas tank can deliver about as much in an hour as a 50-gallon electric tank. The calculator allows for this when it suggests a starting tank size.
- Gas or propane: fast recovery, so a smaller tank often covers the same busy hour.
- Electric: slower recovery, so most homes go one size up compared to gas.
- Heat pump: very efficient, but slower to recover in heat pump mode. Many people choose one size larger, and the unit needs space and air around it.
Always check the first-hour rating on the yellow EnergyGuide label or the spec sheet before you buy. Two 50-gallon heaters can have very different ratings.
Signs your current water heater is the wrong size
- The last person to shower in the morning gets lukewarm water.
- You cannot run the dishwasher and a shower at the same time.
- Hot water runs out sooner than it used to. This can also be sediment or a failed part, not the size. Our guide to a water heater leaking from the bottom covers the warning signs of a worn tank.
- Your family has grown since the heater was put in.
If your heater used to keep up and no longer does, it may need a repair, not a bigger tank. Our water heater repair service can test the burner, elements and thermostat before you replace anything.
Tank or tankless?
Tankless water heaters do not store water, so the first-hour rating does not apply. They are sized by flow rate: how many gallons per minute they can heat at once. If you are weighing the two, try our tankless water heater sizing calculator, and see our tankless water heater service for repair and install questions. You can find all our free plumbing tools in one place.
Water Heater Size FAQs
What size water heater do I need for a family of 4?
Many families of four use a 50-gallon tank, gas or electric. The better test is your busiest hour: if two or three showers run back to back with the dishwasher or a load of laundry, look for a first-hour rating that matches that total. Use the calculator above to get the number.
What is a first-hour rating?
The first-hour rating (FHR) is how many gallons of hot water a tank heater can deliver in one hour, starting with a full tank. It is printed on the yellow EnergyGuide label and in the spec sheet. The U.S. Department of Energy suggests choosing a heater with a first-hour rating within 1 or 2 gallons of your peak-hour demand.
Is a bigger water heater always better?
No. A tank that is too big keeps more water hot all day and costs more to run. A tank that is too small runs out of hot water. Matching the first-hour rating to your busiest hour gives you enough hot water without paying to heat water you never use.
Why does a gas tank deliver more hot water than an electric one of the same size?
A gas burner heats water faster than electric elements, so a gas tank refills with hot water faster during the hour. That is why a 40-gallon gas tank often has a higher first-hour rating than a 50-gallon electric one.
Should I switch to tankless instead?
Tankless heaters are sized by flow rate, not gallons. If you are thinking about it, use our tankless water heater sizing calculator to see the flow rate and gas or electric capacity your home would need.
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