Water Heater Size Calculator

Please fill in the required fields with values greater than 0.
–tank size (gallons)
–peak hot water flow (gpm)
–temperature rise (F)
–tankless gas size (BTU/hr)

Rule-of-thumb estimate. Flow uses 2.5 gpm per shower, 1.5 gpm for a dishwasher or sink and 2 gpm for a washer. Tankless size assumes 82% efficiency. Ask a plumber for exact sizing.

How to use this calculator

Enter the number of people and the fixtures that may run at the same time. Add the incoming water temperature and the hot water setting. Press Calculate to see the tank size and the tankless size you need.

The formula

Tank size depends on the number of people.
Peak flow = sum of the flow of fixtures used at once
Tankless BTU/hr = flow (gpm) × 500 × temperature rise ÷ efficiency

Example: A family of 4 needs a 40 to 50 gallon tank. With one shower, a dishwasher and the kitchen sink running together the flow is 5.5 gpm. A 65 F temperature rise needs a gas tankless heater of about 217,988 BTU/hr.

How to size a water heater for your home

The water heater size calculator gives two kinds of results. The tank size suits a storage water heater based on the number of people in the home. The peak flow, temperature rise and BTU figures suit a tankless heater, which must heat water as fast as you use it. Pick the results that match the type you plan to buy.

What changes the size

  • Household size. More people means more showers, dishes and laundry each day.
  • Fixtures used at once. Tankless sizing depends on how many fixtures run at the same time, not daily use.
  • Incoming water temperature. Cold groundwater in northern states means a bigger temperature rise and a larger tankless unit.
  • Hot water setting. A higher setting increases the rise and the heating power needed.

Two examples

A family of four typically suits a 40 to 50 gallon tank. For tankless, one shower at 2.5 gpm, a dishwasher at 1.5 gpm and a kitchen sink at 1.5 gpm add up to 5.5 gpm. With a 65 degree rise, that needs about 217,988 BTU per hour.

Now picture two showers and the clothes washer at once, with 50 F incoming water heated to 120 F. That is 7 gpm with a 70 degree rise, or about 298,780 BTU per hour. That is more than many single gas tankless units provide, so a larger unit, two units or staggered use may be needed.

Tips and common mistakes

  • Check the first hour rating on a tank, not just its gallon size. It shows how much hot water you get during a busy hour.
  • Do not undersize tankless units. A unit that is too small gives lukewarm water when several fixtures run.
  • Low-flow showerheads reduce peak flow and can allow a smaller tankless unit.
  • Gas tankless units often need larger gas lines and special venting. Factor that into the cost.

Cost and planning advice

Tank heaters usually cost less up front and are simpler to replace. Tankless models cost more to buy and install but can save space and energy. Heat pump water heaters are another efficient option where there is room. Water heaters involve gas, venting and high-voltage wiring, so follow local code and have a licensed plumber confirm sizing and do the installation.

Frequently asked questions

What size water heater do I need?
A 30 to 40 gallon tank suits 1 to 2 people, a 40 to 50 gallon tank suits 3 to 4 and a 50 to 66 gallon tank suits 5 to 6 people.
What is the first hour rating?
It is the number of gallons a tank heater can deliver in one hour starting full. It is on the EnergyGuide label.
How do I size a tankless water heater?
Add the flow of the fixtures that may run at the same time and the temperature rise from your incoming water to the hot water setting.
What temperature should I set the water heater to?
120 F is a common setting. It is safer and saves energy.
Is tankless better than a tank?
Tankless gives endless hot water and saves space, but costs more to buy and install.
How long does a water heater last?
A standard tank water heater typically lasts about 8 to 12 years. Tankless units often last longer, commonly 15 to 20 years with regular maintenance. Signs of trouble include rusty water, rumbling noises, leaks around the base and less hot water than before. Flushing a tank yearly can help it last longer.
How long does it take a water heater to heat up?
A gas tank heater usually recovers a full tank in about 30 to 40 minutes. An electric tank often takes longer, sometimes an hour or more. A tankless heater heats water on demand, so it delivers hot water within seconds of the burner firing, though water still takes time to travel through the pipes.
Is a heat pump water heater worth it?
A heat pump water heater can use much less electricity than a standard electric tank, because it moves heat from the air instead of making it. It costs more up front and needs space and air around it, often in a garage or basement. It makes the most sense when replacing an electric tank.