Water Heater Size Calculator

The Water Heater Size Calculator helps you choose the right water heater two ways. In storage-tank mode it recommends a tank size in gallons based on your peak-hour hot water demand, using the U.S. Department of Energy first-hour rating (FHR) method. In tankless mode it recommends a flow rate in gallons per minute (GPM) based on the fixtures you run at once and your local temperature rise, along with the required BTU/hr and kW. That distinction matters: a tank is limited by how much hot water it stores, while a tankless unit is limited by how fast it can heat water as it flows.

How to Size a Storage-Tank Water Heater

Rather than guessing from household size alone, professionals size a tank to the busiest hour of the day. You add up all the hot water drawn during that peak hour, then pick a heater whose first-hour rating is within one to two gallons of that number.

The recovery ratio reflects how quickly the fuel type reheats the tank while you draw from it: gas recovers fastest (about 1.75× the tank size becomes available in the first hour), electric is slower (about 1.25×), and heat-pump models are slowest of all, so they need the largest physical tank for the same demand.

Typical Hot Water per Use

Tank Size by Household (Quick Reference)

Electric tanks are usually a size larger than gas tanks for the same household because they reheat more slowly.

How to Size a Tankless Water Heater

A tankless (on-demand) heater never runs out of hot water, but it can only heat a limited number of gallons per minute. Sizing it takes two numbers working together: the peak flow rate and the temperature rise.

The constant 500 comes from the physics of water: one gallon weighs about 8.34 pounds, and there are 60 minutes in an hour, so \( 8.34 \times 60 \approx 500 \) BTU are needed to raise one GPM by one degree Fahrenheit. Divide the BTU/hr figure by 3,412 to get the equivalent electric power in kilowatts.

Typical Fixture Flow Rates

Why Climate Changes Everything for Tankless

The same tankless unit delivers far fewer gallons per minute in a cold climate than a warm one, because it has to work harder to raise colder incoming water to your target. In Florida, incoming water might be 65°F, giving only a 55°F rise to reach 120°F. In Minnesota, incoming water can be 40°F, an 80°F rise — nearly 50% more heating for every gallon. That is why cold-climate homes need noticeably more powerful tankless units.

Tank vs Tankless: Which Should You Choose?

• Tankless gives endless hot water and lower standby energy loss, but costs more up front and is capped by flow rate. Best where hot water use is spread out and you want to save space and energy.
• Storage tank is cheaper to buy and can deliver a big burst of hot water instantly, but it runs out during heavy use and loses a little heat while sitting idle. Best for tight budgets and homes with short, concentrated peaks.

How to Use This Calculator

1. Choose the heater type: Pick Storage Tank for a size in gallons, or Tankless for a flow rate in GPM.
2. Enter your peak usage: For a tank, count the hot water uses during your busiest hour. For tankless, choose the fixtures that can run at the same time.
3. Set fuel or climate: Select your fuel type for a tank, or your climate zone and target temperature for a tankless unit.
4. Review the recommendation: Read the recommended gallons or GPM, the demand breakdown, BTU/kW, and the full step-by-step math.

Frequently Asked Questions

What size water heater do I need?

For a storage tank, size it to your peak-hour hot water demand: add up the hot water used during your busiest hour and choose a tank whose first-hour rating matches. As a rule of thumb, 1–2 people need about 30–40 gallons, 3–4 people about 50–65 gallons, and 5 or more people about 65–80 gallons. For a tankless unit, size by flow rate (GPM) instead: add the flow of every fixture that can run at once and make sure the unit delivers that flow at your local temperature rise.

How is water heater tank size calculated?

The Department of Energy recommends the first-hour rating method. You total the hot water drawn during the single busiest hour of the day (for example, a shower is about 20 gallons and a hot laundry load about 7 gallons), then choose a heater whose first-hour rating is within one to two gallons of that peak demand. Gas heaters recover faster than electric ones, so a gas tank can be slightly smaller than an electric tank for the same demand.

How many GPM does a tankless water heater need?

Add the flow rate of every fixture that might run simultaneously. A shower uses about 2 GPM, a bathroom faucet about 1 GPM, and a kitchen faucet or dishwasher about 1.5 GPM. A single bathroom often needs 3–4 GPM, a typical family 5–7 GPM, and a large household 8 GPM or more. The unit must deliver that flow at your required temperature rise.

What is temperature rise and why does it matter for tankless?

Temperature rise is the difference between your incoming ground water temperature and your target hot water temperature, usually 120°F. In warm climates incoming water may be 65°F so the rise is only about 55°F, but in cold northern climates it can be 40°F, making the rise 80°F. A larger rise means the same tankless unit delivers fewer gallons per minute, so cold-climate homes need more powerful units.

Is a tankless water heater better than a tank?

Tankless heaters provide endless hot water and use less energy because they only heat water on demand, but they cost more up front and are limited by flow rate rather than storage. Tank heaters are cheaper to buy and can supply a large burst of hot water quickly, but they run out during heavy use and lose some heat through the stored water. The right choice depends on your budget, hot water pattern, and climate.

What temperature should a water heater be set to?

Most manufacturers and the Department of Energy suggest 120°F. This is hot enough for comfort and to limit bacterial growth while reducing the risk of scalding and saving energy. Settings above 130°F increase scald risk, and settings below 120°F can allow bacteria such as Legionella to grow in the tank.

Additional Resources

• Sizing a New Water Heater - U.S. Department of Energy
• Water Heating - Wikipedia
• Tankless Water Heating - Wikipedia