Plumbers

Water Heater Size Calculator

Find the right water heater tank size or tankless flow rate for any household size.

3
People
36 gal
First Hour Rating Needed
40-gallon tank
Tank Recommendation

First Hour Rating (FHR) estimates how many gallons a tank heater delivers in the first hour of heavy use. For tankless units, size is based on simultaneous demand - count showers, faucets, and appliances running at the same time. Cold groundwater temperatures in northern climates require higher-capacity tankless units.

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How to size a water heater

Water heater sizing is based on peak hour demand, which is the most hot water your household uses in any single hour. Think about the morning rush when showers, the dishwasher, and the washing machine might all be running. A standard shower uses about 2 gallons per minute. A dishwasher uses 1 to 2 gallons per cycle. Add it up and that tells you how many gallons of hot water you need available in your busiest hour.

Tank vs tankless water heaters

Tank water heaters store hot water so it is ready when you need it. Sizing is about matching the tank capacity to your peak demand. Tankless water heaters heat water on demand and are sized by flow rate in GPM. A tankless unit that can handle 5 GPM will run two showers at once. Tankless units cost more upfront but take up less space and do not run out of hot water.

Water heater sizing for families

For a household of 1 to 2 people, a 30 to 40 gallon tank usually works. For 3 to 4 people, go with 40 to 50 gallons. For 5 or more people, you want 50 to 80 gallons or consider going tankless. These are rough guidelines. If your family takes long showers or runs the dishwasher and laundry at the same time, size up.

Common Questions

How much does it cost to run a water heater per month?

A gas water heater costs about $30 to $40 a month. An electric tank runs $40 to $60. Tankless units cost less to operate since they only heat water when you turn on the tap.

Electric vs gas water heater which is better?

Gas heaters recover faster and cost less to operate. Electric heaters are cheaper to buy, easier to install, and do not need venting. If you already have a gas line, gas is usually the better deal.

What temperature should a water heater be set to?

Set it to 120 degrees. That is hot enough for showers and dishes but low enough to prevent scalding. Going to 140 wastes energy and increases the risk of burns.

Can a water heater be too big?

Yes. An oversized tank keeps water hot that nobody uses, which wastes energy. A 40 gallon tank is plenty for 1 to 2 people. Do not pay to heat water you are not going to use.

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