Plumbers
Water Heater Size Calculator
Find the right water heater tank size or tankless flow rate for any household size.
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.
Common Questions
What size water heater do I need for a family of 4?
A family of 4 with average usage needs a 50 gallon tank or a tankless unit rated at 7 to 9 GPM. If multiple people shower back to back in the morning, go with a 65 gallon tank or a higher capacity tankless.
What is first hour rating on a water heater?
First hour rating (FHR) tells you how many gallons a tank heater can deliver in the first hour of heavy use. Match the FHR to your peak demand. A 50 gallon tank might have an FHR of 60 to 70 gallons depending on the heating element.
Should I install a tank or tankless water heater?
Tank heaters cost less up front and are simpler to install. Tankless heaters cost more but last longer and never run out of hot water. For cold climates with low groundwater temperatures, size a tankless unit bigger than you think you need.
How long does a water heater last?
Tank water heaters typically last 8 to 12 years. Tankless units last 15 to 20 years or more with proper maintenance. Flushing the tank annually and replacing the anode rod extends the life of a tank heater.
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