Different types of water heaters have varying sizing requirements. To size a tank-style water heater, you need to check its FHR (first hour rating), which will help determine if the water heater can meet your needs during the busiest time period. Sizing a tankless water heater requires you to calculate the flow rate in gallons per minute (GPM) and temperature rise you'll need for its application in your property. Our local experts at Charles Krull & Son, Inc. Plumbing & Heating can help you determine what size water heater is ideal for your needs.