This is a how-to guide to take you through the process of measuring your pool, spa or hot tub to work out what size pool heater you require. To work out the size of pool heater requires that you match your pool to the BTH/Hr rates specified by the pool manufacturer. To do this is fairly straight forward. Matching swimming pools require knowing the surface area and for spas and hot tubs the size in gallons. We have done this, using Hayward H Series Pool Heaters as an example.