Filter systems are sized based on a couple of factors: (1) type and amount of dissolved mineral present in your water; (2) your home's flow rate, which is typically based on the number of people present in the home. For filter systems, this information simply tells us what the fastest rate your water will travel through our units would be, and how much water in gallons per day is being used.
Water softeners are sized based on the total hardness of your water, and the number of people in the home. Most all-residential applications have around an average 5 GPM flow rate. Typically, the higher the flow rate of your water going through the unit, the larger the mineral tank will be to handle the larger water flow rate. With a larger tank, the filtering media or resin will be physically deeper thereby permitting the water flowing down through it to be in contact with the media longer. Contact time is important, as the media/resin inside the tank needs to be in contact with your water for a long enough period of time, ensuring all dissolved impurities are removed before it leaves the tank.