If the label does not mark your specific unit's size, use the filter’s dimensions to narrow your choices.
Your filter model’s size should be marked by the manufacturer, or the installer must do this during installation, but that is not always the case. To identify your filter's size, you can measure specific parts of the filter to determine its size.
For sand filters, measure the diameter of the tank. There are a couple of ways to measure the tank’s diameter: measure the circumference, then solve for the diameter, or you can use two sticks and a measuring tape.
To use the circumference method, wrap a measuring tape around the tank's widest part and write that length down. You can then use the formula for converting circumference to diameter (Diameter = Circumference divided by Pi), or you can search for a circumference to diameter calculator on your favorite search engine.
To use the stick and tape method, secure two yardsticks on the opposite sides of the tank's widest part, then measure the distance between them. You are essentially making goalposts and creating a crossbar with the measuring tape, which should give you an accurate diameter.
For a cartridge filter, take out the cartridge to measure the plastic endcap's diameter and the length(end cap to end cap, not including the spout/tap found on some cartridge models.
For DE filters, measure the length of the curved grids to identify their size. grids come in 5 different sizes 24, 36,48,60, and 72 square feet; their respective lengths are 12, 18, 24, 30, and 36 inches
For older models like the Jacuzzi Earthworks and Avalanche, which use non-curved grids, you will need to measure all grids, as the various model sizes used a combination of two or three grid sizes per model.