Your hot tub's filter system is a vital part of the tub and its efficient operation. Beyond cleaning it regularly to keep both it and the water clean, at some point you will want to replace the filter with a new one. You can usually buy a new one from the same place where you bought your hot tub, but you may wish to shop around or get a different filter than the one it came with.
If you are looking for a hot tub filter that isn't an exact duplicate of the one that came with your hot tub, you'll need to take a few measurements to ensure that you get one that is a perfect fit. Start by measuring the filter's length (from end cap to end cap). This needs to be as exact as you can get, so use something that gives very precise measurements.
Then you'll need to note the outside and inside diameters of the filter end caps. Again, these measurements need to be as exact as possible. Try to get within 1/32 of an inch to be safe. Some hot tub filters that are not actually designed for your system will fit it, but they will not sift the water correctly, so be careful! When in doubt, get a pro to help you.
Next, note the name of the spa filter from which you removed the filter cartridge. It should feature on the housing of the filter system. You may need this when ordering a replacement. Some hot tub sellers have a ready guide at the store, but if you buy the filter online, this is handy info to have at your fingertips.
Another important part of this process is determining which type of hot tub filter end caps you need. Once you've removed the filter to needs to be replaced, look at the ends and note the configuration. There are about 15 different possibilities, and you definitely need to know which one is right for your replacement filter. If you find that you cannot accurately describe it in words, draw a detailed sketch of it and take that to your local hot tub dealer.