Here are some instructions on what you will need to measure when you want to replace or change your spa or pools filter. You will learn how to measure the diameter, opening diameter and length.
Length
Now measure the length, from outside of end cap to outside of end cap. Once again, remember that measurements may vary from specs by about 1/8" or more due to expansion, shrinkage, or manufacturer's variances.
Diameter
Always start by measuring the diameter of your old cartridge across the outside edges of the end cap at its widest point.
Opening Diameter
Your filter may have an opening in one end (with a handle top or closed top), or it may have openings in both ends. Measure the actual hole diameter, excluding taper, if any. If your filter has a slotted-hole, measure the shortest way, not including the slot.
Filter Thread Types
For filters with male threaded fittings, you will first need to determine the type of thread. There are two types of threads which are easily identified by referring to the left photo:
- Fine threads, also called MPT or NPT
- Coarse threads, also called SAE
Male Thread Sizes
After identifying thread type, you will need to determine the thread size. Screw fitting sizes are stated as nominal pipe thread sizes (1-1/2", 2", etc.) NOT the actual thread diameter measurement. If in doubt, use our tables to determine your match. Shown below are the overall outside diameters (O.D.) measured at the widest point.
Male Slip Fittings
Some filter cartridges come with a smooth (non-threaded) male slip fitting on one end. The size dimension is the overall outside diameter (O.D.) measured at the fitting's widest point.
Female (Internal) Coarse Threads
Female screw fitting sizes are stated as nominal pipe thread sizes (such as 1-1/2") which is NOT the actual thread inside diameter measurement. A 1-1/2" nominal female coarse threaded fitting actually measures approx. 1-7/8" inside diameter (I.D.).
- Measure at the widest inside diameter, not including threads.
- Coarse threads are also known as SAE.
