How to measure spa filter cartridge for hot tub

By part number

A genuine quality hot tub filter will have a part number stamped on the end cap (see example). It can be a Darlly, Unicel, Pleatco or Filbur number. It can even be a part number from the manufacturer. Just find the number printed on your hot tub cartridge, type it into the box, and click the Find My Hot Tub Filter button.

By manufacturer

Here’s another way to find your hot tub filter, when you don’t see a part number printed on the end cap, you probably have a filter cartridge from the original equipment manufacturer. We have about 250 spa and jacuzzi manufacturers on the list. Just select your brand from the drop-down menu and click the Find My Hot Tub Filter button.

By dimensions

If these two methods fail you, we have another way to get the right hot tub filter cartridge, but this means getting your tape measurement. We will show you the right way to confirm your correct hot tub filter cartridge by dimensions, so you can find the right replacement from our large selection of hot tub filters from our online store.


Always start by measuring the width (diameter) of your old hot tub filter cartridge through the outer edges of the end cap at its widest point.

Note: Measurements can vary approximately 1/8″ due to expansion, contraction, etc. For example, if you measure as a diameter of 5″ and can’t find a match, it may actually be a 4-15/16″ filter, etc.


Now measure the length of the hot tub filter cartridge, from the outside of the end cap to the outside of the end cap (this is the hard plastic end of the filter). Again, remember that measurements can vary from specifications by about 1/8″ or more due to expansion, contraction, or manufacturer variations.

Note: If your filter has a top and/or an accessory at the other end, DO NOT include them in your length measurement.

Aperture diameter

Your filter may have an opening at one end (with a closed top or top handle), or it may have openings at both ends. Measure the actual diameter of the hole, excluding the cone, if any. If your filter has a slotted hole, measure the shortest path, not including the slot. Closed top filters often have small holes to allow trapped air to escape when installed. Ignore holes for measurement purposes.

Note: Refer to the instructions below for measuring hot tub filter cartridges with threaded or sliding accessories.

Types of filter threads

For hot tub filters with male threaded fittings, you will first need to determine the thread type. There are two types of threads that are easily identified by referencing the photo on the left: Fine threads, also called MPT or NPT Thick threads, also called SAE

Men’s thread sizes

After identifying the type of thread, you will need to determine the size of the thread. Screw adjustment sizes are indicated as nominal pipe thread sizes (1-1/2″, 2″, etc.) NOT the actual measurement of the thread diameter.

Women’s thick threads (internal)

Female screw connection sizes are indicated as nominal pipe thread sizes (such as 1-1/2″), which is NOT the actual measurement of the thread inner diameter. A nominal 1-1/2″ female thick thread connection actually measures approx. 1-7/8″ inside diameter (ID). Measure at the widest inside diameter, not including threads. Thick threads are also known as SAE

Removable threaded fittings

If you replace a Pleatco brand threaded filter cartridge, note that older types had a removable male tube thread attachment that fit into a 1-15/16″ D cartridge opening. You may need to reach out to unscrew the male pipe fitting from the filter housing of hot tubs if you didn’t come out with the old filter (many hot tub owners don’t even know the connection is still inside). Darlly’s screw fittings are permanently attached to the replacement filter cartridge. Some filter cartridges come with a smooth male (not threaded) sliding attachment at one end. The size dimension is the total outer diameter (OD) measured at the widest point of the fixture.

Read a tape measure

Tape measures have fractional lines and inch markings on the face. The scale between the inches marks is divided into 1/16″ increments. There are four different sizes of lines between the inches markings. The longest line represents 1/2″, the second longest 1/4″, then 1/8″ and finally 1/16″.

The value of any line on the scale can be described in sixteenth of an inch, but it is customary to give measurements with the lowest common denominator, so 6/16 “is indicated as 3/8”.

Hopefully you now have the numbers you need to find the right filter for your hot tub. If you’re still having trouble identifying the correct hot tub filter, you’d better email the make and model of your hot tub, current filter measurements, and an image of the filter you want to replace to¬† where our team will direct you directly to the correct hot tub filter required.

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By Catharine Bwana