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How to Measure Hot Tub Spa Filter Cartridge

Updated: May 12, 2020

Choosing the correct hot tub filter cartridge may sometimes be tricky due of the availability of multiple brands available in the UK and each offering different features in their respective products. Here at Penguin Hot Tubs, our team of hot tub experts have put together some tips to help you choose the best filter for your hot tub.

Guide on how to choose the correct hot tub filter for your hot tub
How to Measure Hot Tub Spa Filter Cartridge by Penguin Hot Tubs

By part number

A genuine quality hot tub filter will have a part number stamped into the end cap (see example). It may be a Darlly, Unicel, Pleatco or Filbur number. It can even be a manufacturer part number. Just find the number printed on your hot tub cartridge, and enter 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 an original equipment manufacturer filter cartridge. We have nearly 250 spa and hot tub manufacturers listed. Just select your make from the drop-down menu and click the Find my Hot Tub Filter button.

Hot Tub filter manufacturers - Darlly, Pleatco and Unicel

By dimensions

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

Measuring Hot Tub Filter Width written Guide by Penguin Hot Tubs


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

Note: Measurements may vary by about 1/8" due to expansion, shrinkage, etc. For example, if you measure as a 5" diameter, and can't find a match, it may actually be a 4-15/16" filter, etc.

Measuring Hot Tub Filter Length written Guide by Penguin Hot Tubs


Now measure the length of your hot tub filter cartridge, from outside of end cap to outside of end cap (this is the hard-plastic end of the filter). Once again, remember that measurements may vary from specs by about 1/8" or more due to expansion, shrinkage, or manufacturer's variances.

Note: If your filter has a handle top and/or fitting on the other end, do NOT include these in your length measurement.

Measuring Hot Tub Filter Opening Diameter, written Guide by Penguin Hot Tubs

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. Closed-top filters often have tiny pinholes to allow trapped air to escape when installed. Ignore pinholes for measurement purposes.

Note: See below instructions for measuring hot tub filter cartridges with threaded or slip fittings.

Hot Tub Filter Thread Types: Written Guide by Penguin Hot Tubs

Filter Thread Types

For hot tub 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 photo on the left:

  • Fine threads, also called MPT or NPT

  • Coarse threads, also called SAE

Hot Tub Filter Male Thread Sizes: Written Guide by Penguin Hot Tubs

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.

Hot Tub Filter Female screw fitting sizes: Written Guide by Penguin Hot Tubs

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

Hot Tub Filter removable threaded fittings: Written Guide by Penguin Hot Tubs

Removable Threaded Fittings

If replacing a Pleatco brand threaded filter cartridge, be aware that older types had a removable male pipe thread fitting which slip-fit into a 1-15/16"D cartridge opening.

You may need to reach down to unscrew the male pipe fitting from your hot tubs filter housing if it did not come out with the old filter (many hot tub owners are not even aware that the fitting is still inside). Darlly's screw fittings are permanently attached to the replacement filter cartridge.

Hot Tub Filter male slip fittings: Written Guide by Penguin Hot Tubs

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.

How to measure a hot tub filter correctly: Written Guide by Penguin Hot Tubs

Reading a Tape Measure

Tape measures have fractional lines & inch marks across the face. The scale between inch marks is divided into 1/16" increments. There are four different sizes of lines between the inch marks. 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 sixteenths of an inch, but it's customary to give measurements with the lowest common denominator, so 6/16" is stated as 3/8".

Hopefully you now have the numbers you need to find the correct filter for your hot tub. If you are still having trouble identifying the correct hot tub filter, it’s best if you email the make and model of your hot tub, measurements of current filter and a picture of the filter you wish to replace to where our team will direct you directly to the correct hot tub filter required.

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