Are Hot Tub Filters Universal? (Complete Filter Sizing Guide)

There are hundreds of differently sized filers available on the market, which can be a bit of a headache when it comes to finding a replacement. But is there a one-size-fits-all filter among the options?

Hot tub filters are not universal. Filters vary in size and flow rates and come in a range of dimensions, materials, and thread types. The size of your filter can usually vary a few millimeters without causing problems. However, if the bottom of your filter is threaded, it needs to be exactly the same size.

Without the right filter, you run the risk of exposing yourself to harmful bacteria. This guide will show you:

  • What to look for when identifying your hot tub filter size
  • How to take measurements the right way
  • The 4 types of filter materials

How Do I Know What Size Filter for My Hot Tub?

The best way to ensure that you get the correct replacement filter is to compare it to the old one. If you are looking to buy a spare or a replacement online, you will need to work off the dimensions or the model number of the filter.

General dimensions of the cartridge and method of endcap connection are the two most important features. Measuring the filter is difficult to do since these filters have small tolerances to prevent them from leaking.

Your replacement filter cartridge needs to fit your hot tub filter perfectly. The incorrect size of the thread will prevent the unit from forming a seal, which can lead to leaks or large particles being sucked into the pump.

External threads on the filter will either be Fine threads (MPT) or Coarse threads (SAE). The information on the thread type should be written on the filter.

Hot Tub Filter Part Numbers

You should be able to see printed numbers on the top or bottom of your existing spa filter. Most manufacturers will stamp information on their filters so that you can see the date of production, the model, and the type.

Model numbers tend to begin with the first letter of the name of the manufacturer, then on to the filter type. For example, the part number of a Filbur filter will begin with the initials �FC’. Some filter codes will contain numbers and no letters.

Manufactures may also reference the same part with different codes. This can be confusing, but more model codes also give you more options in terms of where you can source your new filter from. You may be able to find the relevant model equivalent from the manufacturer’s website.

Once you think you have a model match for your filter, it is worth cross-checking the dimensions of your old filter against the one you have found. You may notice that even though the filter has the same dimensions, it is made from different materials.

Original parts tend to be made from higher grades of plastics with better particle mesh. You can make savings by buying third-party filters, but the life span will be shorter. An aftermarket filter may not fit as well and can allow debris to escape into your hot tub pump.

How to Measure a Hot Tub Filter

As an alternative to relying on the model number of the filter, you can try measuring the part and finding a similar-sized replacement. With each dimension, you need to be careful where you take the measurements from.

1. Length

This does not include the extra dimensions of a handle or the thread. The length of the filter is from the top cap to the base cap, including the thickness of the caps. Take your measurement from the outside corner edge of each cap.

2. Width

The width measurement should also be taken from the outside of the cap and include the thickness of the plastic. You need to consider the overhang of the cap over the filter, as this will need to fit inside the filter housing on your pump.

3. Hole Diameter

Your filter will either have a hole going right through its center or a single hole at the bottom and a closed top. The important measurement is the internal diameter of the hole and the tread width. Measure from the base of the thread, where it is thickest. You need to avoid measuring the narrow part of the taper.

4. Identify the Thread

Filters will come with either fine or coarse threads, also known as SAE or MPT. Standard thread sizes are 1�, 2, or 3 � inches. Some filter threads are also interchangeable. Such universal filter fittings will allow you to remove and swap out the bezel so that you can fit the correct thread for your pump.

Inflatable Hot Tub Filter Sizes vs Regular Hot Tub Filter Sizes

Inflatable hot tub filters tend to be smaller than those of a hard-sided tub. A smaller filter is easier to block, and this means that you will be replacing or cleaning it out more often.

Inflatable hot tub filters also tend to be less efficient, so they take longer to clean the water and struggle to keep it clean if the spa gets a lot of use.

The 4 Types of Hot Tub Filters

The filter keeps your hot tub hygienic by pulling out organic matter and screening dirt. You want a filter that catches the most particles while still providing good water flow to reduce the strain on the pump.

There are 4 main types of hot tub filters, each using a different filtering material to clean the water:

Cartridge (Polyester)

Cartridge filters are a common type of filter because of their convenience and price. These are medium-cost filters that are effective at filtering out most particles and offer good flow rates. A cartridge filter will last up to 5 years if you maintain it properly.


Ceramic filters last the longest and are the most versatile type of filter. Ceramic filters can screen out anything from large particles down to bacteria. If looked after, a ceramic filter can last more than 6 years.


Sand filters are the cheaper option, but they need constant maintenance to prevent them from becoming clogged. Sand filters will last up to 7 years, though they do become less efficient as they age.


These are the most common filter choice for hot tub and swimming pool owners. Diatomaceous Earth (DE) uses natural crushed shells to draw dirt out of the water. This is an efficient filter and can last up to 3 years if the internal membrane is maintained.

DE filters are the best for hot tubs though these are also the most expensive of the four options. A DE filter can screen out particles twice as well as a regular cartridge filter and up to 4 times better than sand.

Other than the cost, a DE filter is a potential hazard, and care must be taken when handling them dry. Diatomaceous Earth is a powder that will cause health issues if inhaled for long periods. When installing a DE filter, it is advisable to wear a mask and gloves to prevent skin irritations.

Joshua Milton

Joshua Milton is a seasoned hot tub enthusiast. With many years of experience in the industry, he offers valuable insights on hot tub maintenance, health benefits, and relaxation techniques.

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