How To Measure Hot Tub Filter Uk
Direct answer: start with the part number printed on the old cartridge. If the number is missing or unclear, measure total length, outer diameter and the top and bottom connection style together. A close height match on its own is not enough.
UK suppliers often list filters in both millimetres and inches, and small connection differences can create expensive returns. Hydrospares recommends using identifying marks, diameter, length and end styles. Hot Tubs UK warns that handles and threads should not be included in the plain length measurement, while One Hot Tub stresses that length, outer diameter and connection must match as a set.
Measurement checklist
| What to check | How to record it | Why it matters |
|---|---|---|
| Part number | Photograph any code on the end cap or band | It is usually the cleanest cross-reference route |
| Total cartridge length | Measure end-cap to end-cap, excluding handles or protruding thread | It prevents long/short family mistakes |
| Outer diameter | Measure across the pleated body at the widest point | A cartridge can be the right height and still not fit the housing |
| Top and bottom style | Note open hole, closed cap, handle, coarse thread or fine thread | The seal and connection style decide whether water bypasses the media |
Step-by-step measuring method
- Switch the spa off using the manufacturer-safe process before removing the cartridge.
- Rinse loose debris so you can see the cap shape and any printed code.
- Lay the cartridge on a flat surface and measure the body length end to end.
- Measure the outside diameter across the pleats, not across a handle.
- Photograph the top, bottom and side so a seller can see the connection style.
Who should not guess
Do not order from height alone if the old filter has a threaded end, unusual bayonet fitting, or a cap shape that does not match the listing photo. A cautious buyer is not being slow here; they are avoiding the most common fit error. If two listings use the same cross-reference code but show different end caps, pause and confirm before buying.
The product facts available for this site safely support long and short height families, including 375mm and 275mm snapshots. They do not support inventing a universal diameter, thread type or filtration area. That is why the measuring step stays deliberately conservative.
Next useful checks
If your main uncertainty is height, compare long vs short spa filter cartridges. If you are choosing pack size after the fit is confirmed, see 2-pack vs single filter rotation.