To find the correct filter for your spa pool by length and diameter:

1.Measure the outer diameter of the filter  - Measure across the top or bottom of the filter from the Left outer edge to the right outer edge

2. Measure the inside diameter of the filter  - Measure the daimeter of the opening through the middle of the filter or the inside edges of the thread.  Holes are normally 50mm, 75mm or 100mm. Threads are normally 38mm or 50mm.

3. Measure the length of the filter - Measure the length of the filter from top to bottom -do not include handles or threads.  It is more accurate to measure the length through the hole in the centre of the filter where possible

4. Threads and handles Fine thread (MPT) has from 6-8 fine twists whereas Course thread (SAE) has as little as 3 thicker threads.