Question: How do I know which size filter to order?

Answer:

Industry standard size filters are available in 4 basic filter sizes;

1) 10″ x 2.5″

2) 20″ x 2.5″

3) 10″ x 4.5″

4) 20″ x 4.5″

To determine the size of your filter housing, so that you can purchase the correct filters, remove the sump (lower portion of the filter housing that screws off the cap). Now measure the inside diameter at the top of the sump. If you have either # 1) or # 2) above, your inside measurement will be 3″ to 3 1/2″. The reason you need a 2 1/2″ diameter filter is because the housing tapers as it goes towards the bottom so at the bottom of the sump the inside diameter is very close to the 2 1/2″ measurement. The length of the sump will be approximately 10 1/2″ on # 1) above and the length on # 2) above will be approximately 20 1/2″.

Now to the Big Blue size filters. If you have either item # 3) or # 4) above, the inside diameter will be approximately 5 1/4″. The length of the sump for item # 3) will be approximately 11″ and the length of the sump for item # 4) will be approximately 21″. As with the smaller diameter filters, these Big Blue filter housings taper from top to bottom so the 4 1/2″ diameter filters fit snug at the bottom of the sump housing.