Filtration grade sand used when it is time to replace the media. The amount of bags will depend on the size of the filter you have.
When refilling you sand filter you will need...
3 bags for a 18" Hayward sand filter
4 bags for a 21" Hayward sand filter
6 bags for a 24" Hayward sand filter
6 bags for a 24" Hayward sand filter with 2 inch valves
8 bags for a 27" Hayward sand filter with 2 inch valves