The number of caissons in a structure is the total quantity of piers in that structure's foundation which are built inside shafts that retain the piers' forms against water, sand, or earth. Normally caissons are built by pouring concrete down a preset steel tube, with reinforcing bars set inside to strengthen the concrete.

The shaft retains the form of the pier against the surrounding material. The count concerns the resulting foundation piers, rather than their reinforcing bars or the volume of concrete used.

Caisson quantity describes how many such piers support the structure. Their individual dimensions and the overall size of the foundation are separate measurements.