There are a variety of different base types on the market, and many have different surface areas. Before ordering inserts, please ensure you're picking a type that will fit with the type of base you have on hand.
Beveled Bases - round lipped, beveled bases, like those made by Privateer Press or Wyrd Games.
Straight-Sided Bases - round bases with no beveling, like those made by Games Workshop or Infinity.
Hollow Bases - bases with a sunken impression, like those made by Kingdom Death.