The grid factor (also called bucky factor) defines the requirement to increase the exposure factors to maintain density with the use of a grid.

(B) The Bucky factor is the ratio of x-rays arriving at the grid (incident radiation), and those being transmitted through the grid. The Bucky factor describes how much the output of the x-ray tube must be increased in order to compensate for the removal of incident radiation by a grid.
B = incident radiation / transmitted radiation
For example, if for every two x-rays impinging the grid, only one will pass through, then the grid has a Bucky factor of two and the intensity of the x-ray beam must be doubled.