FastQSL calculate the squashing factor Q at the photosphere, a cross section, or a box volume, given a 3D magnetic field with Cartesian, uniform or stretched grids. It is available in IDL and in an optimized version using Fortran for calculations and field line tracing. Use of a GPU accelerates a step-size adaptive scheme for the most computationally intensive part, the field line tracing, making the code fast and efficient.