The present disclosure describes a method for determining spinal instability, more particularly, a method for quantifying the type and extent of spinal instability in a standardized way. The disclosure also describes a method comprising measurement of the intervertebral rotation and intervertebral translation between a vertebrae pair using simple images, and from these measurements, determining if one or more vertebrae pair of the spine is unstable. Finally, the disclosure describes software that may be used to quantify the type and extent of spinal instability.