The noncomplete compensated calibration errors of a fiber optic gyro (FOG) can remain in the navigation stage, leading to decreased navigation accuracy. A new approach is presented to calibrate the FOG drift and scale factor error based on a double calculating program. Using an analysis of the relationship between the FOG error and misalignment, a FOG calibration error model is derived, and a three-step six-position calibration path is analyzed. In this method, the FOG drift and scale factor error are calculated from two groups of attitude information, which are resolved, respectively, from calculating programs A and B in the navigation computer. The input signal of these two calculating programs is measured from one FOG. Its effectiveness is demonstrated by simulation and turntable experiments. The results show that this method can further compensate the residual calibration errors with misalignments, thereby improving the FOG calibration accuracy.