LEEDCal provides a powerful algorithm to determine and correct systematic distortions in LEED images. Easy to operate via its graphical user interface, LEEDCal will automatically generate a correction matrix upon analyzing a calibration sample exhibiting a well-known surface structure. This calculated matrix can be saved and applied later on, by using LEEDCal_corr, to rectify all future diffraction patterns recorded with the same LEED device. This works also in batch mode! The results allow accurate interpretation of lattice constants/epitaxial relations when used in combination with the LEEDLab.