GD-Calc, a grating diffraction calculator offered by KJ Innovation, computes diffraction efficiencies of biperiodic diffraction gratings comprising linear, isotropic and nonmagnetic optical media, based on a generalized variant of rigorous coupled-wave theory. It is implemented in MatLab and runs on Windows, Macintosh, Unix and Linux MathWorks platforms. The calculator features optimized algorithms and code, and provides unrestricted control over diffraction order selection as well as advanced parameterization capability. A free demo, tutorial code and documentation provide technical and instructional information. Applications include aerospace and defense, optics and semiconductors.