  The calibration of an advanced constitutive law for soil is a challenging
task. This work describes GA-cal, a Fortran software for automatically
calibrating constitutive laws using Genetic Algorithms (GA) optimization. The
proposed approach sets the calibration problem as a regression, and the GA
optimization is used to adjust the model parameters so that a numerical model
matches experimental data. This document provides a user guide and a simple
tutorial. We showcase GA-cal on the calibration of the Sand Hypoplastic law
proposed by von Wolffersdorff, with the oedometer and triaxial drained test
data. The implemented subroutines can be easily extended to solve other
regression or optimization problems, including different tests and constitutive
models. The source code and the presented tutorial are freely available at
\url{https://github.com/FraJoMen/GA-cal}.
