Ferroelectrics are insulating materials with a nonzero electric polarization switchable by an applied electric field. Ferroelectric materials have been extensively studied using DFT and the modern theory of polarization. In this framework, the electronic polarization of a periodic crystal is computed from the Kohn-Sham wavefunctions as a Berry phase. This polarization is a multivalued quantity, defined only modulo a quantum of polarization. The quantum of polarization is an integer multiple of a lattice vector, multiplied by the ratio of charge and unit cell volume. In short, the polarization is a lattice. In practice, only polarization differences are experimentally relevant, and the spontaneous polarization of a crystal is defined as a change in polarization relative to a nonpolar structure. Therefore, in computing the difference in polarization between two structures, one must select polariza-between the polar and nonpolar structures are obtained following the same process. Finally, the output of all calculations is collected and the polarization is computed by finding the common branch. Other outputs include the electronic and ionic contribution to the polarization and the quantum of polarization. The workflow is in principle general and any ab initio DFT code can be used, however at present only VASP implementation exists. The workflow was originally developed in atomate by Smidt et al. and extensively used to build two databases of candidate ferroelectrics. We note that determining an appropriate nonpolar reference structure for a given polar structure can be obtained by using the PSEUDO tool 129 in the Bilbao Crystallographic Server (BCS), as recently done to build the ferroelectrics database in Ref. 128. Alternatively, if a nonpolar reference is already known, it can be transformed into the polar setting by using the Structure Relations tool in BCS, as was done to build the ferroelectrics database in Ref. 127.