The implementation of a front and back grating in ultra-thin photovoltaic cells is a promising approach towards improving light trapping. A simple design rule was developed using the least common multiple (LCM) of the front and back grating periods. From this design rule, several optimal period combinations can be found, providing greater design flexibility for absorbers of indirect band gap materials. Using numerical simulations, the photo-generated current (J(ph)) for a 10-pm-thick crystalline silicon absorber was predicted to be as high as 38 mA/cm(2), which is 11.74% higher than that of a single front grating (J(ph) =34 mA/cm(2)). (C) 2016 Elsevier B.V. All rights reserved.