Leisure boats market has constantly grown during the latest years entailing an increasing need of design tools, dealing with technical problems like seakeeping of planing boats, whose complexity still present a high challenge for designers. The present paper describes a methodology for the calculation in time domain of the vertical motions of a planing boat in head waves, capable of predicting the non-linear vertical motions of the boat in both regular / irregular head waves. The formulation makes recourse to a ‘strip-theory’ approach to get the 3D hydrodynamic forces as a linear superposition of the sectional forces. The determination of the 2D planing forces is based on ‘momentum theory’ using Wagner’s wedge-theory to model the sectional characteristics, while non-linear Froude-Krylov forces are directly evaluated accounting the relative motions at each cross-section. Moreover, from the time-series of the vertical motions an overall passenger comfort index and slamming pressures are evaluated. A validation of the code is presented against the model-test results available in literature.