It is important to predict the added resistance in waves already in early stage of ship design. The increase in resistance due to waves affects the ship speed and fuel consumption and can represent significant problem from the aspect of environmental protection. The added resistance in waves is the nonlinear hydrodynamic force of the second order which depends on the ship motions and is caused by the interaction of the ship wave system and the incoming waves. The paper provides a theoretical basis for the added resistance in waves and the results of the numerical calculation of the added resistance in regular waves for three different ship forms. The numerical calculations are carried out utilizing HydroSTAR program package, based on the linear theory of potential flow. The numerical results of the added resistance in waves are compared with the experimental data available in the literature. In addition to regular head waves, the added resistance is also determined for different incident angles. The ship mass characteristics are necessary input parameters for calculation of the added resistance in waves and they are often unknown in the preliminary design stage. Therefore, they need to be accurately evaluated to ensure that the results of the added resistance in waves are as reliable as possible. For this reason, the methods for the determination of center of gravity and the gyration radii are presented within this paper and their effect on the added resistance in waves is analysed. Since ship spends approximately 40-50% of her total voyage time in the ballast condition, the added resistance in waves is calculated for the ballast condition as well. The obtained results are compared with the results of added resistance in waves for the full load condition.