Added resistance in waves is the second-order steady force dependant on the ship motions. It significantly affects ship operability and causes speed loss. Therefore the required power and fuel consumption of a ship advancing in a seaway increase. In this paper, additional fuel consumption is approximated in relation to the increased resistance, i.e. the difference between the ship resistance in calm water and resistance while operating in waves. Ship advancing in regular head waves with different speeds is examined. Calm water resistance is calculated according to the potential flow theory and ITTC 1957 model-ship correlation line. The added resistance in waves is calculated using three-dimensional boundary element method based on the potential flow theory. Motion induced resistance due to heave and pitch motions as well as reflection induced resistance are taken into account. Therefore, overall assessment of added resistance in regular waves is achieved through evaluation of the ship drift forces and quadratic transfer function. The so-called near-field formulation is used to determine wave loads acting on the ship hull. Evaluation of increased fuel consumption is made on the basis of estimated added resistance for different speeds and wave frequencies. Correlation of fuel consumption with various wave frequencies that the ship may encounter in its voyage is obtained.