This paper focuses on mathematical modelling of cutting force with tool wear effect for milling by milling heads. With regard to a small number of experiments with sufficient accuracy, universality, speed and applicability in practice, a mathematical model based on the specific cutting force and cutting area was created. The proposed mathematical model is more refined than other models owing to more accurate calculation of the specific cutting force and cutting area. The calculation of the specific cutting force is more precise due to consideration of the influence of the cutting speed, more accurate calculation of the undeformed chip thickness and implementation of the correction factor of the tool flank wear, which has a significant impact on the cutting force. The calculation of the cutting area is achieved through more accurate calculation of the chip width as well as undeformed chip thickness. The chip width and undeformed chip thickness are refined by consideration of the straight and rounded parts of the cutting edge. For the proposed mathematical model, material constants for the workpiece material made of DIN C45 steel were obtained.