This paper proposes a method for calculating the tilt angle of the tool and the machining strip width when the tool is inclined to the feed direction. Tilt angle of the tool and machining strip width are important factors which affect feed rate and machining quality in five-axis flat-end milling for free surface. There are some methods to calculate the tilt angle of the tool in five-axis flat-end milling for free surface, but mathematically complicated algorithm is applied to the calculation of tilt angle of the tool, so it is difficult to apply it in practice. We considered the geometry of the surface and the tool as well as the scallop height to determine the tool tilt angle, thus ensuring the tool to be contacted with the surface at two points. This allows us to calculate the tool tilt angle and the machining strip width by solving quadric equations based on the contact circle. Moreover, tool tilt angle and machining strip width are calculated analytically. Thus the speed of calculation is quick and easy to implement. An experiment of machining on the biquantic B-spline surface was performed and the results show that the proposed method has considerably higher machining efficiency than the CMM.