<p>A computational methodology for determining the tooth thickness of a face gear is proposed systematically. According to the tooth thickness of a face gear on its reference plane at its nominal radius, a symmetric coordinate system with respect to its one tooth can be determined. The equation of the tooth addendum curve is obtained in the symmetric coordinate system, and the change law of the addendum thickness is analyzed both qualitatively and quantitatively. Based on this, a formula to calculate the tooth addendum thickness can be established by means of symmetry. The methodologies are put forward to implement the design computation of the outer radius in line with the addendum thickness and to implement the verifying computation of the addendum thickness in the light of the radial position. A comparative study was performed between the methods suggested and the traditional approach. Abundant numerical results are provided fully.</p>