Common definition and calculation formula of transerse contact ratio for all types of gears are put forward.The calculation formulae for involute gear

micro-segments profile gear

and sine-curved profile gear are discussed.The transerse contact ratio of gear pair is defined as the ratio of the action angle （the rotation angle of gear from gear-in to gear-out for one pair of teeth） to the rotation angle per pitch（or central angle per tooth）.According to the theory of gearing

equation of the meshing line can be deduced from the tooth profile of basic rack.According to the equation of the meshing line and the addendum outline of the gear

the contact ratio can be calculated with the calculation formulae in this paper.For the involute gear

this definition has same effect as the well-known definition-ratio of the contact line to the base pitch.This definition of contact ratio is also suitable to other non-involute gear

such as micro-segments profile gear

sine-curved profile gear.