© 2023, The Author(s), under exclusive licence to Springer Nature B.V.Compliant mechanisms have increasing popularity. However, very few studies are available on transmission characteristics of compliant mechanisms in the literature. For conventional rigid-body mechanisms, the transmission angle is used to define motion quality. On contrary, the classic transmission angle formula cannot easily be employed for compliant mechanisms, unlike rigid-body mechanisms. This paper, a generalized equation that defines the transmission angle of all types of compliant four-bar mechanisms is introduced. For all configurations of partially compliant four-bar mechanisms, the formula is altered. Four theorems in some special cases are devised. Deviation of the transmission angle of a fully compliant four-bar mechanism from its rigid-body counterpart is discussed. Finally, a prototype is built and the theoretical approaches are compared with experimental results. To the best of our knowledge, this is the first study on the transmission angle of a compliant four-bar mechanism.