An apparatus for measuring a bending angle (a) between two portions (51, 52) of a workpiece (50) in a bending machine (100; 110) comprises at least a motion sensor unit (2) provided with a gyroscope sensor (3), a processing unit (10) connected to said gyroscope sensor (3) and coupling means (12; 11; 211) for removably coupling said motion sensor unit (2) to a portion (51, 52) of said workpiece (50); during a bending operation of said workpiece (50) said gyroscope sensor (3) measures angular velocities (ωχ, ωγ, ωz) and calculates related rotation angles (θχ, θγ, θz) of said portion (51, 52) in a local coordinate system (X, Y, Z) of said gyroscope sensor (3) and said processing unit (10) receives from said gyroscope sensor (3) data regarding at least said rotation angles θχ, θγ, θz) so as to calculate, whatever a position and/or an orientation of said local coordinate system (X, Y, Z) is on said portion (51, 52), further rotation angles (θχ, θγ, θz) of said portion (51, 52) in a global coordinate system (x, y, z) associated to said bending machine (100; 110) in order to estimate said bending angle (a) as a function of said further rotation angles (θχ, θγ, θz).