Abstract: A vernier gauge includes a first arc member having a first radially inner side and a first radially outer side, a second arc member having a second radially inner side and a second radially outer side slidably engaged with the first radially inner side, having a second free end and a second clamped end and provided with a second scale, a third arc member having a third radially inner side slidably engaged with the first outer side and a third radially outer side, having a third free end and a third clamped end and provided with a third scale, the third arc member together with the first and second arc members defining a free space at the center of the first, second and third arc members, two intermediates each of which has a first end provided on one of the second and third free ends and a second end and is capable of moving radially with respect to the arc members and two measuring pieces each of which has a first engaging end pivotally connected to the second end and a second engaging end pivotally intercon