A vernier callipers gives with an auxiliary (or vernier) scale in addition to the major measurement scale. The vernier scale may be slide with the main scale. The vernier scale is so marked that the length of total number of divisions on it is lesser by length of one division on major scale.

The least count of vernier scale is computed by using the following formula

Least count of vernier scale (or vernier constant)= 

or

Least count (vernier constant) = 1 M.S.D.(Major measurement division) - 1 V.S.D.(vernier scale division).

Zero error of vernier callipers: If the zero marking of major scale and vernier callipers does not coincide, essential correction has to be created for this error which is called as zero error of the instrument. If the no error of the vernier measurement scale is to the right of the zero of the major scale the zero error is said to be positive & the correction will be negative or  vice versa.