Digital Vernier Caliper

Reading the scale

How to read a vernier caliper

A vernier caliper splits one measurement across two scales. The main scale gives you the whole millimetres; the vernier scale recovers the fraction the main scale is too coarse to show. Once you see how the two fit together, the procedure is the same every time.

  1. 1

    Check the zero first

    Close the jaws completely. The vernier zero should sit exactly on the main scale zero. If it does not, note the offset — every reading you take will carry it until you correct for it.

  2. 2

    Bring the jaws onto the work

    Seat the part against the fixed jaw and bring the sliding jaw up to it with the thumb roller, not by squeezing. You want light, repeatable contact; more pressure than that measures the deflection of the part.

  3. 3

    Read the main scale

    Find the last whole millimetre mark that the vernier zero has already passed. That number is the main scale reading. The commonest slip in the whole procedure is taking the mark just after the vernier zero instead of just before it, which puts you out by a full millimetre.

  4. 4

    Find the coinciding vernier division

    Run along the vernier scale until you find the one division whose line is exactly level with a main scale line. Its neighbours will sit visibly high or low. That division number is the vernier scale reading.

  5. 5

    Add them together

    Multiply the vernier division by the least count and add it to the main scale reading. Total reading = MSR + (VSR × LC). Apply the zero correction from step one, and that is your measurement.

The formula

TR = MSR+ (VSR× LC)

Worked through: a shaft on a 0.02 mm caliper shows the vernier zero just past the 25 mm mark, so MSR = 25. The sixth vernier division lines up with a main scale line, soVSR = 6. WithLC = 0.02, the total is 25 + (6 × 0.02) = 25.12 mm. Set the simulator to 25.12 and the same three numbers appear in the readout.

Diagnostics

Six mistakes that spoil a reading

Nearly every bad caliper reading comes from one of these. Reproduce each one deliberately in the simulator and you will recognise it in your own work.

  • Reading the main scale one division too far

    Taking the millimetre mark just after the vernier zero rather than the one it has passed. The result is exactly 1 mm too large, and it looks perfectly plausible.

  • Settling for a division that is nearly aligned

    Only one vernier line truly coincides. Its neighbours sit visibly above or below. If two look equally good, the true reading is between them and your least count cannot resolve it.

  • Ignoring a zero error entirely

    An uncorrected offset shifts every measurement in a batch by the same amount, so nothing looks anomalous. Check the zero before you start and again when you finish.

  • Applying the zero correction the wrong way round

    Positive error, subtract. Negative error, add. Getting the sign backwards doubles the error rather than removing it.

  • Squeezing the jaws

    On aluminium, plastic or thin sheet, hand pressure can compress the part by more than the least count. Use the thumb roller until you feel contact, then stop.

  • Reading the scale at an angle

    Parallax makes a division that is genuinely aligned look offset, and vice versa. Look square at the scale, or use the zoom in the simulator above.

Try it

Everything on this page is easier to believe with the instrument in front of you.

The simulator has draggable jaws, three least counts, imperial mode, zero-error simulation and a practice mode that marks your readings.

Open the caliper simulator

Keep reading