How do I read a vernier or digital caliper?

A caliper is the tool that answers questions a tape cannot: the exact diameter of a dowel, the thickness of a sheet of glass, the depth of a rebate, the size of a bolt. It measures to a hundredth of a millimetre, which means the way you hold it matters as much as the way you read it.

What you'll need

• Vernier, dial or digital caliper
• A gauge block or a known-size item to check it against (optional)

Step by step

1. Learn the four jaws.The large lower jaws measure external sizes — a diameter, a thickness. The small upper jaws measure internal sizes — a bore, a slot. The rod that slides out of the tail measures depth. And the stepped face at the end of the beam measures a step or a shoulder. One tool, four measurements, and most people only ever use one of them.
2. Close it and zero it.On a digital, close the jaws gently and press zero. On a dial, close them and rotate the bezel so the needle reads nought. On a vernier, close them and check the two zeros align; if they do not, note the offset and apply it to every reading. Do this at the start of every session, not once a year.
3. Read the main scale first.Find where the vernier scale's zero mark falls on the main scale and read the last whole millimetre before it. That is the coarse part of your answer.
4. Then find the one vernier mark that lines up.Run your eye along the vernier scale until you find the single graduation that sits exactly opposite a main-scale graduation. Its number is the fraction — on a common 0.02mm vernier, the marks are numbered so you read the value directly as hundredths.
5. Add the two together.Main scale 23mm, vernier mark 34 lining up, gives 23.34mm. If two adjacent vernier marks look equally aligned, take the value between them; that is the limit of what the tool can tell you.
6. Use light, consistent pressure.A caliper will happily compress soft material, spring its own frame, and read whatever you squeeze it to. Close it until it just contacts, then read. The thumbwheel exists so that the same person applies the same force every time.
7. Sit the jaws square on the work.Rock the caliper slightly while closing on a round object and watch for the smallest reading — that is the true diameter. On a flat object, look for the largest. A jaw that is not square reads long on a shaft and short on a bore.
8. Measure the same feature three times.Three readings within a hundredth or two means you are measuring rather than squeezing. A spread of a tenth means your technique is the limiting factor, not the tool.

Tips

• The little upper jaws have a thickness of their own, which is already accounted for in the scale. Do not add anything to an internal measurement.
• Digital calipers eat button cells because many keep drawing current with the display off. Take the battery out if it is going in a drawer for months.
• A cheap plastic composite caliper is fine for reading a bolt diameter or a board thickness. It is not fine for anything where the last two decimal places matter, because the frame flexes.
• Wipe the beam and the jaw faces before use. A single chip of swarf between the jaws is worth more than the tool's entire accuracy specification.

Common mistakes

• Never zeroing a digital caliper — The zero drifts with temperature, battery state and any knock. An unzeroed caliper produces readings that are precise, repeatable and wrong by a constant amount.
• Reading the vernier by looking for the nearest mark — The whole principle is that exactly one pair of graduations coincides. Choosing the nearest rather than the coincident one introduces an error of several hundredths and defeats the tool.
• Squeezing to make it feel tight — The frame deflects and soft materials compress. On rubber, plastic or timber a heavy hand can easily cost half a millimetre.
• Using the depth rod without seating the beam end — The end of the beam has to sit flat across the opening. Tilted, it measures a diagonal into the hole and reads deep.

Questions people ask