Close the jaws until you can comfortably fit them inside the object.

Gradually open the jaws until both sides touch the object. Don’t pull too hard, yet firm enough to make sure both jaws sit flat against the object’s inside edges.

Read the measurement.

Take the measurement a few times and make sure it's the same—or very close—each time. If you get inconsistent readings, zero the calipers and try again.

Zero the calipers before making another measurement.