Inside measurements. Select the caliper blades with flat edges that face away from one another. Set the blades into the opening and slide the caliper jaws apart until they press firmly against the walls of the opening. For the vernier-style caliper shown here, you’ll determine the opening width by reading off two scales on the tool handle.
Outside measurements. Close the larger caliper jaws (those with flat edges facing one another) around a workpiece. To determine distance between the jaws on a dial caliper, read tenths of an inch off a scale on the caliper handle and hundredths/thousandths of an inch off the pointer in the dial.
Depth measurements. Most vernier and dial calipers have a narrow extension probe that slides out of the end of the tool handle to measure depth. Set an edge of the tool handle next to the recess, and open the caliper jaws to extend the probe. Measure depth off the vernier scales or dial just as you would for inside or outside measurements.