Loosen the locking screw found on the top of a vernier caliper's vernier. The vernier, the sliding clamp that wraps around the ruler, has precision measurement marks printed along a scale under the ruler. Most vernier calipers use metric rulers with numbers above each centimeter and dashes for each millimeter.
Place one edge of a measurable object against the flat part of a vernier caliper's fixed jaw. The fixed jaw connects to the end of the ruler that reads zero. Outside measurements use the larger jaws found on the bottom of the ruler and have the flat part of the jaws facing each other. Inside measurements use the smaller jaws located on top of the ruler and have the flat part of the jaws facing out.
Close the vernier until the flat part of the sliding jaw nearly touches the measurable object. Keep the sliding jaw off of the measurable object.
Turn the fine adjustment wheel until the sliding jaw lightly touches the surface of the item you are measuring. Do not bind the jaws or the adjustment wheel.
Tighten the locking screw. This will secure the vernier in place.
Remove the object from the vernier caliper's jaws without moving the jaws.
Calculate the whole number, usually in millimeters. Find the zero-mark on the vernier's scale. Follow that mark to the ruler. If the zero-mark ends on a whole number mark, then the object's measurement equals that whole number. If the zero-mark finishes between two dashes on the ruler, the use the smaller whole number as a base measurement and add the vernier calculation.
Calculate the vernier's precise measurement. Each dash on the vernier's scale represents a fraction, usually 1/10 or 1/100 mm. Find the dash on the vernier's scale that lines up exactly with a dash on the ruler. Count the dashes between zero and the one that lines up with a ruler's dash. This total represents the fractional part of the measurement.
Add the whole number and fraction together for the object's precise measurement. For example, if the whole number ends between the 16 and 17 mm marks and the third dash on the vernier lines up evenly with a dash on the ruler, then the object's precise measurement would equal 16.30 mm.