A caliper is a precision measuring tool used to determine the exact distance between two opposite sides of an object. Unlike a ruler, which you lay flat against something, calipers have two adjustable arms that can grip around an object or reach into tight spaces. You might use calipers to measure the thickness of a coin, the diameter of a marble, or the width of a small pipe.

Machinists, engineers, and scientists rely on calipers when measurements need to be extremely accurate, sometimes down to hundredths of a millimeter. The two arms of the caliper can slide closer together or farther apart, and once they touch the object being measured, you read the measurement from a scale or digital display. Some calipers can measure the inside of a tube, the outside of a cylinder, and even the depth of a hole.

Calipers come in different styles. Vernier calipers use a clever sliding scale to show precise measurements, while digital calipers display numbers on a small screen. Even simple calipers made of wood or plastic can work better than rulers for measuring round or irregular objects.