Calipers or callipers are an instrument used to measure the linear dimensions of an object or hole; namely, the length, width, thickness, diameter or depth of an object or hole. The word "caliper" comes from a corrupt form of caliber.

Many types of calipers permit reading out a measurement on a ruled scale, a dial, or an electronic digital display. A common association is to calipers using a sliding vernier scale.

Some calipers can be as simple as a compass with inward or outward-facing points, but with no scale (measurement indication). The tips of the caliper are adjusted to fit across the points to be measured, and then kept at that span while moved to separate measuring device, such as a ruler, or simply transferred directly to a workpiece.

Calipers are used in many fields such as mechanical engineering, metalworking, forestry, woodworking, science and medicine.

Caliper is the American spelling, while calliper (double "L") is the British spelling.

A single tool might be referred to as a caliper or as calipers – a plural only (plurale tantum) form, like scissors or glasses.

Colloquially, the phrase "pair of verniers" or just "vernier" might refer to a vernier caliper. In loose colloquial usage, these phrases may also refer to other kinds of calipers, although they involve no vernier scale. In machine-shop usage, the term "caliper" is often used in contradistinction to micrometer, even though outside micrometers are a form of caliper. In this usage, caliper implies only the form factor of the instrument.

The earliest caliper has been found in the Greek Giglio wreck near the Italian coast. The ship's find dates to the 6th century BC. The wooden piece already featured a fixed and a movable jaw. Although rare finds, calipers remained in use by the Greeks and Romans.