A ruler, sometimes called a rule, line gauge, or scale, is an instrument used to make distance measurements whereby a user estimates a distance by reading from a series of markings called "rules" along an edge of the device. Commonly the instrument is rigid and the edge itself is a straightedge ("ruled straightedge") which additionally allows one to draw straight lines. Some rulers such as cloth or paper tape measures are non-rigid. Specialty rulers exist that have flexible edges that retain a chosen shape and these find use in sewing, arts, and crafts.

Rulers have been used since ancient times. They are commonly made from metal, wood, fabric, paper, and plastic. They are crucially important in the design and construction of buildings. Their ability to quickly and easily measure lengths makes them important to textile industry and to retailers where amounts of string, fabric, and paper goods can be cut to size. Children learn basic use of rulers at the elementary school level and are often part of a student's school supplies. At the high school level rulers are often used as straightedges for geometric constructions in Euclidean geometry. Rulers are ubiquitous in the engineering and construction industries, often in the form of a tape measure, and are used for making and reading technical drawings. Since much technical work is now done on computer, many software programs implement virtual rulers to help the user estimate virtual distances.