The invention provides a linear size measuring tool, especially calipers, and belongs to measuring tools. The calipers include an inside-calipers state and an outside-calipers state; the calipers comprise a pair of caliper arms, the two caliper arms are arranged symmetrically and are hinged to each other in an intersected manner via a rotational pin, the side, of the rotational pin, of each caliper arm includes a clamping segment, and the other side of the caliper arm includes a reading segment; the end portion of each clamping segment is provided with a clamping head, the included angle between the clamping head and the caliper arm in a plane defined by the pair of caliper arms can be adjusted, two clamping heads whose front ends include tips respectively are arranged symmetrically, when the clamping heads incline inwardly, the calipers serve as inside calipers, and when the clamping heads incline outwardly, the calipers serve as outside calipers; the reading segments are provided with a reading device which reflects the lengths of the inside calipers and the outside calipers; when the clamping heads incline inwardly to limit positions, the included angle between the axis of the clamping heads and the axis of the clamping segments is 12 degree; when the clamping heads incline outwardly to limit positions, the included angle between the axis of the clamping heads and the axis of the clamping segments is 25 degree; and the front ends of the clamping heads form a wedge shape.