A measuring instrument includes a ruler unit including an elongated plank portion extending along a first direction, two spaced apart positioning members disposed fixedly on the elongated plank portion, and a first measuring portion extending from the elongated plank portion along a second direction. A movable seat is movably mounted on the elongated plank portion and is movable relative to the elongated plank portion along the first direction between a reset position, where the movable seat abuts against one of the positioning members, and a normal position, where the movable seat abuts against the other one of the positioning members. The movable seat has a second measuring portion. A resilient member is mounted between the movable seat and the ruler unit, and biases the movable seat toward the normal position.