Johansson gauge blocks, also known as jo blocks, are precision measuring instruments used in metrology to determine length or distance. They are made of high carbon steel or ceramic material, and are carefully ground to an exact size, usually in millionths of an inch.

The blocks are stacked together to form the desired length and their flatness allows accurate and repeatable measurements in many different applications, such as inspection, machine tool setup, and testing and calibration of other measuring instruments.

Johansson gauge blocks are available in sets that include blocks of various sizes, ranging from 1 millimeter to 300 millimeters or more, and are used in a variety of industrial and scientific applications, including manufacturing, engineering, and quality control. They are considered a fundamental tool in dimensional metrology and their accuracy and versatility make them a widely used and essential tool in many precision measurement applications.