Gage blocks (also called gauge blocks, Jo blocks, Hoke blocks, and Johansson gauges) are precision ground and lapped metal or ceramic blocks, used for calibrating precision measuring equipment and as a length standard in manufacturing. Gage blocks are square or rectangular in shape, with flat and parallel measuring surfaces, and usually have an engraved unique serial number and nominal length (provided in either inches or millimeters). They often come in sets with blocks of varying lengths, so that many different standard lengths can be created by stacking and wringing them. Wringing is the process of sliding blocks together with a small amount of pressure, in order to create a longer gage block stack.

Gage block accessories are used to assist with the assembly, use, care, and maintenance of gage blocks. Gage block accessory sets allow blocks to be joined together in a stack, used as a scriber, or as Go-NoGo gages. Gage block accessories include half round and plain jaws, screws, holders, base block, tie rods, and straight edges, among others. Care and maintenance items include cleaning supply kits and stones to remove burrs. Optical parallels are used to check for flatness on the working surface of gage blocks.

Gage blocks should be kept within a protective case, cleaned thoroughly before wringing and after each use to prevent corrosion, and kept free of burrs and other damage that would decrease their accuracy. Temperature is another important consideration. Blocks are susceptible to thermal expansion and must be used at room temperature (68 degrees F) to ensure their accuracy.

Gage blocks are available in different grades, which refer to their tolerance or allowable deviation from their nominal dimension. Grades are defined by the American Society of Mechanical Engineers (ASME) B89.1.9M for both length and flatness. ASME B89.1.9M also defines standards for parallelism and surface finish. Different grades are available depending on the application and the required level of accuracy. Grade 00 blocks are generally used to calibrate and check the accuracy of instruments, grade 0 blocks for setting and checking gages in quality control, grade AS-1 blocks for tool rooms and production purposes, and grade AS-2 are similar to AS-1 blocks but are slightly less accurate. Grade K blocks are used for calibrating and checking the accuracy of other gage blocks within a temperature-controlled inspection or calibration room. A certificate of inspection and calibration is provided with each gage block or set, specifying the deviation of the individual block from its nominal length.