International Tolerance Grade, also known as IT Grade, identifies what tolerances a specific process can produce for a given dimension defined in ISO 286. It establishes the magnitude of the tolerance zone or allowed amount of part size variation for internal and external dimensions. There are totally 18 grades of tolerance, the smaller the International Tolerance Grade number, the smaller the allowable tolerance zone, the higher the precision level.
The IT Grade is used to indicate how precise an industrial process is. In designing, the engineer will determine a key dimension and relative tolerance for a particular feature. The tolerance for a particular IT grade can be calculated through the formula as below. The calculation formula can help engineers to know what IT Grade is necessary for producing a part with certain specifications. If the manufacturing process can’t reach the grade, you should consider another method.
T: tolerance in micrometers [μm]
D: geometric mean dimension in millimeters [mm]
ITG: IT Grade, a positive integer