What are the different grades of gage blocks and what do they mean? The grade of a gage block is a specific rating given to the gage block that represents the degree of tolerance it has. Gage blocks used to come in grades depicted by letters – A, AA, AAA, B. Now the standard labeling is in the form of numbers ranging from 0.5 to 3. Each grade has a different purpose, but generally, the higher the grade, the tighter the tolerance. Tighter tolerances, and therefore higher grades, will result in a greater amount of accuracy and precision in your measurement. Depending on the country and the company you are working with there will be different ways to label grades. Higher grades, representing smaller degrees of tolerance (or higher degrees of tolerance tightness; ±0.05 μm) are often used to establish standards and calibrate, while higher grades, representing slightly larger degrees of tolerance (or lower degrees of tolerance tightness; 0.25 μm to − 0.15 μm) are used as shop standards for precision measurement purposes.