Definition
The diameter symbol marks a dimension as the diameter of a round feature, such as a hole, a shaft or a bore, rather than a radius or a straight length.
What it means for the calculation
The diameter and its tolerance decide which operation is needed: drilling, turning or milling, and sometimes an extra grinding or reaming step for a tight fit. A tight diameter tolerance usually pushes up cycle time and machine choice.
Where it appears on the drawing
Directly before a number, for example ⌀12, sometimes inside a dimension line or as part of a feature control frame.
Common mistakes
Mixing up a diameter callout with a radius callout (⌀ versus R) halves or doubles the actual size if misread. Confusing a diameter tolerance with a position tolerance on the same hole is another common mistake.
The diameter and its tolerance decide which operation is needed: drilling, turning or milling, and sometimes an extra grinding or reaming step for a tight fit. A tight diameter tolerance usually pushes up cycle time and machine choice.
See how Blake reviews a partFrequently asked questions
Is ⌀12 the same as R12?
No, ⌀12 is a 12 mm diameter (6 mm radius); R12 is a 12 mm radius (24 mm diameter).
Does a diameter symbol also control roundness?
No, roundness (circularity) is a separate GD&T control; diameter only sets the size.
Can a diameter appear without a tolerance?
Yes, a general tolerance from the title block or a standard such as ISO 2768 then applies.