Definition
Circularity is a geometric tolerance that controls the roundness of a single cross-section of a round feature, without reference to a datum.
What it means for the calculation
A tight circularity requirement on a single cross-section usually points to a turning or grinding process capable of holding roundness at that station, and can require slower cutting speeds or multiple passes.
Where it appears on the drawing
In a feature control frame with the ○ symbol and a tolerance value; no datum, since circularity only controls the feature's own cross-sectional shape.
Common mistakes
Confusing circularity with cylindricity (circularity checks a single cross-section, cylindricity the whole length) or with a diameter tolerance (which controls size, not roundness).
A tight circularity requirement on a single cross-section usually points to a turning or grinding process capable of holding roundness at that station, and can require slower cutting speeds or multiple passes.
See how Blake reviews a partFrequently asked questions
Is circularity the same as cylindricity?
No, circularity checks one cross-section; cylindricity checks roundness and straightness over the entire length.
Does circularity need a datum?
No, it only controls the feature's own shape at that cross-section.
Can a round feature pass circularity but fail cylindricity?
Yes, each individual cross-section can be round while the feature as a whole is tapered or bent along its length.