Circularity, also called roundness, is a GD&T form control that keeps each cross-section of a round feature between two concentric circles. Each slice is judged on its own, with no datum, so it controls out-of-round shapes such as ovals and lobes but not taper or bend.
Picture slicing a shaft, cone or sphere perpendicular to its axis. Circularity looks at one slice at a time and asks how far the outline strays from a perfect circle. An egg-shaped slice, a three-lobed slice from centreless grinding and a slice with a flat spot are all caught.
Because every slice gets its own pair of circles, centred wherever they fit best, circularity does not care whether neighbouring slices are the same size or share a centre. A tapered pin can have perfect circularity. Control taper with size limits or cylindricity, and wobble with circular runout.
Circularity is typical on bearing journals, O-ring grooves, seal diameters and turned parts where an oval shape would leak or vibrate.
At each cross-section, the zone is the ring between two concentric circles whose radii differ by the tolerance value. Note that it is a radial value: a circularity of 0.01 means the two circles are 0.01 apart in radius, not diameter.
The circles may be any size and centred anywhere for each slice, as long as the measured outline fits between them.
Circularity never references a datum. Each cross-section is compared only with its own best-fit pair of concentric circles.
| Modifier | With circularity |
|---|---|
| MMC or LMC | Not allowed. Circularity always applies to surface elements, never to a feature axis. |
| Free state | Common on thin-walled rings and plastic parts that go oval when unclamped. The drawing may give one value in the free state and another when restrained. |
| Average diameter | Often paired with a free-state circularity on non-rigid parts, so size is judged from an averaged diameter. |
Example: a Ø25 mm bearing journal with a circularity of 0.02 mm
Result: All sections are round within 0.02 mm, so the journal passes circularity.
Every GD&T symbol page: GD&T symbols chart
Yes. Circularity is the ASME term and roundness is the common shop and ISO term. Both mean the same two-concentric-circles control on each cross-section.
No. Circularity is a form control and is checked against the section itself.
Yes. On a cone the sections are perpendicular to its axis. On a sphere every section through the centre is checked.
Circularity checks one cross-section at a time. Cylindricity checks the whole surface at once between two coaxial cylinders, so it also catches taper, barrel shape and bend.
A part with an odd number of lobes, typical of centreless grinding, has the same two-point diameter at every angle. Only a measurement around the full circle shows the lobes.
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