Circularity

Controls how close each circular cross-section of a round feature is to a true circle.

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0.04

Circularity, also called roundness, controls the form of a circular cross-section so the surface is not too oval, lobed, or out of round.

It is a form control, so it is independent of any datum reference.

Use it for shafts, holes, bearings, bushings, and other round features where shape matters beyond simple two-point diameter size.

It can allow a looser size tolerance while still limiting out-of-round form error.

Round Shaft Section Checked For Roundness

Circularity is evaluated one cross-section at a time and does not reference a datum axis.

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0.04

Two concentric circles separated by the circularity tolerance in a cross-section plane.

Rotate the feature and record surface variation in a single cross-section using roundness equipment or an indicator setup.

Multiple cross-sections may be checked along the length when the whole feature must behave consistently.

A shaft cross-section has circularity 0.04 mm. The roundness trace fits between circles separated by 0.031 mm.

Trace spread

0.031 mm

This is the radial separation needed to enclose the cross-section.

Compare

0.031 <= 0.04

The section is inside the circularity zone.

Repeat sections

As needed

Circularity does not automatically evaluate the full cylinder length.

The checked section passes circularity. Cylindricity or runout may be needed for full-length or datum-related control.

Control	Datum	Zone	Scope
Circularity	No	Two concentric circles	One section
Cylindricity	No	Two coaxial cylinders	Full cylinder
Runout	Yes	Circular variation to axis	Rotating section

Circularity is the 2D version of cylindricity.

A size tolerance already limits some form error through the envelope principle, but circularity can provide a clearer tighter limit.