Runout, concentricity, and circularity are three of the most frequently confused terms in dimensional inspection. They all relate to cylindrical features, they’re all measured with similar equipment, and in casual shop conversation they often get used as though they mean the same thing. They don’t. Each controls something different, and knowing which one a drawing is actually asking for changes both how you measure and whether a part passes. Here’s a practical breakdown.
Why These Three Get Confused
The confusion is understandable. All three describe ways a round feature can fail to be perfectly round and perfectly positioned, and a single dial indicator sweep can be used — correctly or otherwise — in the measurement of each. In practice, many shops use a runout check as a general-purpose screen and only distinguish further when a drawing forces the issue.
That works until it doesn’t. A part can pass a runout check and still violate a circularity tolerance, or vice versa, because the two describe different geometric properties. Reading the specific callout on the drawing is what resolves it, which is one more reason familiarity with drawing notation pays off across all inspection work.
Circularity (Roundness)
Circularity is a form tolerance. It asks a single question: at a given cross-section, how close is the profile to a perfect circle? It says nothing about where that circle sits or whether it’s aligned with anything else. A feature can be perfectly circular and badly off-centre, and still satisfy a circularity tolerance.
The tolerance zone is the radial distance between two concentric circles that just contain the actual profile. Because circularity is a form control referenced only to the feature itself, it doesn’t need a datum — which distinguishes it from the other two and makes it conceptually the simplest of the three.
Concentricity
Concentricity is a location tolerance, and it’s the strictest of the three. It controls the position of the median points of a feature relative to a datum axis — meaning it looks at where the centre of the feature lies at each cross-section along its length, and requires those centres to fall within a cylindrical tolerance zone around the datum axis.
That definition makes true concentricity genuinely difficult to verify with simple equipment, because it requires determining median points rather than surface positions. This is why concentricity callouts are relatively uncommon on production drawings, and why many designers specify runout or position instead when the intent allows. Establishing the datum axis reliably is itself a precondition, which brings the underlying datum concept into play.
Runout: Circular and Total
Runout is a composite control, referenced to a datum axis, and it comes in two forms. Circular runout applies at individual cross-sections: as the part rotates about its datum axis, the indicator’s total variation at that one position must stay within tolerance. It captures both out-of-roundness and off-centre position together.
Total runout extends this along the whole feature: the indicator traverses axially while the part rotates, and the entire collected variation must fall within a single tolerance zone. Total runout therefore also captures taper and other lengthwise variation. Both forms are practical to measure with a bench centre and an indicator, which is a large part of why runout is so widely specified.
How Each Is Measured in Practice
Circularity is properly measured by capturing a full profile at a cross-section and fitting reference circles to it — a roundness tester or a coordinate measuring machine does this well. An indicator sweep on a bench centre approximates it only if the part is already known to be well-centred, since otherwise off-centre position contaminates the reading.
Runout is the most straightforward: mount the part so it rotates about its datum axis, sweep an indicator, and read total variation. Concentricity, done rigorously, generally needs a CMM capable of determining median points. Whichever method applies, the setup rests on a stable reference surface and a properly organised measurement environment, since instrument capability alone doesn’t guarantee a valid result.
Choosing the Right Callout
From a design standpoint, the guiding question is what actually needs controlling. If a bearing seat must run true so a shaft doesn’t vibrate, runout is usually the appropriate and most economical callout, because it controls the combined effect the assembly actually experiences.
If the concern is specifically that a sealing surface be round regardless of position, circularity is the right control. Concentricity should be reserved for the narrow cases where median-point location genuinely matters, since specifying it imposes real inspection cost. Choosing the loosest callout that still protects function is generally sound practice.
There’s also a communication benefit to choosing carefully. A drawing that specifies concentricity where runout would have served invites exactly the confusion this article started with — an inspector reasonably measures runout, records a pass, and the record doesn’t actually demonstrate conformity to what was drawn. Specifying what you can verify keeps inspection records meaningful.
A Quick Summary of the Differences
Circularity is a form tolerance, needs no datum, and asks only whether a cross-section is round. Concentricity is a location tolerance, requires a datum axis, and controls where the feature’s median points lie. Runout is a composite tolerance, requires a datum axis, and controls the combined effect of roundness and position as the part rotates.
In equipment terms, circularity properly needs a roundness tester or CMM, concentricity generally needs a CMM, and runout is comfortably measured with an indicator and a rotating fixture. That last point explains a great deal about why runout dominates production drawings — it’s the control that aligns best with equipment most shops already have on the inspection bench.
Frequently Asked Questions
Is runout the same as total indicator reading?
Circular runout is often measured as a total indicator reading at one cross-section, so the terms get used loosely together. But runout is the tolerance being controlled, while TIR describes the measurement result.
Can a part pass runout but fail circularity?
Yes. Runout tolerance may be generous enough to accommodate an out-of-round profile that still violates a tighter circularity limit, since the two controls have different tolerance values and different purposes.
Why is concentricity considered hard to measure?
Because its definition is based on median points of the feature rather than surface positions, which generally requires collecting full cross-sectional data rather than a single indicator sweep.
Does circularity need a datum?
No. Circularity is a form tolerance evaluated against the feature itself, so no datum reference is required — unlike runout and concentricity, which are both referenced to a datum axis.