Guide
Circularity vs cylindricity in GD&T: which does a shaft or bore need?
Cylindricity is a form tolerance that holds an entire cylindrical surface between two coaxial cylinders, so it limits roundness, straightness and taper along the whole length at once. Circularity is its cross-section-only sibling: it holds each circular slice between two concentric circles and says nothing about how one slice relates to the next. Both are datum-free form controls, and choosing between them is the main cylindricity GD&T decision on a turned shaft or bore.
By Saola Manufacturing engineering team · Published · Updated
Designers usually reach for one of these when a bearing, bushing or seal starts misbehaving, and the wrong pick fails in one of two ways. Circularity on a bearing journal lets a tapered or barrel-shaped seat through inspection, so the bearing ring is distorted in service. Cylindricity on a short seal land or a chamfered pilot buys an expensive scan for no functional gain. This guide covers the two tolerance zones, how they interact with the size tolerance, which turning errors each one catches, how each is verified, and the callouts to put on a drawing and an RFQ. Both controls are defined in ASME Y14.5-2018 and ISO 1101:2017 (where circularity is called roundness); only their public descriptions are paraphrased here. Datum-referenced controls on the same features, such as runout and position, are covered in the separate circular vs total runout guide and the concentricity, runout and position guide.
On this page
How do the circularity and cylindricity tolerance zones differ?
Circularity gives every cross-section its own ring-shaped zone, and the rings are free to shift and change size from one section to the next. Cylindricity gives the whole surface one zone, the space between two coaxial cylinders, so every section must also line up with every other.
Both zones float. Neither one references a datum, so neither cares where the axis sits relative to the rest of the part; that is the job of runout, position or profile. What they judge is shape alone. The width of the zone is a radial value: a circularity tolerance of a given number means the difference between the largest and smallest radius in that slice, measured from a common center placed to make that difference as small as possible, may not exceed that number. It is not a diameter difference, which is the first source of confusion between design and inspection.
Because each circularity ring is independent, a cone, a taper, a bent shaft or an hourglass can all pass circularity as long as every individual slice is round. Cylindricity rejects all four. It is often described as a composite of three refinements: circularity of each slice, straightness of the surface lines, and the absence of taper between opposite lines. A surface that meets a cylindricity value therefore meets circularity of the same value automatically, and calling both with the same number on one feature adds nothing.
| Question | Circularity (roundness) | Cylindricity |
|---|---|---|
| Zone shape | Two concentric circles in each cross-section | Two coaxial cylinders over the full surface |
| Sections linked? | No; each slice judged alone | Yes; one zone for every slice |
| Catches lobing and ovality | Yes | Yes |
| Catches taper, barrel, hourglass, bow | No | Yes |
| Datum reference | Never | Never |
| MMC or LMC modifier | Not allowed | Not allowed |
| Can apply to cones and spheres | Yes | No; cylinders only |
| Usual verification | Roundness instrument at chosen heights | Roundness instrument with axial travel, or dense coordinate scan |
How do circularity and cylindricity interact with the size tolerance?
On an ASME drawing, the size limits of a shaft or bore already control its form through the envelope rule, so a circularity or cylindricity callout only does something if its value is smaller than the size tolerance. On an ISO drawing, size and form are independent by default, so an explicit form callout is often the only thing limiting lobing or taper.
Under ASME Y14.5 Rule #1, a shaft at its maximum material size must be a perfect cylinder, and form error may only grow as the actual size departs toward minimum material. A journal with a total size band therefore cannot be tapered or lobed by more than that band. A designer who adds a cylindricity value equal to or larger than the size band has written a callout that the envelope already enforces; inspection still has to report it, so it costs money without adding control. Make the form value a fraction of the size band, or leave it off.
ISO drawings follow the independency principle of ISO 8015:2011, under which a size tolerance, by default a two-point diameter, does not limit form. A three-lobed bore can measure in tolerance with a micrometer at every angle and still be measurably out of round. ISO users close this gap either with the envelope symbol (a circled E after the size) or with an explicit roundness or cylindricity frame. The general geometric tolerances of ISO 2768-2, still cited in many title blocks although ISO has withdrawn it in favor of ISO 22081:2021, set a default roundness equal to the diameter tolerance, capped by the general circular runout value, and state that general tolerances on cylindricity are not specified, which is one more reason to state it where it matters. The differences between the two systems are summarized in the ASME Y14.5 vs ISO GPS guide, and the general-tolerance notes themselves in the ISO 2768 and NADCA tolerance guide.
Which turning errors does each control catch?
Circularity catches the errors that live inside one revolution: ovality, lobing from clamping, and chatter. Cylindricity also catches the errors that build up along the axis: taper from tool wear or deflection, barreling of a slender shaft, and bow.
It helps to know where each shape comes from, because the cause decides whether a tighter callout is realistic or just a rejected lot. A thin-walled tube or sleeve clamped in a three-jaw chuck is pushed inward at three points; the bore is cut round while clamped and springs back into a three-lobed shape when released. That shape fails circularity and cylindricity alike, and it is invisible to a two-point micrometer because a three-lobed form has the same width in every direction. Bored soft jaws, a collet or full-grip pie jaws spread the clamping load and reduce it.
Along the axis the story is different. A long, slender shaft turned between a chuck and a free end deflects away from the tool where it is least supported, so it comes out larger in the middle or at the tip. Gradual tool wear over a long pass tapers a diameter. Residual stress released by heavy stock removal can bow a shaft after it leaves the machine. Every one of those can leave each slice perfectly round, so circularity passes and only cylindricity, straightness or total runout will find them. Sliding-headstock (Swiss-type) lathes cut close to a guide bushing that supports the bar right at the tool, which is why they are favored for long, thin turned parts; the trade-offs are covered in the Swiss lathe vs CNC lathe guide.
Surface texture sits underneath both. A form tolerance that is only a few times larger than the peak-to-valley roughness of the surface leaves little room for genuine shape error, so a tight cylindricity value usually needs a finer finish to go with it. Roughness callouts are covered in the surface roughness guide.
When does a bearing seat or bore need a cylindricity GD&T callout instead of circularity?
Use cylindricity where a mating part bears on the full length of the surface: rolling bearing seats, press-fit hub seats, bushings that must slide over their whole length, and piston or spool bores. Use circularity where only individual sections matter: lip-seal tracks, O-ring lands, short pilots, chamfers and cones.
The test is to ask what the mating part feels. A bearing inner ring pressed onto a tapered journal is loaded harder at one end, its raceway is distorted to match, and the bearing runs hot; bearing manufacturers' manuals such as the Timken Engineering Manual give seat form limits for exactly this reason, and those limits are the right starting point for the drawing. A spool valve or hydraulic piston that strokes the full length of a bore leaks or sticks wherever the bore barrels or tapers. These are cylindricity problems.
A rotary lip seal, by contrast, touches a narrow band of the shaft. What it cannot tolerate is a lobed or out-of-round track that makes the lip chase the surface every revolution; a taper across a band only a few millimeters wide is irrelevant. An O-ring land, a short location pilot and a seat for a thin washer behave the same way. Circularity, often paired with circular runout to a datum axis, is the proportionate control. Cones and chamfers can only take circularity, because cylindricity is not defined for them.
For the value itself, industry guidance commonly keeps a form tolerance to roughly half of the size tolerance or less, and bearing makers commonly express seat form limits as an ISO IT grade, or a fraction of one, finer than the grade of the seat's size tolerance. The table below lists the standard tolerance grades so that arithmetic can be done from the drawing. Short features, roughly one diameter long or less, usually gain little from cylindricity over circularity, which is a common reason to downgrade the callout and save the inspection time described in the cost of tight tolerances guide. Fits themselves, including interference values for bearing seats, are worked through in the press fit tolerance guide.
| Nominal size (mm) | IT5 (µm) | IT6 (µm) | IT7 (µm) |
|---|---|---|---|
| over 3 up to 6 | 5 | 8 | 12 |
| over 6 up to 10 | 6 | 9 | 15 |
| over 10 up to 18 | 8 | 11 | 18 |
| over 18 up to 30 | 9 | 13 | 21 |
| over 30 up to 50 | 11 | 16 | 25 |
How are circularity and cylindricity measured, and why should the method be agreed?
Circularity is properly measured by rotating the part, or a probe around it, and recording radial deviation over a full turn; cylindricity repeats that over many heights and fits one pair of coaxial cylinders to all the data. A micrometer or a part rolled in a V-block gives only an estimate, and two shops using different methods can disagree on the same part.
A two-point micrometer reading is blind to odd-lobed shapes, as described above. A V-block with a dial indicator sees some lobe counts and misses others depending on the block angle, and its reading has to be corrected by a factor that depends on that angle and the lobe count. A rotating-spindle roundness instrument or a coordinate measuring machine scanning the surface gives the real profile, but even then the answer depends on choices the drawing rarely states: the reference circle used (minimum zone or least squares), the filter that separates form from roughness, and how densely the surface is sampled. In ISO GPS, roundness is defined in the two parts of ISO 12181 and cylindricity in the two parts of ISO 12180, with Part 1 of each giving the vocabulary and parameters (for example ISO 12180-1:2011) and Part 2 the default specification operators; ASME drawings usually leave them to the inspection plan.
Cylindricity is the costlier of the two to verify because it needs many sections, aligned to one axis, plus straightness lines along the length. Where no suitable instrument is available, inspectors sometimes approximate it with circularity at several heights plus a diameter check at each end for taper. That is a reasonable screening method, but it is not the same requirement, so if the drawing calls cylindricity the supplier and the buyer should agree in writing how it will be measured before first articles are cut. The ballooned drawing guide shows how to tie each callout to a line on the inspection report.
What should a drawing and RFQ say about circularity and cylindricity?
Put the form control only on the functional surfaces, size it below the size tolerance, and say how it will be verified. If the method is not stated, write it on the RFQ so the supplier quotes the inspection you actually expect.
The examples below show the usual pattern on a turned shaft and bore. The common mistakes are a datum letter in a form frame (form controls never take one), a material condition modifier on circularity or cylindricity (not permitted), cylindricity on a cone or chamfer, a form value equal to or larger than the size band on an ASME drawing, and cylindricity on every diameter of the part when only two of them seat anything. A plating or coating step after machining also changes the surface, so state whether the form callout applies before or after finishing, as discussed in the before-or-after-coating guide.
| Feature | Size callout | Form callout |
|---|---|---|
| Bearing journal, inch drawing | Ø0.9844–0.9847 in | Cylindricity 0.0002 in |
| Bearing journal, ISO drawing | Ø25 k5 Ⓔ | Cylindricity 0.004 mm |
| Spool bore, full stroke | Ø12 H6 | Cylindricity 0.003 mm |
| Lip-seal track | Ø30 h9 | Roundness 0.005 mm plus circular runout to A |
| Short pilot, under one diameter long | Ø20 g6 Ⓔ | None, since the envelope already limits form; add roundness 0.005 mm only if the size band is too loose |
On the RFQ, list which features carry form controls, the drawing standard and edition, whether a specific measuring method or filter is required, and whether those values should be treated as process-capability control points through the run rather than first-article checks only. The machining RFQ guide covers the rest of the package.
Saola Manufacturing turns shafts, bushings and inserts on the conventional CNC lathes at its lead CNC plant, described on the CNC turning page. A diameter tolerance there is a size tolerance; a circularity or cylindricity value is a separate requirement, and our published pages do not list a roundness-measuring instrument, so tell us the value and the verification method before tooling and we will say plainly whether we can hold and prove it. Bores in castings are bored and reamed to print on the CNC milling line. Per the quality page, every toleranced dimension is measured at first article, and a dimension you mark as a process-capability control point is watched through the run. To check a specific part, send the drawing for a quote.
Frequently asked
Is circularity the same as roundness?
Yes. ASME Y14.5 uses the word circularity and ISO 1101 uses roundness for the same control, drawn with the same single-circle symbol. Cylindricity uses a circle between two slanted lines in both systems.
Is cylindricity the same as total runout?
No. Total runout is measured about a datum axis, so it adds coaxiality to the form check, while cylindricity has no datum and judges shape only. A perfectly cylindrical diameter that sits off-center fails total runout but passes cylindricity. The runout guide covers when the datum-referenced control is the better choice.
Can circularity be applied to a bore?
Yes. Both controls apply to internal and external features. On a bore, clamping distortion of a thin wall is the usual cause of lobing, so it is worth saying on the RFQ that the bore must meet the callout in the free state, which is the default for a rigid part.
Does cylindricity control the diameter?
No. Cylindricity limits shape, not size. The diameter is still controlled by its size limits, and the cylindricity zone floats inside them; the two zones are independent widths checked separately.
Which costs more to inspect?
Cylindricity, because it needs data from many heights aligned to one axis rather than individual slices. The GD&T symbols chart ranks inspection effort across all the symbols.
Sources
- ASME Y14.5-2018, Dimensioning and Tolerancing, ASME, 2018 (reaffirmed 2024)
- ISO 1101:2017, GPS — Geometrical tolerancing — Tolerances of form, orientation, location and run-out, ISO, 2017
- ISO 8015:2011, GPS — Fundamentals — Concepts, principles and rules, ISO, 2011
- ISO 12180-1:2011, GPS — Cylindricity — Part 1: Vocabulary and parameters of cylindrical form, ISO, 2011
- ISO 286-1:2010, GPS — ISO code system for tolerances on linear sizes — Part 1, ISO, 2010
- ISO 2768-2:1989, General tolerances — Part 2: Geometrical tolerances for features without individual tolerance indications, ISO, 1989 (withdrawn; replaced by ISO 22081:2021)
- Timken Engineering Manual, The Timken Company, accessed 2026