GD&T Symbol Quick Reference

Fourteen GD&T characteristics from ASME Y14.5 and ISO 1101, in one place — what each controls, whether it needs a datum, and what it does to machining cost and assembly fit. Tap any symbol to see the detail. Basis: ASME Y14.5 / ISO 1101.

Read GD&T for Buyers

Why This Reference Exists

You're reading a drawing with a frame around a number and a symbol you half-remember, and you need to know whether it will cost you a setup or just a line on the inspection report. GD&T is the language of fit and function, not a way to make parts expensive — but the wrong callout does exactly that. This reference is the cheat sheet we wish every buyer had: every characteristic, what it actually constrains, whether it needs a datum, and the cost and fit trade behind it. Use it before you release a drawing, or before you question a quote that cited a GD&T callout.

Quick filter

Every GD&T Characteristic

Type a keyword — flatness, datum, runout, position — to narrow the list, or open any item for the detail.

⌓StraightnessForm · no datum

Controls: how straight a line or axis is — a 2D element or the axis of a cylinder.

Cost / fit: cheap in isolation; on a critical bore axis it forces a controlled turning setup. Loosen it on non-functional edges to save inspection.

Basis: ASME Y14.5, ISO 1101 (form tolerance, no datum required).

▱FlatnessForm · no datum

Controls: how flat a single surface is, independent of any other feature.

Cost / fit: one of the cheapest callouts — but on a sealing face it drives the whole stock-removal and inspection plan. State it only where the seal actually depends on it.

Basis: ASME Y14.5, ISO 1101.

○CircularityForm · no datum

Controls: roundness — the variation of a single circular cross-section from a perfect circle.

Cost / fit: measured per slice; on a bearing or piston surface it adds roundness checks but no datum setup. Asking for it everywhere is the classic over-spec.

Basis: ASME Y14.5, ISO 1101.

⌭CylindricityForm · no datum

Controls: the complete form of a cylinder — combining roundness, straightness and taper into one zone.

Cost / fit: stricter and pricier than circularity because it captures the whole element; reserve it for precision shafts and spindles.

Basis: ASME Y14.5, ISO 1101.

⌒Profile of a LineForm · optional datum

Controls: the 2D outline of a feature against a true contour.

Cost / fit: ideal for curved or contoured profiles that a ± chain can't describe cleanly; adds programming but replaces several dimensions.

Basis: ASME Y14.5, ISO 1101.

⌔Profile of a SurfaceForm · optional datum

Controls: the full 3D boundary of a surface — the workhorse for complex organic shapes.

Cost / fit: very flexible; on a functional aerodynamic or sealing surface it's worth the inspection. On a simple face it's overkill versus flatness.

Basis: ASME Y14.5, ISO 1101.

⊥PerpendicularityOrientation · datum

Controls: the angle between a feature and a datum plane or axis — within 90° ± a tolerance zone.

Cost / fit: common and moderate; on a mounting face it's expected. Tighten it only where the assembly truly sits on that angle.

Basis: ASME Y14.5, ISO 1101 (orientation, needs a datum).

∠AngularityOrientation · datum

Controls: a feature at any specified angle to a datum — not just 90°.

Cost / fit: same cost family as perpendicularity; useful when a part seats at an angle. Often replaced by a basic angle plus a ± if the fit is loose.

Basis: ASME Y14.5, ISO 1101.

∥ParallelismOrientation · datum

Controls: that two surfaces or axes stay a constant distance / direction relative to a datum.

Cost / fit: cheap when the datum is already being held; pairs naturally with flatness. Watch stacking it on top of flatness on the same face.

Basis: ASME Y14.5, ISO 1101.

⌖PositionLocation · datum

Controls: where a feature's axis or centre sits relative to datums — a diametral/2D zone, not separate ± directions.

Cost / fit: the most powerful callout: it earns bonus tolerance at MMC, which can relax inspection. Tight position on multiple datums is where cost climbs — but it's also what makes parts actually fit.

Basis: ASME Y14.5 (MMC bonus), ISO 1101.

◎ConcentricityLocation · datum

Controls: that a feature's median axis coincides with a datum axis.

Cost / fit: expensive — it's measured on the derived median, not the surface, so it needs careful CMM work. Often replaceable by position or runout at lower cost.

Basis: ASME Y14.5, ISO 1101.

◉SymmetryLocation · datum

Controls: that a feature is centred on a datum plane.

Cost / fit: like concentricity, it's median-based and inspection-heavy; usually replaced by position at lower cost unless true median control is required.

Basis: ASME Y14.5, ISO 1101.

↗Circular RunoutRunout · datum

Controls: the combined wobble of one circular slice — roundness plus coaxiality — as the part turns.

Cost / fit: moderate; right for a sealing or bearing surface that must stay true at one section. Specifying it along the whole length should be total runout instead.

Basis: ASME Y14.5, ISO 1101.

⌰Total RunoutRunout · datum

Controls: cumulative variation along the entire surface from end to end, relative to the datum axis.

Cost / fit: the strictest, most expensive runout — it forces a controlled turning setup and full-length checks. Reserve it for shafts and seals that must stay true along their length.

Basis: ASME Y14.5, ISO 1101.

One rule that saves money

Form First, Then Datum-Linked — and Only Where It Matters

The single biggest GD&T cost mistake is calling out a datum-linked tolerance (position, runout) on a feature that only needed a form tolerance (flatness, straightness). Form tolerances need no datum and no extra setup; location and runout tolerances pull in datums, extra inspection and sometimes a second operation. Before you tighten, ask what actually has to fit — and free the rest. The full decision logic and the tolerance-cost curve are in the tolerance chart, and the buyer's plain-language version is in GD&T for Buyers.

FAQ

GD&T Symbols — Common Questions

Do all GD&T symbols need a datum?

No — only orientation, location and runout callouts reference a datum. The four form tolerances (straightness, flatness, circularity, cylindricity) control a feature's own shape and need no datum. When it does not apply: a form tolerance can reference a datum only in the special case of a derived median line or plane, which is advanced usage. Basis: ASME Y14.5, ISO 1101.

Is a position tolerance the same as a ± dimension?

No — a position tolerance defines a diametral (or 2D) zone the axis or centre must sit inside, and it earns bonus tolerance as the feature grows toward its MMC limit, which a ± dimension cannot do. A ± callout controls each direction separately and gives no bonus. When it does not apply: for a feature whose only job is a size, a ± or limit dimension is simpler and sufficient. Basis: ASME Y14.5 position and MMC rules.

Which GD&T callouts add the most cost?

The expensive ones are tight location and runout callouts that force extra setups, datums and 100% CMM inspection — total runout and tight position on multiple datums cost more than a simple flatness or straightness. A form tolerance on a non-critical surface adds little; a three-datum position at ±0.01 mm adds a lot. When it does not apply: tighten only the feature that actually affects fit, and relax the rest. Basis: DFM practice; see the tolerance chart.

What's the difference between circular and total runout?

Circular runout limits the wobble of one circular cross-section as it turns; total runout limits the cumulative variation along the whole surface from end to end. Total runout is stricter and more expensive because it controls the entire element, not a single slice. When it does not apply: for a sealed or bearing surface that must stay true along its length, circular runout is not enough — you need total. Basis: ASME Y14.5, ISO 1101.

Does this reference cover both ASME Y14.5 and ISO 1101?

The characteristic set is the same under both standards — ISO 1101 defines the symbols and tolerances, and ASME Y14.5 uses the same symbols with its own rule details (datums, MMC, etc.). The symbols and what each controls are listed here for either system. When it does not apply: the default rule for bonus tolerance and datum precedence differs slightly between the two, so confirm the standard on your drawing. Basis: ASME Y14.5, ISO 1101.

Not sure which callout your drawing needs?

Send us the drawing with the features that must fit flagged. We'll flag over-specified GD&T, propose the cheapest callout that still holds the assembly, and return DFM feedback and a per-drawing quote within 24 hours — before any metal is cut.

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Sources

Where This Comes From

ASME Y14.5 — dimensioning and tolerancing; the reference for GD&T symbols, datums and MMC rules.
ISO 1101 — geometrical product specifications (GPS); GD&T symbols and rules.

The 14 characteristics and their datum requirements follow the standard symbol set. Specific tolerance values and bonus-tolerance behaviour are defined by the standard cited on your drawing, not by this reference.