GD&T explained: a buyer's guide to geometric tolerancing
What GD&T is, why it beats plus/minus tolerances, the 14 symbols grouped by type, datums, and how the callouts you choose drive machining cost.
GD&T (geometric dimensioning and tolerancing) is a drawing language, defined by ASME Y14.5, that controls the form, orientation, location and runout of features relative to datums, instead of just plus/minus on each dimension. It tells the shop what actually has to be true for the part to work, and it lets a feature move within a real tolerance zone rather than a box. Used well it makes parts cheaper and inspection unambiguous. Used as decoration it just raises your quote.
Why plus/minus is not enough
Coordinate (plus/minus) tolerances stack up and square off the tolerance zone, which can reject good parts and accept bad ones. GD&T defines a tolerance zone tied to datums, so a hole can sit anywhere in a round zone (true position) rather than a square one, which is both more honest about function and about 57% more area to work in. It also separates the features that matter (the mating bore) from the ones that do not (a clearance edge), so the shop spends money where the function is.
The 14 symbols, grouped
| Type | Controls | Symbols |
|---|---|---|
| Form | A surface by itself | Flatness, straightness, circularity, cylindricity |
| Orientation | A feature to a datum angle | Perpendicularity, parallelism, angularity |
| Location | Where a feature sits | True position, concentricity, symmetry |
| Profile | A whole surface or line | Profile of a surface, profile of a line |
| Runout | Rotation about an axis | Circular runout, total runout |
True position and profile do most of the real work; the rest are specialized. If you learn two, learn those.
Datums: the reference frame
A datum is the surface or feature everything is measured from, labelled A, B, C. The datum reference frame (primary, secondary, tertiary) locks the part in space the way it actually functions or mounts. Picking datums that match how the part is used, not just a convenient flat face, is the single biggest thing that makes a drawing inspectable.
How GD&T drives cost
| Callout choice | Effect on cost |
|---|---|
| Looser position zones where function allows | Lower: more parts pass, faster machining |
| Maximum material condition (MMC) modifier | Lower: bonus tolerance as features depart from MMC |
| Tight flatness or cylindricity on large faces | Higher: grinding or lapping, slow metrology |
| Tight tolerances with no datums | Higher and ambiguous: the shop guesses the setup |
The cheapest path is to tolerance only what the function needs and let everything else open up. See the tolerances and surface finish reference for the plus/minus side of the same decision.
What to send for a quote
A fully dimensioned drawing with datums defined and GD&T on the features that matter, plus a STEP model. If you are not sure which callouts your part needs, send the model and the function (what mates to what) and the shop can advise. Request quotes from Canadian shops that machine to GD&T every day, including AS9100 shops for aerospace drawings where first-article inspection against GD&T is mandatory.
FAQ
What does GD&T stand for?
Geometric dimensioning and tolerancing. It is a symbolic language, standardized in ASME Y14.5, for defining the allowable geometry of a part relative to datums.
Is GD&T better than plus/minus tolerances?
For functional features, yes: it defines a tolerance zone tied to how the part works, avoids tolerance stack-up, and is unambiguous to inspect. For simple, non-critical dimensions, plus/minus is fine and simpler.
What is a datum in GD&T?
The reference surface or feature that other features are measured from, labelled A, B, C. The datum reference frame should match how the part mounts or functions.
What is true position?
A location control that defines a round (or cylindrical) tolerance zone for a feature's axis, instead of a square plus/minus box. It gives more usable tolerance and models function better, especially for hole patterns.
Does GD&T make parts more expensive?
Only if you over-apply it. Tolerancing only the functional features and opening up the rest usually makes parts cheaper, because more parts pass and the shop machines to the real requirement.
Last reviewed: October 2026. Related reading: CNC tolerances and surface finish and what is CNC machining.
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