GD&T is the language engineers use to say how much a part's shape, orientation and position can vary. Here is what it is, how to read it, and when it is worth using.
IN SHORT
- GD&T (geometric dimensioning and tolerancing) is a symbol language that defines how much a feature's form, orientation, location and runout may vary.
- It is defined by ASME Y14.5 in the USA and by the ISO GPS standards, mainly ISO 1101, elsewhere.
- Use it on features that affect fit and function, measured from datums that match how the part is located in use.
Plus and minus tolerances say how big a feature can be. They do not say much about its shape, or exactly where it must sit relative to the rest of the part. GD&T fills that gap. It describes the allowed variation in a way that matches how the part actually fits and works, and it gives the inspector a clear way to check it.
What does GD&T stand for?
Geometric Dimensioning and Tolerancing. It is a system of symbols, rules and definitions used on engineering drawings to control the geometry of features: how flat, straight, round, square, parallel or accurately positioned they must be.
Which standards define GD&T?
In the USA the main standard is ASME Y14.5, most recently revised in 2018. In Europe and much of the world, geometric tolerancing follows the ISO GPS (Geometrical Product Specifications) family, with ISO 1101 as the core standard. The symbols look almost the same, but the default rules differ. For example, ASME applies the envelope principle (Rule #1) by default, while ISO applies the independence principle (ISO 8015) unless the drawing says otherwise. Always state which standard a drawing follows.
How to read a feature control frame
Each geometric tolerance sits in a feature control frame: a row of boxes attached to the feature. Read it left to right.
- .01Box 1, the symbol: which characteristic is controlled, for example position or flatness
- .02Box 2, the tolerance: the size of the tolerance zone, with a diameter symbol if the zone is cylindrical, and any modifier such as maximum material condition
- .03Boxes 3 to 5, the datums: the reference features the tolerance is measured from, in order of priority (primary, secondary, tertiary)
The main GD&T characteristics
| CATEGORY | CHARACTERISTICS | WHAT IT CONTROLS |
|---|---|---|
| Form | Straightness, flatness, circularity, cylindricity | The shape of a single feature, with no datum |
| Orientation | Parallelism, perpendicularity, angularity | The angle of a feature relative to a datum |
| Location | Position (plus concentricity and symmetry in ISO) | Where a feature sits relative to datums |
| Profile | Profile of a line, profile of a surface | The shape, and often size and location, of a contoured feature |
| Runout | Circular runout, total runout | Variation of a surface as the part rotates about a datum axis |
NOTE · ASME Y14.5-2018 REMOVED CONCENTRICITY AND SYMMETRY. ISO STILL USES THEM. MOST OF THEIR JOBS ARE NOW DONE WITH POSITION, PROFILE OR RUNOUT.
What are datums?
Datums are the reference features a part is measured from, usually the faces, holes or shafts that locate it in its assembly. Choosing datums that match how the part is really held is the single most important GD&T decision. Good datums make the drawing describe function; poor ones make good parts fail inspection.
Why use GD&T instead of plus and minus tolerances?
- .01It describes function: the tolerance zone follows how the part fits, not just its size
- .02More usable tolerance: a round position zone through the corners of a square plus-and-minus zone is about 57% larger, so more good parts pass
- .03Bonus tolerance: with maximum material condition, a hole that is made larger gets extra position tolerance, while still guaranteeing assembly
- .04Clear inspection: datums tell the inspector exactly how to set up the part
- .05One interpretation: suppliers and inspectors read the drawing the same way
When to use GD&T, and when not to
Use GD&T on features that mate with other parts or that affect performance: bolt patterns, bearing seats, sealing faces, locating holes. Leave everything else on a general tolerance. A drawing covered in geometric tolerances is slower to make and inspect, without being any better.
Our related note, GD&T that machinists actually use, covers the handful of controls that do most of the work on real drawings.
Questions people ask
What is GD&T in simple words?
GD&T is a symbol language on engineering drawings that says how much a feature's shape, angle and position can vary, measured from defined reference features called datums.
What is the difference between ASME Y14.5 and ISO GD&T?
The symbols are very similar, but the default rules differ. ASME applies the envelope principle by default, ISO applies the independence principle, and ASME Y14.5-2018 no longer uses concentricity and symmetry. A drawing should state which standard it follows.
How many GD&T symbols are there?
ASME Y14.5-2018 uses 12 geometric characteristics across form, orientation, location, profile and runout. ISO also includes concentricity and symmetry, which ASME removed in 2018.
What is a datum in GD&T?
A datum is a reference feature, such as a face, hole or shaft, from which other features are measured. Datums should match how the part is located in its assembly.
What does MMC mean in GD&T?
Maximum material condition: the size at which a feature contains the most material, for example the smallest hole or largest shaft. A tolerance at MMC allows bonus tolerance as the feature moves away from that size.