Geometric Dimensioning & Tolerancing: A Practical Guide with Case Studies

What is GD&T?

GD&T or Geometric Dimensioning and Tolerancing is a symbolic language used by designers to communicate their design intent to colleagues in production and metrology. The symbols are a language designed to convey a precise understanding of the dimensions of a part to ensure it meets its intended function. There are two widely-used standards internationally, the ASME standard uses the term GD&T, while the ISO standards prefer the term ‘GPS’ or ‘Geometric Product Specification’ – but there is a wide overlap between the two standards systems. Many of the controls used have similar meanings and symbols; these symbols are used within a ‘feature control frame’ or ‘tolerance frame’.

The purpose of this guide is to provide an introduction to and overview of concepts in GD&T. In this series of articles and case studies, our aim is to cover some of the more practical challenges of applying the GD&T standards in the real world, giving example measurements on real parts, and discussing the pitfalls to avoid and the opportunities to get the best possible data for your application.

GD&T symbols

Common GD&T symbols are listed in the table below. These are grouped into symbols relating to form, profile, orientation, runout and location. In some cases, different terms may refer to the same concept (e.g. roundness and circularity). In other cases, terms were used for controls in the past but have been withdrawn; these may still be encountered on many engineering drawings. Finally, some terms are used in industry but do not have dedicated GD&T controls (e.g. sphericity). But all of them are good to be aware of as a practical metrologist.

Please use the links in the table below to dive into a specific GD&T symbol.

TypeControlSymbol(s)Applicable StandardsDescription
FormRoundness or CircularityGDnt Roundness or Circularity SymbolASME Y14.5 to current (2018)
ISO 12181
A two-dimensional  measure, comparing a part or measured profile to a perfect circle
CylindricityGDnT Cylindricity SymbolASME Y 14.5 to current (2018)
ISO 12180
A three-dimensional measure, comparing a part or measured profile to a perfect cylinder
StraightnessGDnT Straightness SymbolASME Y 14.5 to current (2018)
ISO 12780
A measure in two or three dimensional space, reporting deviation from a perfect straight line for a feature (e.g. a part surface or an axis)
FlatnessGDnT Flatness SymbolASME Y 14.5 to current (2018)
ISO 12781
A three-dimensional measure, comparing a part or measured profile to a perfect 2D plane
ProfileProfile of a LineGDnT Profile of a Line SymbolASME Y 14.5 to current (2018)
ISO 1660
A measure along a 1D profile in 2D or 3D space, reporting deviation from the ideal design intent (which can be any freeform path)
Profile of a SurfaceGDnT Profile of a Surface SymbolASME Y 14.5 to current (2018)
ISO 1660
A measure over a 2D surface in 3D space, reporting deviation from the ideal design intent (which can be any freeform surface)
OrientationParallelismGDnT Parallelism SymbolASME Y 14.5 to current (2018)
ISO 1101
A measure of how parallel a line, axis or plane is to another line, axis, surface datum / datum system
PerpendicularityGDnT Perpendicularity SymbolASME Y 14.5 to current (2018)
ISO 1101
A measure of the deviation from a 90° orientation between a line, axis or plane and some other line/axis/surface datum/datum system.
AngularityGDnT Angularity SymbolASME Y 14.5 to current (2018)
ISO 1101
An orientation measure which can control any required angle between
RunoutCircular RunoutGDnT Run-out (circular) SymbolASME Y 14.5 to current (2018)
ISO 1101
A control which limits local change in surface position as a part is rotated about an axis. Each local measure is independent of each other measure.
Total RunoutGDnT Run-out (total) SymbolASME Y 14.5 to current (2018) A control which limits total change in surface position across an entire surface simultaneously as a part is rotated about an axis.
LocationConcentricity MeasurementGDnT Concentricity SymbolGDnT Position SymbolASME Y14.5 up to 2009 (withdrawn 2018)
ISO 1101
A control which ensures the ‘centre’ points of a profile lie on a defined datum point or axis; this requires a clear definition for the centre points. Note that position control is now used instead in the ASME standard.
SymmetryGDnT Symmetry SymbolGDnT Position SymbolASME Y14.5 up to 2009 (withdrawn 2018)
ISO 1101
A control to ensure the mid-points of a nominally symmetric profile lie in the datum line/plane defining the mirror symmetry Note that position control is now used instead in the ASME standard.
PositionGDnT Position SymbolASME Y 14.5 to current (2018)
ISO 5458
Controls the location of a point, centre axis or centre plane of a feature, relative to a datum feature or (for a feature pattern) relative to other pattern instances
Feature of Sizeno symbolASME Y 14.5 to current (2018)
ISO 14405, 14406
Controls the absolute size of a part within tolerances. May/may not influence other controls depending on standards used. Can be used to define an axis/midplane for datum referencing.
RoughnessGDnT Roughness SymbolASME B 46.1, ISO 1302 & related standardsControls local surface profile at the smaller scale (short spatial wavelength)
Special Cases
SphericityCovered as a special case of circularity or form profile; application-specific standards also exist (e.g. ISO 7206 for implants)A three-dimensional measure comparing a part of measured profile to a perfect sphere (ball)

Feature control frame or tolerance frame

Many of the common symbols used in technical drawings make use of a graphical callout structure known in the ASME standard as a ‘feature control frame’ (FCF) and in the ISO GPS family of standards as a ‘tolerance frame’. Despite the difference in name, both do broadly the same thing; they break out one particular inspection requirement for the finished part, and provide information on the parameters for that inspection. The illustration below breaks down the typical content of these drawing frames:

Note that not all of these elements will be present in every frame callout; some may only feature a symbol and a single number. But it is important to account for all the information in the frame, as every detail can significantly change how the tolerance is evaluated, and thus whether a part will pass or fail inspection.