GD&T Reference Guide — Geometric Dimensioning & Tolerancing (ASME Y14.5)

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Geometric Dimensioning & Tolerancing (GD&T)

GD&T is the language of engineering drawings. If you’re reading prints or designing parts, you need to understand these symbols. This reference covers the 14 geometric characteristic symbols defined in ASME Y14.5, with explanations and tolerance zone definitions.

GD&T replaces ambiguous plus/minus tolerancing with precise geometric controls. Instead of saying “this hole is 1.000 ± 0.005,” you can say “this hole’s axis must fall within a 0.010 cylindrical tolerance zone positioned exactly 2.000 from datum A.” Much more precise, much less open to interpretation.

The 14 Geometric Symbols

Geometric Dimensioning and Tolerances: Symbols (ASTM Y14.5-2009)

Gdt Angularity Angularity – The condition of a surface, centerplane or axis being exactly at a specified angle
Gdt Circularity Circularity – A condition where all points of a surface of revolution, at any section perpendicular to a common axis, are equidistant from that axis.
Gdt Concentricity Concentricity – The condition where the median points of all diametrically opposed elements of a cylinder (or a surface of revolution) are congruent with the axis of a datum feature.
Gdt Cylindricity Cylindricity – A condition of a surface of revolution in which all points of the surface are equidistant from a common axis.
Gdt Flatness

Flatness – The condition of a surface having all of its elements in one plane.

Gdt Parallel Parallelism – The condition that results when a surface, axis or centerplane is exactly parallel to a datum.
Gdt Perpendicularity Perpendicularity – The condition that results when a surface, axis, or centerplane is exactly 90° to a datum.
Gdt Position Position – The zone within which the axis or center plane of a feature is permitted to vary from true (theoretically exact) position.
Gdt Profline Profile of a Line Control – A geometric tolerance that limits the amount of error for line elements relative to their true profile.
Gdt Profsurf Profile of a Surface Control – A geometric tolerance that limits the amount of error a surface can have relative to its true profile.
Gdt Runout Circular Runout – A composite control that affects the form, orientation, and location of circular elements of a part feature relative to a datum axis.
Gdt Straightness Straightness (Axis or Centerplane) – The condition where an axis is a straight line (or, in the case of a centerplane, each line element is a straight line).
Gdt Symmetry Symmetry – The condition where the median points of all opposed elements of two or more feature surfaces are congruent with the axis or centerplane of a datum feature.
Gdt Totalrunout Total Runout – A composite control affecting the form, orientation, and location of all surface elements of a diameter (or surface) relative to a datum axis.

Geometric Dimensionsing and Tolerances: Modifiers (ASTM Y14.5-2009)

Gdt Allaround All-Around Symbol – A circle placed on the bend of the leader line of a profile control.
Gdt Basicdim Basic Dimension – A numerical value used to describe the theoretically exact size, true profile, orientation, or location of a feature or datum target.
Gdt Between Between Symbol – A double ended arrow that indicates the tolerance zone extends to include multiple surfaces.
Gdt Controlledradius Controlled Radius – A radius with no flats or reversals allowed. The symbol for a controlled radius is “CR.”
Gdt Datumfeature Datum Feature – A part feature that contacts a datum.
Gdt Datumtarget Datum Target – A symbol that describes the shape, size, and location of gage elements that are used to establish datum planes or axes.
Gdt Fcf Feature Control Frame – A rectangular box that is divided into compartments within which the geometric characteristic symbol, tolerance value, modifiers, and datum references are placed.
Gdt Lmc Least Material Condition – The condition in which a feature of size contains the least amount of material everywhere within the stated limits of size.
Gdt Mmc Maximum Material Condition – The condition in which a feature of size contains the maximum amount of material everywhere within the stated limits of size.
Gdt Projected Projected Tolerance Zone – A tolerance zone that is projected above the part surface.
Gdt Radius Radius – A straight line extending from the center of an arc or circle to its surface.

Reference Specification for Geometric Dimensioning and Tolerances:
ASTM Y14.5-2009