Geometric Dimensioning and Tolerances (GD&T) is the practice of communicating the geometric relationships of a design beyond simple dimensions.
It is developed and outlined by the American Society of Mechanical Engineers in the specification ASTM Y14.5.
Geometric Dimensionsing and Tolerances Modifiers (ASTM Y14.5-2009) include the following:
 |
Angularity – The condition of a surface, centerplane or axis being exactly at a specified angle |
 |
Circularity – A condition where all points of a surface of revolution, at any section perpendicular to a common axis, are equidistant from that axis. |
 |
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. |
 |
Cylindricity – A condition of a surface of revolution in which all points of the surface are equidistant from a common axis. |
 |
Flatness – The condition of a surface having all of its elements in one plane. |
 |
Parallelism – The condition that results when a surface, axis or centerplane is exactly parallel to a datum. |
 |
Perpendicularity – The condition that results when a surface, axis, or centerplane is exactly 90° to a datum. |
 |
Position – The zone within which the axis or center plane of a feature is permitted to vary from true (theoretically exact) position. |
 |
Profile of a Line Control – A geometric tolerance that limits the amount of error for line elements relative to their true profile. |
 |
Profile of a Surface Control – A geometric tolerance that limits the amount of error a surface can have relative to its true profile. |
 |
Circular Runout – A composite control that affects the form, orientation, and location of circular elements of a part feature relative to a datum axis. |
 |
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). |
 |
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. |
 |
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. |
 |
All-Around Symbol – A circle placed on the bend of the leader line of a profile control. |
 |
Basic Dimension – A numerical value used to describe the theoretically exact size, true profile, orientation, or location of a feature or datum target. |
 |
Between Symbol – A double ended arrow that indicates the tolerance zone extends to include multiple surfaces. |
 |
Controlled Radius – A radius with no flats or reversals allowed. The symbol for a controlled radius is “CR.” |
 |
Datum Feature – A part feature that contacts a datum. |
 |
Datum Target – A symbol that describes the shape, size, and location of gage elements that are used to establish datum planes or axes. |
 |
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. |
 |
Least Material Condition – The condition in which a feature of size contains the least amount of material everywhere within the stated limits of size. |
 |
Maximum Material Condition – The condition in which a feature of size contains the maximum amount of material everywhere within the stated limits of size. |
 |
Projected Tolerance Zone – A tolerance zone that is projected above the part surface. |
 |
Radius – A straight line extending from the center of an arc or circle to its surface. |
GD&T Reference Guide (ASME Y14.5) Geometric Dimensioning and Tolerancing symbols and definitions. 1. FORM (No Datums) Symbol Name & Definition Straightness: 2D straightness of a line element. Flatness: Surface must lie between two parallel planes. Circularity: Roundness of a 2D cross-section. Cylindricity: Roundness + Straightness of a 3D cylinder. 2. ORIENTATION (Datums Required) Symbol Name … Read more
What is ISO 286? ISO 286 defines the system of limits and fits for holes and shafts. If you’ve ever seen a callout like H7/g6 on a drawing and wondered what it actually means, this is your guide. The metric system uses a code-based approach instead of explicit ± tolerances. Once you understand the logic, … Read more
What is Tolerance Stack-Up? When multiple parts assemble together, each dimension has a tolerance. Those tolerances accumulate. Tolerance stack-up analysis tells you whether your assembly will work — or whether you’ll be forcing parts together on the shop floor. There are two fundamental approaches: Worst Case (everything goes wrong simultaneously) and RSS/Statistical (realistic probability-based analysis). … Read more
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 … Read more
Why Surface Finish Matters Surface finish (or surface roughness) describes the texture of a machined surface. It affects sealing, wear, fatigue life, coating adhesion, and appearance. If you’re designing parts that mate, seal, or slide against each other, you need to specify surface finish on your drawing. Common Surface Finish Values Ra (µin) Ra (µm) … Read more