# GD&T Symbols – A Beginner’s Guide

Geometric dimensioning and tolerancing (GD&T) consists of a set of symbols and rules for applying them that communicates the requirements of an engineering blueprint.

GD&T controls variations of size, form, orientation, location and runout individually or in combination.

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## GD&T and Engineering Blueprint Symbols

### True Position

Controls how far a feature can deviate from a given location.

### Straightness

A tolerance given that defines how far from a perfect line a feature can be. A 2D (line) requirement.

### Flatness

Similar to straightness but the callout applies over the entire surface (plane).

### Perpendicularity

Defines the maximum allowed deviation of a feature from 90°.

### Parallelism

Describes two features that are equally distanced from each other over their entire surfaces.

### Angularity

Angularity is the allowed deviation from a theoretically perfect feature at a specified angle other than 90° or 180° from another feature.

### Circular Runout

Controls the runout of a rotated circular at any single location on the feature.

### Total Runout

Controls the runout of a rotated circular at all locations on the feature.

### Profile of a Line

A tolerance that identifies how close to a specified design a feature is at specific cross section.

### Profile of a Surface

A tolerance that identifies how close to a specified design a whole feature is.

### Circularity

A tolerance that defines how round a feature must be at a single location.

### Cylindricity

Similar to circularity but it applies over the entire cylinder instead of at a single point.

### Concentricity

A requirement that the centerline or axis of two features are located within a specified tolerance of each other.

### Symmetry

Controls the location of a feature such as a slot to a datum.

### Envelope Requirement

Specifies that the feature may not exceed the minimum or maximum total size requirement. This applies at each cross section as well as over the entire feature.

### Free State

For non-rigid parts, the specified GD&T requirements apply when the part is in a free state, where only the force of gravity is affecting the part.

### Independency

Removes the form control for a feature. When independency is called out the feature size is controlled only by the attached requirement.

### Least Material Condition (LMC)

A modifier that can give bonus tolerance based how close or far the features are from nominal size.

### Maximum Material Condition (MMC)

A modifier that can give bonus tolerance based how close or far the features are from nominal size.

### Projected Tolerance Zone

Used when a tolerance zone applies outside the boundaries of the physical part.

### Regardless of Feature Size (RFS)

The tolerances do not change based on the size of the features. This is an older symbol that is no longer used. This condition is assumed unless specified otherwise now.

### Tangent Plane

Creates a plane based on the highest contact points of a feature.

### Unequally Disposed Profile

Profile tolerance zones are normally centered on the nominal dimensions. When this is not the case, the unequally disposed tolerance symbol is used to move the tolerance zone based off the nominal dimensions.

### Continuous Feature

Identifies two or more separate features that should be treated as one such as two planes separated by a keyway.

### Statistical Tolerance

Specifies that statistical process controls can be used which may be more cost effective in some situations.

### Slope

Identifies the rise over run of a feature. Shown as a ratio.

### Conical Taper

The ratio of size difference between two diameter sections.

### Datum Target Point

A single point that is to be used as a datum or as part of a datum.

### Basic Dimension

A basic dimension is a theoretically exact dimension used to calculate GD&T tolerances. Shown enclosed in a box. Older blueprints may identify the feature with BSC instead.

### Reference Dimension

Reference dimensions are shown in parentheses. They are not blueprint requirements. They are shown for informational purposes only.

### Arc Length

Describes the length along an arc. Measures the distance around the outside of the arc. It is not a chord length.

### Dimension Not to Scale

Features are not shown with the same relative scale for all features.

### Square

Identifies a feature that has equal sizes for both length and width.

### Diameter

The distance across the center of a circle. Two times the radius.

Half of the diameter. The distance from the center of a circle to the edge.

### Spherical Diameter

A sphere or ball shaped feature specified by the diameter of the sphere.

A sphere or ball shaped feature specified by the distance from the center of the sphere to the outside.

A radius without flats or reversals.

### Common Tolerance Zone

Identifies two or more separate features that should be treated as one such as two planes separated by a keyway.

### Datum

A feature which is used for location and measurement of other features.

### Counterbore

A flat bottomed, recessed hole

### Countersink

A round, angled feature that allows a fastener such as a screw head to sit flush or below the surface.

### Spotface

A shallow counterbore. Shown as on simply SF on some blueprints.

### Depth of

Identifies how deep a feature is. Commonly applied to holes, counterbores, etc.

### Surface Finish

Describes the surface texture or roughness of a surface. Can be specified as a max allowable roughness as  shown above or a range.

### Number of Places

An identifier that lists the number of instances of a repeated feature.

### All Around

Identifies that a requirement applies around the entire perimeter (edge) of the part.

### All Over

Identifies that a requirement applies to all surfaces and features of a part.

### Datum Translation

Allows the identified datum locating feature to be adjusted so that the feature is fully engaged.

### Movable Datum Target

Identifies datum targets that may be shifted in a specified direction.

### Dynamic Profile Tolerance Zone

Controls the form of a feature. When the dynamic profile modifier is used, the profile callout does not control the size of the feature.

### Datum Target

The top section is the size and shape of the target area. The lower section lists the identifying letter and number.

### From-To

Indicates a gradual change in tolerance between two points. The arrow points in the direction of the tolerance change.

### Between

Feature requirements apply between two specified locations.

### Dimension Origin

Indicates the feature that another feature should be measured from. Often used to indicate a measurement should be taken from a shorter surface.

### What does GD&T stand for?

GD&T stands for geometric dimensioning and tolerancing.

### What is GD&T used for?

GD&T is a way for engineers and designers to more accurately control features and tolerances on manufacturing drawings.

GD&T allows designers to place only the needed controls and tolerances on features which reduces cost and complexity of components.

GD&T is critical for designing mating parts and assemblies effectively.

### What are the 5 categories of GD&T callouts?

The categories of GD&T symbols are form, profile, orientation, location and runout.

In addition there are a large amount of supplementary symbols which are used to identify blueprint requirements.

### What standards control GD&T use?

There are two main standards which govern the use of various GD&T symbols.

ASME Y14.5 is the main standard referenced and the current revision is the 2018 version of the standard. The standard is commonly referred to as ASME Y14.5-2018 or on occasion simply, Y14.5-2018.

There are multiple ISO standards which govern the use of symbols and their interpretation. ISO 8015, ISO 1101 and ISO 2768 are the three most common standards that detail GD&T use and interpretation.

### How to read a GD&T callout

A GD&T callout comes in the form of a feature control frame.

Feature control frames are read from left to right.

It reads “Type of control” of “Tolerance” to Datum. It should be noted that if a diameter symbol is present before “Tolerance” then it indicates the shape of the tolerance zone is cylindrical.

Here are a couple examples with description of how to read them:

True position of 0.2 to datums A and B

Perpendicularity of 0.001 to datum A

### What is the best way to learn GD&T?

GD&T is a complicated subject and understanding it correctly can be the difference between a perfect part and scrap.

The best way to learn GD&T is from experienced teachers who can break down the material into manageable pieces.

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