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 Blueprint GD&T Symbol target

True Position

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

Straightness Blueprint GD&T Symbol line

Straightness

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

Flatness Blueprint GD&T Symbol parallelogram

Flatness

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

Perpendicularity Blueprint GD&T Symbol two lines perpendicular to each other

Perpendicularity

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

Parallelism Blueprint GD&T Symbol two parallel lines

Parallelism

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

Angularity Blueprint GD&T Symbol two lines at an angle

Angularity

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

Runout Blueprint GD&T Symbol single arrow

Circular Runout

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

Total Runout Blueprint GD&T Symbol two arrows

Total Runout

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

Profile of a Line Blueprint GD&T Symbol half circle not connected

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 Blueprint GD&T Symbol half circle

Profile of a Surface

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

Circularity Blueprint GD&T Symbol circle

Circularity

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

Cylindricity Blueprint GD&T Symbol circle with two lines coming off it

Cylindricity

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

Concentricity Blueprint GD&T Symbol two concentric circles

Concentricity

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

Symmetry Blueprint GD&T Symbol three lines

Symmetry

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

Envelope Requirement Blueprint GD&T Symbol e in a circle

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 Blueprint GD&T Symbol f in a circle

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 Blueprint GD&T Symbol i in a circle

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 Blueprint GD&T Symbol l in a circle

Least Material Condition

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

Maximum Material Condition Blueprint GD&T Symbol m in a circle

Maximum Material Condition

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

Projected Tolerance Zone Blueprint GD&T Symbol p in a circle

Projected Tolerance Zone

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

Regardless of Feature Size Blueprint GD&T Symbol s in a circle

Regardless of Feature Size

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 Blueprint GD&T Symbol t in a circle

Tangent Plane

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

Unequally Disposed Profile Blueprint GD&T Symbol U in a circle

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 Blueprint GD&T Symbol cf in a hexagon

Continuous Feature

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

Statistical Tolerance Blueprint GD&T Symbol st in a hexagon

Statistical Tolerance

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

Slope Blueprint GD&T Symbol triangle

Slope

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

Conical Taper Blueprint GD&T Symbol triangle with line through it

Conical Taper

The ratio of size difference between two diameter sections.

Target Point Blueprint GD&T Symbol x

Datum Target Point

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

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

Arc Length Blueprint GD&T Symbol number with an arc over it

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 Blueprint GD&T Symbol number with line underneath

Dimension Not to Scale

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

Square Blueprint GD&T Symbol

Square

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

Diameter Blueprint GD&T Symbol o with line through it

Diameter

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

Radius Blueprint GD&T Symbol r

Radius

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

Spherical Diameter Blueprint GD&T Symbol s and o with a line through it

Spherical Diameter

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

Spherical Radius Blueprint GD&T Symbol SR

Spherical Radius

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

Controlled Radius Blueprint GD&T Symbol cr

Controlled Radius

A radius without flats or reversals.

Common Tolerance Zone Blueprint GD&T Symbol cz

Common Tolerance Zone

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

Datum Blueprint GD&T Symbol a in a box with arrow

Datum

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

A flat bottomed, recessed hole

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

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

Depth Blueprint GD&T Symbol line with arrow pointing down

Depth of

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

Surface Finish Blueprint GD&T Symbol check mark

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.

Places Blueprint GD&T Symbol 4x or 4 pl.

Number of Places

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

All Around Blueprint GD&T Symbol a bent arrow with a circle at the bend

All Around

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

All Over Blueprint GD&T Symbol a bent arrow with two circles at the bend

All Over

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

Translation Blueprint GD&T Symbol triangle

Datum Translation

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

Movable Datum Target Blueprint GD&T Symbol circle with A1 in it and lines coming off

Movable Datum Target

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

Dynamic Profile Blueprint GD&T Symbol triangle

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 Blueprint GD&T Symbol circle with diameter 3 in top and A1 in bottom half

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 Blueprint GD&T Symbol arrow

From-To

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

Between Blueprint GD&T Symbol line with arrows on both ends

Between

Feature requirements apply between two specified locations.

Dimension Origin Blueprint GD&T Symbol cross hair with arrow

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.

Frequently asked questions

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

feature control frame description with parts identified

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 callout

True position of 0.2 to datums A and B

perpendicularity callout example with feature control frame

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.

Luckily, we know someone.

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