Overview:

GD&T Symbols, Basics & Full Form

What is GD&T?

GD&T = Geometric Dimensioning & Tolerancing

A standard language used on engineering drawings
Defines the allowable variation in a part
Controls form, orientation, location & runout
Helps achieve accurate fit, function & interchangeability

Why is GD&T Important?

Reduces manufacturing ambiguity
Improves part quality and consistency
Simplifies inspection and measurement
Supports better communication between Design → Manufacturing → Quality

Let’s understand GD&T — symbol by symbol.

GD&T Symbols, Basics, and Full Forms

GD&T stands for Geometric Dimensioning and Tolerancing. It is a standardized engineering drawing language used to define the size, form, orientation, location, and allowable variation of a component’s features.

Unlike conventional ± tolerancing, GD&T uses symbols and rules to communicate design requirements accurately.

It is widely used in mechanical engineering, manufacturing, CNC machining, automotive, aerospace, inspection, and quality control.

1. GD&T Full Form

GD&T = Geometric Dimensioning and Tolerancing

Term	Meaning
Geometric	Relates to the geometry/form of a feature
Dimensioning	Specifies the required size and location
Tolerancing	Specifies the permissible variation

GD&T is commonly based on standards such as ASME Y14.5 in the United States and ISO GPS standards internationally.

2. Why Is GD&T Used?

GD&T provides a precise method of communicating design requirements between:

Designer → Manufacturer → Inspector → Quality Department

Major advantages

Improves drawing clarity
Controls manufacturing variation
Ensures proper assembly
Reduces unnecessary tight tolerances
Improves interchangeability
Reduces manufacturing cost
Makes inspection easier
Provides a common engineering language

3. Basic Concepts of GD&T

Before learning the symbols, it is important to understand a few fundamental terms.

3.1 Feature

A feature is a physical portion of a component that can be identified on a drawing.

Examples:

Hole
Shaft
Slot
Surface
Cylinder
Step
Plane

3.2 Feature of Size

A feature that has a measurable size, generally associated with:

Diameter
Width
Thickness
Length

Examples:

Hole diameter
Shaft diameter
Slot width

3.3 Datum

A datum is a theoretically exact reference from which another feature is located or oriented.

Datums are usually identified by letters such as:

A, B, C

For example:

Datum A = primary reference surface
Datum B = secondary reference
Datum C = tertiary reference

A datum system can therefore establish the component’s reference coordinate system.

4. The Three Major Categories of GD&T

GD&T controls can broadly be divided into:

A. Form

Controls the shape of an individual feature.

B. Orientation

Controls the angular relationship between features.

C. Location

Controls where a feature is located.

There is also profile and runout, which provide additional geometric controls.

5. Complete GD&T Symbol Chart

The following are the major GD&T symbols used in engineering drawings.

Category	Control	Symbol	What it controls
Form	Straightness	⏤	Straightness of a line/axis
	Flatness	▱	Flatness of a surface
	Circularity / Roundness	○	Roundness of a circular feature
	Cylindricity	⌭	Entire cylindrical form
Profile	Profile of a Line	⌒	Profile of a line
	Profile of a Surface	⌓	Profile of a surface
Orientation	Angularity	∠	Angular orientation
	Perpendicularity	⟂	90° orientation
	Parallelism	∥	Parallel orientation
Location	Position	⌖	Exact location of a feature
	Concentricity*	◎	Relationship of median points/axes
	Symmetry*	⌯	Symmetry about a datum
Runout	Circular Runout	↗	Circular variation during rotation
	Total Runout	⌰	Entire surface variation

*Concentricity and symmetry are retained in some standards/drawing practices but are no longer included as active controls in current ASME Y14.5 practice; position and other controls are generally preferred where applicable.

Note: Symbol appearance can vary slightly depending on font, CAD software, standard, and drawing convention.

6. Form Tolerances

Form tolerances control the shape of an individual feature. They generally do not require a datum reference.

There are four principal form controls:

Straightness
Flatness
Circularity
Cylindricity

Straightness

Symbol: ⏤

Straightness controls how much a line or axis may deviate from a theoretically perfect straight line.

Example

A shaft may be specified as:

Straightness | 0.05 mm

This means the controlled line element must remain within the specified straightness tolerance zone.

Applications

Shafts
Edges
Centerlines
Guide rails

Important point

Straightness does not control:

Parallelism
Perpendicularity
Location

It controls only straightness.

Flatness

Symbol: ▱

Flatness controls how much a surface can deviate from a theoretically perfect plane.

A flatness tolerance creates two parallel planes separated by the specified tolerance.

Example

Flatness | 0.03 mm

The entire controlled surface must lie between two parallel planes separated by 0.03 mm.

Applications

Mounting surfaces
Gasket surfaces
Machine bases
Flanges

Important point

Flatness does not require a datum.

Circularity / Roundness

Symbol: ○

Circularity controls the roundness of individual circular cross-sections.

The actual circular profile must lie between two concentric circles separated radially by the specified tolerance.

Example

Circularity | 0.02 mm

This means each applicable cross-section must meet the specified roundness requirement.

Applications

Shafts
Bearings
Holes
Cylindrical components

Important point

Circularity does not directly control:

Axis location
Orientation
Cylindricity

Cylindricity

Symbol: ⌭

Cylindricity controls the complete cylindrical form of a surface.

It simultaneously controls the surface’s:

Roundness
Straightness
Cylindrical form

Example

Cylindricity | 0.05 mm

The entire cylindrical surface must lie between two coaxial cylinders separated by 0.05 mm.

Applications

Precision shafts
Hydraulic cylinders
Bearing seats
Rotating components

Difference between circularity and cylindricity

Circularity	Cylindricity
Controls individual cross-sections	Controls the entire cylindrical surface
2D control	3D control
No datum	No datum
Usually simpler	More comprehensive

7. Profile Tolerances

Profile controls are useful for controlling complex curves and surfaces.

There are two main profile controls:

Profile of a line
Profile of a surface

Profile of a Line

Symbol: ⌒

Profile of a line controls the shape of a line element or cross-sectional profile.

It is particularly useful for:

Curved surfaces
Radii
Contoured sections
Airfoil profiles

Profile of a Surface

Symbol: ⌓

Profile of a surface controls the entire three-dimensional surface.

It can control:

Form
Orientation
Location

depending on how the tolerance is referenced and applied.

Applications

Cast components
Forged components
Automotive body panels
Complex machined surfaces
Molded parts

9. Orientation Tolerances

Orientation controls the angular relationship of a feature relative to a datum.

The major orientation controls are:

Parallelism
Perpendicularity
Angularity

Parallelism

Symbol: ∥

Parallelism controls how parallel a surface, line, or axis is to a datum.

Example

Parallelism | 0.04 | A

This means the controlled feature must maintain the specified parallel relationship with datum A.

Applications

Parallel faces
Shafts
Guide surfaces
Machine slides

Important

Parallelism controls orientation, not necessarily the feature’s location.

Perpendicularity

Symbol: ⟂

Perpendicularity controls whether a feature is oriented at 90° to a datum.

Example

Perpendicularity | 0.03 | A

The controlled feature must remain within the specified perpendicularity tolerance zone relative to datum A.

Applications

Holes
Shafts
Mounting faces
Vertical walls

Angularity

Symbol: ∠

Angularity controls a feature at a specified angle other than the standard 0° or 90° relationships.

For example:

30° ± 0.5°

may define a basic angle, while GD&T angularity controls the allowable geometric variation around that theoretically exact angle.

Applications

Tapered surfaces
Inclined holes
Angled faces
Chamfers

10. Location Tolerances

Location controls specify where a feature should be positioned.

The most important modern location control is:

Position

Symbol: ⌖

Position is one of the most widely used GD&T controls.

It controls the location of a feature relative to one or more datums.

Position Tolerance

Suppose a drawing specifies:

⌀0.10 | A | B | C

for a hole.

The position tolerance establishes a cylindrical tolerance zone around the theoretically exact position of the hole axis.

This controls the hole’s location and, depending on the application, can also establish orientation requirements.

Why position is important

Instead of specifying separate X and Y ± tolerances, position can define a more functional tolerance zone.

Common applications

Bolt holes
Mounting holes
Pin holes
Hole patterns
Shafts
Slots

Concentricity

Symbol: ◎

Concentricity controls the relationship between the median points of a feature and a datum axis.

It is a highly specialized control and can be difficult to inspect.

In many practical applications, position or runout is preferred because they often provide a more functional way of controlling rotational features.

11. Symmetry

Symbol: ⌯

Symmetry controls whether the median plane or median points of a feature are symmetrically located about a datum plane.

Like concentricity, symmetry is relatively uncommon in modern practical GD&T applications and may be replaced by more functional controls such as position or profile.

12. Runout Tolerances

Runout is generally used for rotating components.

There are two main types:

Circular runout
Total runout

Circular Runout

Symbol: ↗

Circular runout controls variation at individual circular sections as the part is rotated about a datum axis.

It can control variation caused by combinations of:

Roundness
Coaxiality/orientation effects

Applications

Shafts
Rotors
Bearing surfaces
Wheels
Pulleys

Total Runout

Symbol: ⌰

Total runout controls variation over the entire surface while the part is rotated around its datum axis.

It is more comprehensive than circular runout.

Example applications

Precision shafts
Rotating seals
Bearing journals
Spindles

13. GD&T Feature Control Frame

A Feature Control Frame (FCF) is the rectangular box used to specify a geometric tolerance.

A simplified example is:

| ⌖ | ⌀0.10 | A | B | C |

It can be understood as:

Section	Meaning
⌖	Geometric characteristic
⌀0.10	Tolerance zone
A	Primary datum
B	Secondary datum
C	Tertiary datum

14. Datum Reference System

A datum reference system normally consists of:

Primary Datum — A

Establishes the first reference.

Secondary Datum — B

Establishes the second reference.

Tertiary Datum — C

Establishes the final reference.

A useful way to visualize it is:

A → B → C

This establishes the part’s coordinate reference for inspection and manufacturing.

15. Material Condition Modifiers

One of the most important GD&T concepts is material condition.

Common symbols include:

MMC — Maximum Material Condition

Symbol: Ⓜ

MMC represents the condition where a feature contains the maximum amount of material.

For a hole, MMC corresponds to the smallest allowable hole.

For a shaft, MMC corresponds to the largest allowable shaft.

LMC — Least Material Condition

Symbol: Ⓛ

LMC represents the condition where a feature contains the least amount of material.

For a hole, this is the largest allowable hole.

For a shaft, this is the smallest allowable shaft.

RFS — Regardless of Feature Size

Symbol: none

RFS means the geometric tolerance applies regardless of the actual size of the feature.

RFS is generally the default condition unless another material-condition modifier is specified by the applicable standard.

16. Tolerance Zone

A tolerance zone is the region within which the actual feature must lie.

Examples:

Straightness

Two parallel lines or planes.

Flatness

Two parallel planes.

Position

Usually a cylindrical zone for a cylindrical feature.

Circularity

Two concentric circles.

Understanding the shape of the tolerance zone is essential for interpreting GD&T.

17. GD&T vs Traditional ± Tolerancing

Traditional Tolerancing	GD&T
Uses mostly ± dimensions	Uses geometric symbols
Can require many dimensions	Often communicates requirements more clearly
Less direct functional definition	Strong functional relationship
Can lead to unnecessarily tight tolerances	Can allow appropriate manufacturing variation
Limited control of form/orientation	Controls form, orientation, location, profile and runout
Inspection can be less structured	Works well with CMM inspection

18. GD&T Symbol Categories — Easy Memory Trick

You can remember the major controls as:

F-O-L-P-R

F — Form

Straightness
Flatness
Circularity
Cylindricity

O — Orientation

Parallelism
Perpendicularity
Angularity

L — Location

Position
Concentricity*
Symmetry*

P — Profile

Profile of a line
Profile of a surface

R — Runout

Circular runout
Total runout

This is a useful structure for interviews and examinations.

32. Frequently Asked Questions (FAQ)

Q1. What is the full form of GD&T?

GD&T = Geometric Dimensioning and Tolerancing.

Q2. What is the main purpose of GD&T?

To communicate the required geometrical accuracy and allowable variation of manufactured components clearly and functionally.

Q3. How many major types of GD&T controls are there?

The controls are commonly grouped into:

Form
Profile
Orientation
Location
Runout

Q4. Which GD&T controls do not require a datum?

The four form controls generally do not require a datum:

Straightness
Flatness
Circularity
Cylindricity

Q5. Which GD&T symbol is most commonly used for hole location?

Position (⌖) is commonly used to control the location of holes.

Q6. What is a datum?

A datum is a theoretically exact reference used to establish the location and/or orientation of other features.

Q7. What does MMC mean?

MMC = Maximum Material Condition.

For a hole, MMC is the smallest hole size.
For a shaft, MMC is the largest shaft size.

Q8. What does LMC mean?

LMC = Least Material Condition.

For a hole, LMC is the largest hole size.
For a shaft, LMC is the smallest shaft size.

Q9. What is RFS?

RFS = Regardless of Feature Size.

It means the geometric tolerance applies independently of the actual feature size, unless another condition is specified.

Q10. What is the difference between flatness and parallelism?

Flatness controls a surface by itself.

Parallelism controls the orientation of a feature relative to a datum.

Q11. What is the difference between circularity and cylindricity?

Circularity controls individual circular cross-sections.

Cylindricity controls the complete cylindrical surface.

Q12. What is the difference between perpendicularity and angularity?

Perpendicularity controls a 90° relationship.

Angularity controls a specified angle other than the standard perpendicular relationship.

Q13. What is a Feature Control Frame?

It is the rectangular box on a drawing that contains the:

Geometric characteristic symbol
Tolerance value
Datum references
Applicable modifiers

Q14. Does flatness require a datum?

No. Flatness is a form control and normally controls the surface without reference to a datum.

Q15. Why is GD&T important in CNC machining?

GD&T helps define the functional requirements of machined components so that manufacturers know how much variation is acceptable while still ensuring proper assembly and operation.

Conclusion

GD&T (Geometric Dimensioning and Tolerancing) is a standardized engineering language used to control the form, orientation, location, profile, and runout of component features.

Also Read: Purpose and rules of GD&T

The most important concepts to understand are:

Form: Straightness, flatness, circularity, cylindricity
Profile: Profile of line and surface
Orientation: Parallelism, perpendicularity, angularity
Location: Position, plus specialized controls such as concentricity and symmetry
Runout: Circular and total runout
Datums: Establish the reference system
Feature Control Frame: Communicates the geometric requirement
MMC/LMC/RFS: Define material-condition relationships

For a beginner, the best learning sequence is:

GD&T basics → Symbols → Datums → Feature Control Frames → Tolerance zones → MMC/LMC/RFS → Position → Advanced applications → Drawing interpretation.

Mastering these concepts is especially useful for mechanical design, production engineering, CNC machining, metrology, CMM inspection, quality control, and engineering interviews.