GDT symbols, more commonly written as GD&T symbols, are standardized symbols used on engineering drawings to communicate geometric requirements for manufactured parts. GD&T stands for Geometric Dimensioning and Tolerancing. These symbols help designers, engineers, machinists, manufacturers, and inspectors understand how accurately a feature must be formed, positioned, oriented, or controlled.
A normal dimension can tell you the size of a feature, such as the diameter of a hole. However, size alone does not always explain whether the hole is in the correct location, whether a surface is flat, or whether two surfaces are properly aligned. GD&T symbols provide this additional information.
This guide explains the most common GDT symbols and their meanings, how they are organized, how to read them on engineering drawings, and how concepts such as datums, feature control frames, MMC, LMC, and RFS work.
What Are GDT Symbols?
GDT symbols are graphical representations of geometric characteristics used in engineering drawings. Each symbol identifies a particular geometric requirement.
For example, a designer may need to control:
- The straightness of a feature
- The flatness of a surface
- The roundness of a circular feature
- The orientation of a surface
- The position of a hole
- The profile of a curved surface
- The amount of runout on a rotating component
Instead of describing these requirements with lengthy sentences, GD&T uses standardized symbols and tolerance information.
The term “GDT symbols” is commonly used in searches, while “GD&T symbols” is the standard abbreviation for Geometric Dimensioning and Tolerancing.
GD&T Symbols Chart
The following chart provides a quick reference to common geometric characteristics.
| GD&T Symbol/Characteristic | Category | What It Controls |
| Straightness | Form | Straightness of a line or surface |
| Flatness | Form | Flatness of a surface |
| Circularity | Form | Roundness of a circular element |
| Cylindricity | Form | Overall cylindrical form |
| Parallelism | Orientation | Parallel relationship to a datum |
| Perpendicularity | Orientation | 90-degree relationship to a datum |
| Angularity | Orientation | A specified angular relationship |
| Position | Location | Location of a feature |
| Concentricity | Location | Relationship of median points to a datum axis |
| Symmetry | Location | Symmetrical relationship to a datum |
| Profile of a Line | Profile | Two-dimensional profile |
| Profile of a Surface | Profile | Three-dimensional surface profile |
| Circular Runout | Runout | Variation at individual circular sections |
| Total Runout | Runout | Variation across an entire surface |
The exact graphical presentation of GD&T symbols should be checked against the applicable engineering standard used for the drawing.
Categories of GDT Symbols
GD&T geometric characteristics are commonly grouped into five major categories:
- Form
- Orientation
- Location
- Profile
- Runout
Understanding these categories makes it easier to learn what each symbol does.
Form Symbols
Form controls describe the shape of a feature itself. They generally do not require a datum reference because they control the feature’s basic form.
Straightness
Straightness controls how much a line element or derived axis can deviate from perfect straightness.
For a surface, the requirement can control the straightness of individual line elements. When applied to an axis, straightness can control the location of the derived axis within a specified tolerance zone.
Straightness is useful for features such as shafts, edges, and surfaces that need to remain straight.
Flatness
Flatness controls how much a surface can vary from a perfectly flat plane.
A flatness tolerance establishes a zone between two parallel planes. The entire controlled surface must remain inside that zone.
Unlike parallelism, flatness does not establish the surface’s orientation relative to another datum. It simply controls how flat the surface itself must be.
Circularity
Circularity, also known as roundness, controls how closely each circular element of a feature approaches a perfect circle.
It can be useful for cylindrical or spherical features where the individual cross-sections need to maintain a controlled circular shape.
Circularity is a form control, so it does not establish the feature’s location or orientation relative to a datum.
Cylindricity
Cylindricity controls the overall form of a cylindrical surface.
It considers the complete cylindrical feature rather than only individual circular sections. It can control variations involving the overall cylindrical form of the feature.
Cylindricity is useful when a cylindrical surface needs comprehensive form control.
Orientation Symbols
Orientation controls define how one feature is oriented relative to a datum reference.
The main orientation controls are parallelism, perpendicularity, and angularity.
Parallelism
Parallelism controls whether a surface, center plane, line, or axis maintains a parallel relationship with a datum.
For example, a machined surface may need to remain parallel to datum A within a specified tolerance.
Parallelism controls orientation rather than simply controlling the flatness of the surface.
Perpendicularity
Perpendicularity controls whether a feature is oriented at 90 degrees to a datum.
For example, a hole axis may need to be perpendicular to a reference surface. A perpendicularity requirement can help ensure that the mating component fits correctly.
Perpendicularity is widely used for machined surfaces, holes, shafts, and other features requiring a precise right-angle relationship.
Angularity
Angularity controls a feature at a specified angle relative to a datum.
Unlike perpendicularity, which establishes a 90-degree relationship, angularity can specify other angles.
For example, a surface could be required to maintain a particular angle relative to datum A.
Location Symbols
Location controls describe where a feature is positioned relative to a theoretically exact location or datum reference system.
Common location controls include position, concentricity, and symmetry.
Position
Position is one of the most commonly used GD&T controls.
A position tolerance controls the location of a feature, such as a hole, slot, pin, or cylindrical feature.
For example, an engineering drawing may specify a position tolerance for a hole pattern. This tells the manufacturer how far the actual feature is permitted to deviate from its theoretically exact location.
Position is especially useful for hole patterns because it provides a controlled tolerance zone around the intended location.
Concentricity
Concentricity controls the relationship between the median points of a feature and a datum axis.
It is a specialized geometric control and can be relatively difficult to inspect.
Concentricity should not simply be treated as another word for “same center.” It has a specific GD&T meaning related to median points.
Symmetry
Symmetry controls the relationship between the median points of opposing feature elements and a datum plane.
The objective is to maintain a symmetrical relationship around the specified datum.
Like concentricity, symmetry is a specialized control and should be applied when its specific geometric requirement is needed.
Profile Symbols
Profile controls are used when the shape of a line or surface needs to be controlled.
The two main profile controls are profile of a line and profile of a surface.
Profile of a Line
Profile of a line controls the shape of a two-dimensional line element.
It is useful when a particular cross-sectional shape needs to remain within a defined tolerance zone.
For example, it can be applied to a curved section where the manufactured profile needs to remain close to the theoretically exact profile.
Profile of a Surface
Profile of a surface controls a complete three-dimensional surface.
It is particularly useful for complex curves, contours, and irregular surfaces.
Depending on how the requirement is specified, profile of a surface can control aspects such as form, orientation, and location.
This makes profile particularly valuable for complex manufactured components.
Runout Symbols
Runout controls are commonly associated with rotating components.
The two main runout controls are circular runout and total runout.
Circular Runout
Circular runout controls the variation of a feature at individual circular cross-sections as the part rotates around a datum axis.
It is commonly used for rotating cylindrical features.
For example, it can help control the variation of a shaft surface during rotation.
Total Runout
Total runout provides more comprehensive control across the entire surface of a rotating feature.
It considers variation across multiple circular sections as the component rotates.
Total runout can therefore provide broader control of a cylindrical or other rotational surface than circular runout.
What Is a Feature Control Frame?
A feature control frame is the rectangular box used on a GD&T drawing to communicate a geometric tolerance requirement.
A typical feature control frame can contain several pieces of information.
Geometric Characteristic
The first section identifies the geometric control being applied.
For example, it may specify position, perpendicularity, parallelism, or another geometric characteristic.
Tolerance Value
The next section specifies the permitted geometric variation.
The tolerance value establishes the size of the applicable tolerance zone.
Diameter Symbol
A diameter symbol may appear before the tolerance value when the tolerance zone is cylindrical or otherwise requires diameter interpretation.
Material Condition Modifier
A modifier may specify a material condition requirement, such as MMC or LMC.
These modifiers can affect how the tolerance is interpreted.
Datum References
One or more datum references may follow the tolerance.
For example, a feature control frame could reference datum A, B, and C to establish the reference framework used for evaluating the feature.
How to Read GDT Symbols on Engineering Drawings
Reading GD&T becomes easier when you examine the information in a consistent order.
Step 1: Identify the Symbol
Start by identifying the geometric characteristic.
Ask whether the drawing is controlling form, orientation, location, profile, or runout.
Step 2: Read the Tolerance
Look at the tolerance value specified in the feature control frame.
This tells you the permitted geometric variation.
Step 3: Check for a Diameter Symbol
If a diameter symbol appears with the tolerance, the tolerance zone has a specific cylindrical interpretation.
This is particularly common with position tolerances for holes.
Step 4: Check the Datums
Look for datum references such as A, B, or C.
These references establish the part’s theoretical reference framework.
Step 5: Look for Modifiers
Check whether the feature control frame includes modifiers such as MMC, LMC, or other applicable symbols.
The modifier can affect how the tolerance is applied.
What Are Datum Symbols in GD&T?
A datum provides a theoretical reference from which other geometric requirements can be established.
Datums are often identified by letters.
For example:
- Datum A can act as a primary reference
- Datum B can act as a secondary reference
- Datum C can act as a tertiary reference
Together, these references can establish a datum reference frame.
Primary, Secondary, and Tertiary Datums
A primary datum establishes the first major reference relationship.
A secondary datum adds another constraint or orientation reference.
A tertiary datum completes the reference framework.
The exact function of each datum depends on the design and the way the drawing defines the datum reference frame.
GD&T Modifiers: MMC, LMC, and RFS
Modifiers are important when interpreting GD&T requirements.
Maximum Material Condition (MMC)
MMC refers to the condition in which a feature contains the greatest amount of material within its applicable size limits.
For an internal feature such as a hole, MMC corresponds to the smallest permitted hole size.
For an external feature such as a shaft, MMC corresponds to the largest permitted shaft size.
MMC is particularly important when bonus tolerance or functional requirements are involved.
Least Material Condition (LMC)
LMC refers to the condition in which a feature contains the least amount of material within its applicable size limits.
For a hole, this generally corresponds to the largest permitted hole size.
For a shaft, it generally corresponds to the smallest permitted shaft size.
Regardless of Feature Size (RFS)
RFS means the geometric tolerance applies regardless of the actual size of the feature unless another material condition modifier is specified.
Understanding MMC, LMC, and RFS is important because they can change how a geometric tolerance is interpreted during manufacturing and inspection.
GDT Symbols vs. Traditional Dimensions
Traditional dimensional tolerances primarily communicate acceptable size variation.
For example, a drawing may specify the permitted diameter of a hole.
However, a hole could have the correct diameter and still be incorrectly positioned.
GD&T can address this additional requirement.
For example:
- A size dimension controls the hole’s size.
- A position tolerance controls its location.
- A perpendicularity requirement can control its orientation.
- A datum reference establishes the relationship to other features.
This combination allows an engineering drawing to communicate design requirements more precisely.
GDT Symbols in Engineering and Manufacturing
GD&T is used across many industries where precision and interchangeability are important.
CNC Machining
CNC machinists use GD&T requirements to understand how accurately a component must be manufactured.
Position, flatness, perpendicularity, profile, and runout requirements can all affect machining and inspection.
Automotive Manufacturing
Automotive components often contain holes, shafts, mounting surfaces, and other features that need to align accurately.
GD&T provides a standardized method for communicating these requirements.
Aerospace Manufacturing
Aerospace components can contain complex surfaces and tightly controlled interfaces. Profile, position, orientation, and datum requirements can be important for manufacturing and inspection.
Quality Control
Inspectors use GD&T specifications to determine whether manufactured parts meet engineering requirements.
Measurement equipment such as coordinate measuring machines can be used to evaluate many geometric requirements.
Common Mistakes When Reading GDT Symbols
Confusing Flatness With Parallelism
Flatness controls the form of a surface.
Parallelism controls orientation relative to a datum.
They are therefore not interchangeable.
Treating Position as Simple Plus-or-Minus Tolerance
Position creates a geometric tolerance zone around the theoretically exact location. It should not simply be interpreted as an ordinary plus-or-minus dimension.
Ignoring Datums
Datums can determine how a geometric requirement is oriented and located.
Ignoring a datum can lead to an incorrect interpretation of the drawing.
Confusing Circularity With Cylindricity
Circularity controls individual circular elements, while cylindricity controls the overall cylindrical form.
Ignoring Material Condition Modifiers
MMC, LMC, and RFS can affect the interpretation of a geometric tolerance.
They should always be considered when present.
How to Learn GDT Symbols More Easily
The easiest way to learn GD&T is to study the symbols by category rather than memorizing every symbol randomly.
Start with the four form controls:
- Straightness
- Flatness
- Circularity
- Cylindricity
Then learn the orientation controls:
- Parallelism
- Perpendicularity
- Angularity
After that, study:
- Position
- Profile
- Runout
- Datums
- Feature control frames
- MMC, LMC, and RFS
Finally, practice reading complete engineering drawing callouts.
The goal is not just to recognize a symbol but to understand what geometric requirement it creates.
Frequently Asked Questions About GDT Symbols
What are GDT symbols?
GDT symbols are graphical symbols used to communicate geometric dimensioning and tolerancing requirements on engineering drawings. The technically standard abbreviation is GD&T.
What does GD&T stand for?
GD&T stands for Geometric Dimensioning and Tolerancing.
What are the five main categories of GD&T?
The commonly used categories are form, orientation, location, profile, and runout.
What is the most commonly used GD&T symbol?
Position is one of the most frequently used GD&T controls, especially for locating holes and other features.
What is the difference between GDT and GD&T?
“GDT” is commonly used as a shortened search term, while GD&T is the standard abbreviation for Geometric Dimensioning and Tolerancing.
What is a datum in GD&T?
A datum is a theoretical reference used to establish the framework for orienting and locating features.
What is a feature control frame?
A feature control frame is a rectangular box on an engineering drawing that communicates a geometric characteristic, tolerance, modifiers, and, when required, datum references.
What does MMC mean in GD&T?
MMC means Maximum Material Condition. It describes the feature size condition containing the greatest amount of material within its permitted size limits.
What does RFS mean in GD&T?
RFS means Regardless of Feature Size. It means the specified geometric tolerance applies regardless of the actual size of the feature unless another material condition modifier is specified.
Are GD&T symbols used in CNC machining?
Yes. GD&T requirements are commonly found on engineering drawings used for CNC machining and precision manufacturing.
GDT Symbols Quick Reference
For quick study, remember the main groups:
Form: Straightness, Flatness, Circularity, Cylindricity
Orientation: Parallelism, Perpendicularity, Angularity
Location: Position, Concentricity, Symmetry
Profile: Profile of a Line, Profile of a Surface
Runout: Circular Runout, Total Runout
Learning these categories first provides a useful foundation for understanding engineering drawings.
Conclusion
GDT symbols, commonly referred to as GD&T symbols, provide a standardized language for communicating geometric requirements in engineering and manufacturing. They help define not only the size of a feature but also its form, orientation, location, profile, and runout.
The most important concepts to understand include the five major groups of geometric controls, feature control frames, datum references, and material condition modifiers such as MMC, LMC, and RFS.
Whether you are a student, mechanical engineer, CNC machinist, designer, or quality inspector, understanding GDT symbols and their meanings can make engineering drawings much easier to interpret and apply correctly.
I am Lisa Reynolds, a linguist fascinated by the ever-changing world of slang. I enjoy researching how slang reflects trends, technology, and social connections. I wrote “Text Talk: Understanding Digital Slang” and “From Street to Screen: Slang in Media”, exploring the influence of digital communication on language. I love sharing my discoveries in a way that feels like a conversation with friends, so you feel the culture and humor behind every phrase.
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