A Beginner’s Guide to Common GD&T Symbols

Geometric dimensioning and tolerancing (GD&T) is a set of specialized symbols used by part designers and machinists. These symbols create a universal language that makes it easy to understand the specifications of a part, including how much variance is allowed in the size and shape. We’re taking a deeper dive into GD&T symbols, including what they are and the five main tolerance categories—so let’s get started.

GD&T Symbols Chart

GD&T symbols create a universal language between designers and machinists. The most common GD&T symbols are straightness, flatness, circularity, cylindricity, line profile, surface profile, angularity, perpendicularity, parallelism, true position, concentricity, symmetry, total runout, and circular runout.

Section 1 of 8:

What is geometric dimensioning and tolerancing (GD&T)?

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GD&T is a universal language for specifying manufacturing instructions. These instructions are added to technical drawings (or engineering drawings) to make it easier for designers and manufacturers to ensure that parts are engineered precisely. Because GD&T removes guesswork from the engineering process, it helps improve the overall quality of a product and lowers production costs. There are five tolerance categories in GD&T:
- Form: Defines acceptable variations in the shape of a feature
- Orientation: Describes the required angle dimensions for the object
- Location: Where the feature’s axis or center should sit
- Profile: Boundary in which the feature’s surface must fall
- Runout: Defines the allowable variance when a feature is rotated 360°.

Section 2 of 8:

Form Tolerance Symbols

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Straightness (▬) Straightness controls how much the straightness of a feature can vary along its axis or center compared to its “perfect form” (or theoretical ideal). The tolerance zone (the range in which variation is allowed) is defined by two parallel lines a set distance apart.^{[1]
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- Symbol description: Thick, straight line or narrow rectangle
- Commonly used for: Straight items like shafts and pins
- Unicode: 25AC
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Flatness (▱) The flatness of a feature refers to how much deviation is allowed between the highest and lowest points on a surface. The tolerance zone for flatness is also defined by two parallel lines a pre-determined distance apart.^{[2]
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- Symbol description: Parallelogram
- Commonly used for: Joining surfaces
- Unicode: 25B1
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Circularity (◯) Imagine taking a cross-section of a round object, then comparing it to a true circle. The circularity tolerance, or roundness, specifies how much the shape can deviate from that perfect circle. The tolerance zone is defined by the space between two concentric circles.^{[3]
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- Symbol description: Open circle
- Commonly used for: Bores; bearings
- Unicode: 25EF
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Cylindricity (⌭) The cylindricity tolerance defines how much a feature can deviate from a true cylinder shape. It’s similar to circularity, but it applies to both the axis and the round surface.^{[4]
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Research source} The tolerance zone is defined by two concentric cylinders.^{[5]
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- Symbol description: Circle bounded by two parallel, diagonal lines
- Commonly used for: Precision guides; sleeves
- Unicode: 232D

Section 3 of 8:

Profile Tolerance Symbols

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Profile of a line (⌒) The profile of a line (also called the line profile) of a feature is a two-dimensional tolerance zone. It determines the feature’s profile if you were to take a cross-section at a specific point.^{[6]
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- Symbol description: A curved line, like an open semi-circle
- Commonly used for: Cams; blades
- Unicode: 2312
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Profile of a Surface (⌓) The profile of a surface (or surface profile) of a feature is a 3D tolerance zone. It demonstrates where the surface of a feature should be—so in essence, it controls the size and shape of the form. This tolerance can be applied to either flat or curved surfaces.^{[7]
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- Symbol description: A closed half-circle
- Commonly used for: Castings
- Unicode: 2313

Section 4 of 8:

Orientation Tolerance Symbols

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Angularity (∠) The angularity refers to the angle of a surface in relation to the datum feature (a reference point on the feature that helps determine orientation). This is used when a feature needs to sit at an angle other than 90°. The tolerance zone for angularity is defined by two parallel planes that intersect the datum feature at the required angle.^{[8]
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- Symbol description: Two lines that meet at a 45° angle
- Commonly used for: Slopes & tapers
- Unicode: 2220
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Perpendicularity (⟂) The perpendicularity of a feature describes the tolerance of variation allowed between a surface that is meant to sit at a 90° angle to the datum feature or another surface. The tolerance zone is defined by two parallel planes that run perpendicular to the datum feature.^{[9]
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- Symbol description: Two perpendicular lines
- Commonly used for: Mounting faces
- Unicode: 27C2
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Parallelism (∥) The parallelism of a feature is similar to straightness, but it determines how much a parallel surface can deviate from a datum feature or another parallel planbe.^{[10]
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- Symbol description: Two parallel lines
- Commonly used for: Bearing rails
- Unicode: 2225

Section 5 of 8:

Location Tolerance Symbols

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True Position (⌖) The true position (or simply position) tolerance defines how much the location of a feature can vary relative to the rest of the form. In other words, it specifies exactly where a feature (like a hole) must be located. The tolerance zone is typically a circle or cylinder around the feature.^{[12]
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- Symbol description: Bullseye; a circle intersected by a vertical and horizontal line
- Commonly used for: Assemblies; bolt holes
- Unicode: 2316
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Concentricity (◎) The concentricity of a feature defines the variation in two cylinders that are meant to have the same central axis. It’s used to ensure two round features (like a bolt and a hole) are aligned. The central axis is used as the datum feature.^{[13]
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- Symbol description: Two concentric circles
- Commonly used for: Gears; rotating parts
- Unicode: 25CE
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Symmetry (⌯) The symmetrical tolerance of a feature refers to the variance allowed in two planes in relation to a central datum feature. This is useful when two parts need to be a mirror image of each other.
- Symbol description: Three parallel, horizontal lines, with shorter lines on the top and bottom and a longer line in the middle
- Commonly used for: Supports; brackets
- Unicode: 232F

Section 6 of 8:

Runout Tolerance Symbols

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Total runout (⌰) The total runout (sometimes just called runout) of a feature determines how much the surface of a rotating part can vary around the central axis (which is used as the datum feature for this tolerance). The tolerance zone is made up of two concentric cylinders.^{[14]
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- Symbol description: Two parallel, diagonal arrows connected by a straight line at the bottom
- Commonly used for: Crankshafts; rotors
- Unicode: 2330
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Circular runout (↗) The circular runout is similar to the total runout, but two-dimensional. It determines how much the circumference of a cylindrical part can vary at any point along its length, based on the distance from the surface to the central axis when viewed as a cross-section.^{[15]
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- Symbol description: A single diagonal arrow
- Unicode: 2197

Section 7 of 8:

How to Read a Feature Control Frame (FCF)

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A feature control frame includes the GD&T instructions on a technical drawing. The FCF is a rectangular box. On the left side of the box, you’ll find the GD&T symbol that indicates what the instructions are referring to. Beside that, from left to right, you’ll find:^{[16]
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- Tolerance value: How much variation is allowed for that feature
- Modifiers: Additional information, like maximum material condition (MMC)—a feature when the feature has the most material, like the largest peg or smallest hole—or least material condition (LMC)—when the feature has the least material, like the largest hole.
- Datum feature references (if needed): A reference point for what you’re measuring against. May include a primary datum, secondary datum, and tertiary datum.

Section 8 of 8:

Why is GD&T important?

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GD&T allows for precision when manufacturing complex parts. It gives the designers and manufacturers a language they can use to precisely communicate the specifications of a part and all its features. It also defines allowable variances, making it easy to determine whether or not a part meets specifications. This ensures that a part will fit perfectly with other parts in a finished product.^{[17]
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- GD&T is often used in the automotive, aerospace, and medical device industries.^{[18]
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