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GD&T Symbols Reference Table and Meanings

GD&T is a symbolic language on engineering drawings that controls the form, orientation, location and runout of features. This table lists the 14 geometric characteristic symbols and the common modifiers, with the category, datum requirement and tolerance zone shape for each.

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Symbol / characteristic	Category	What it controls	Datum reference	Tolerance zone
Straightness	Form	Line elements or a derived axis	Never	Two parallel lines, or a cylinder when applied to an axis with a diameter symbol
Flatness	Form	Surface flatness	Never	Two parallel planes
Circularity (roundness)	Form	Each circular cross section	Never	Two concentric circles in each section
Cylindricity	Form	Whole cylindrical surface: round, straight, taper	Never	Two coaxial cylinders
Profile of a line	Profile	2D profile at cross sections	Optional	Boundary band of constant width along a line profile
Profile of a surface	Profile	3D surface shape, can also control size and location	Optional	Boundary band of constant width around the true surface
Angularity	Orientation	Surface or axis at a stated angle other than 90 degrees	Required	Two parallel planes at the basic angle
Perpendicularity	Orientation	Surface or axis at 90 degrees to a datum	Required	Two parallel planes, or a cylinder for an axis
Parallelism	Orientation	Surface or axis parallel to a datum	Required	Two parallel planes, or a cylinder for an axis
Position	Location	Location of features of size such as holes, slots, pins	Usually required	Cylinder with diameter symbol, or two parallel planes for a slot
Concentricity	Location	Median points of a feature about a datum axis	Required	Cylinder about the datum axis (removed from ASME Y14.5-2018)
Symmetry	Location	Median points of a feature about a datum plane	Required	Two parallel planes about the datum plane (removed from ASME Y14.5-2018)
Circular runout	Runout	Each circular element as the part rotates on the datum axis	Required	Full indicator movement at each cross section
Total runout	Runout	Entire surface as the part rotates on the datum axis	Required	Full indicator movement over the whole surface
Maximum material condition (MMC)	Modifier	Tolerance applies at the size with the most material; bonus tolerance as size departs from it	n/a	Applied to the feature or to a datum reference
Least material condition (LMC)	Modifier	Tolerance applies at the size with the least material; bonus tolerance as size departs from it	n/a	Applied to the feature or to a datum reference
Regardless of feature size (RFS)	Modifier	Default under ASME Y14.5-2009 and later: the tolerance does not change with feature size	n/a	No bonus tolerance
Projected tolerance zone	Modifier	Extends the zone above the part to control a mating fastener or pin	n/a	Zone height is stated after the symbol
Diameter symbol in a tolerance frame	Modifier	Makes the zone cylindrical instead of two planes	n/a	Cylindrical
Free state	Modifier	Tolerance applies with restraint forces removed, used for flexible parts	n/a	Applied to the feature
Tangent plane	Modifier	The tolerance applies to the plane touching the high points of the surface	n/a	Applied to the feature
Statistical tolerance	Modifier	Allows statistical process control to verify the tolerance	n/a	Applied to the feature

Summary for quick reference, not a substitute for ASME Y14.5 or ISO 1101. Rules differ between the two standards, notably on envelope principle and the default material condition.

Structure GD&T data for inspection planning

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How to use this table

Search by name, category or zone shape. The category column groups symbols the way the standard does: form, profile, orientation, location and runout. Form controls never reference a datum. Orientation, location and runout controls need datums. Profile can be used either way. When you are reading a drawing, find the symbol in the first compartment of the feature control frame, then read the tolerance value, any modifier, and the datum references in priority order.

Reading a feature control frame

A frame reads left to right: the characteristic symbol, an optional diameter symbol, the tolerance value, an optional material condition modifier, then primary, secondary and tertiary datums. For example, position, diameter 0.2 at MMC, A, B, C means a cylindrical zone of 0.2 mm at the MMC size, located from datum A first, then B, then C. The datum order tells the inspector how to fix the part on the CMM or fixture, so changing the order changes the measurement.

Form controls: straightness, flatness, circularity, cylindricity
Orientation controls: angularity, perpendicularity, parallelism
Location control: position, with concentricity and symmetry in older drawings
Runout controls: circular and total

Standards differences to know

ASME Y14.5 and ISO 1101 share the symbols but not every rule. ASME Y14.5-2018 removed concentricity and symmetry, replacing them with position or profile of a surface. Under ASME, Rule 1 requires perfect form at MMC unless a note says otherwise, while ISO uses independence by default. A drawing that mixes both can be ambiguous, so state the governing standard in the title block.

Common mistakes

Frequent errors include using concentricity where position or runout is cheaper and clearer, applying flatness to a feature of size, forgetting that an MMC modifier adds bonus tolerance that the inspection plan must calculate, and omitting datums from an orientation control. Another is specifying a tight position tolerance without datums that can actually be repeated on a fixture.

Frequently Asked Questions

How many GD&T symbols are there?

There are 14 geometric characteristic symbols in ASME Y14.5 and ISO 1101 combined, grouped as form, profile, orientation, location and runout. Concentricity and symmetry were removed from ASME Y14.5-2018, but they still appear on older drawings and in ISO.

Which GD&T symbols need a datum?

Orientation, location and runout controls need datum references. Form controls never use datums. Profile of a line or surface may be used with or without datums depending on whether you want to control shape only or shape and location.

What is the difference between MMC and LMC?

MMC is the size at which a feature holds the most material, the smallest hole or largest pin. LMC is the opposite. Under either modifier the tolerance can grow as the actual size departs from that condition, giving bonus tolerance.

Why is concentricity no longer used in ASME Y14.5-2018?

Concentricity requires evaluating median points, which is hard to measure and rarely what the designer needs. Position, profile or runout controls usually achieve the function more directly and can be verified with common CMM routines.

If you want GD&T callouts captured as structured data in your ERP or PLM so inspection plans generate themselves, we can design that with you.

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