GD&T Symbols Chart: Complete Guide
Complete GD&T symbols chart guide — the 14 geometric characteristics across form, profile, orientation, location, and runout, with MMC/LMC modifiers.
What Is GD&T?
Geometric Dimensioning and Tolerancing (GD&T) is a symbolic engineering language that defines the allowable variation of a part's geometry. Instead of relying only on the traditional plus/minus dimensions that control a feature's size, GD&T controls form, orientation, location, and runout with a compact set of standardized symbols defined by ASME Y14.5 and ISO 1101.
It exists because size tolerances alone cannot guarantee that features relate to each other the way the design intends. Two holes drilled to the correct diameter can still be misplaced, tilted, or out-of-round relative to one another. GD&T puts these geometric relationships into a machine-readable, inspection-repeatable form so that a CNC shop, a fixture designer, and a CMM programmer all interpret the drawing identically.
At the center of the system is the **feature control frame** — a rectangular box that states the geometric characteristic to be controlled, the tolerance value, any material condition modifiers, and the datum references that establish the coordinate system. Every symbol in this chart is used inside these frames.
The GD&T Symbols Chart
The 14 geometric characteristics are grouped into four families: **form**, **profile**, **orientation**, and **location** (plus the related runout controls). The table below lists the symbol, its type, and the geometric error it controls.
Why it matters: selecting the right symbol tells the machinist and the inspector exactly which geometric error to control and which ones are free. Over-constraining with form controls inflates cost; under-specifying risks assemblies that do not fit.
Reading the Feature Control Frame
A feature control frame is read from left to right in up to five compartments.
1. **Geometric characteristic symbol** — the GD&T symbol being applied.
2. **Tolerance value** — the total allowable error, in the drawing units; a diameter symbol (⌀) before the value means a cylindrical tolerance zone.
3. **Material condition modifier (optional)** — Ⓜ (Maximum Material Condition), Ⓛ (Least Material Condition), or Ⓢ (Regardless of Feature Size).
4. **Datum references** — the datum letters (A, B, C) in order of precedence, separated by separators; a modifier after a datum letter allows datum shift.
5. **Datum feature modifiers (optional)** — specify how the datum is established.
Example: `⌀0.5|A|B|C` means the controlled axis must lie inside a cylindrical tolerance zone 0.5 mm in diameter, located relative to the datum reference frame established by datums A, B, and C.
Skipping the datum block means the tolerance is a *form* control applied without a reference frame; including datums makes it an *orientation*, *location*, or *runout* control anchored to the part's datums.
Form, Profile, Orientation, Location: How to Choose
The four families differ in what they constrain and whether they need a datum.
A practical rule: use a *form* control when the feature stands alone, an *orientation* control when its angle to another feature matters, and a *location* control (most commonly **true position**) when the center of a hole or pin must sit in a precise spot. Position with Ⓜ is the most cost-effective way to control hole patterns because it grants bonus tolerance as the hole grows away from its smallest (MMC) size.
GD&T in Practice: Why Precision Manufacturers Rely on It
For a precision CNC and metal stamping shop, GD&T is not paperwork — it is the contract that makes first-article and CMM inspection repeatable. A CMM establishes the datum reference frame from the datum features, then measures every other feature relative to that frame, in exactly the order the callouts specify.
Poor datum selection is the most common failure: picking decorative surfaces instead of functional ones as primary datums leads to ambiguous inspection and rejected parts. The primary datum should always be the largest, most stable, functionally mated surface.
GD&T Symbols and Modifiers: FAQ
What are the 14 GD&T symbols?
The 14 characteristics are straightness, flatness, circularity, cylindricity, line profile, surface profile, angularity, perpendicularity, parallelism, position, concentricity, symmetry, circular runout, and total runout.
What does Ⓜ (MMC) do in a feature control frame?
Maximum Material Condition grants bonus tolerance: as an internal feature grows larger than its smallest (MMC) size, or an external feature shrinks smaller than its largest, the position tolerance increases by the difference.
What is the difference between position and true position?
True position is the formal name of the position control in GD&T practice — it locates a feature's center relative to a datum system, typically with a ⌀ tolerance zone and MMC bonus.
Does every GD&T callout need a datum?
No. Form controls (flatness, straightness, circularity, cylindricity) do not need datums. Orientation, location, and runout controls reference datums.
Which standard is used — ASME Y14.5 or ISO 1101?
ASME Y14.5 is the North American standard and ISO 1101 the international one. The symbols are mostly shared; the differences are in modifiers, datum configuration, and some tolerance-zone geometry. State the standard on your drawing.
Conclusion
GD&T symbols form a precise, universal language that moves a design from intent to a manufacturable, inspectable part. Mastering the characteristic symbol, the feature control frame, and the MMC/LMC modifiers pays off in fewer rejected parts, lower cost, and faster quotes.
At MetalBizz, our engineers read and apply full ASME Y14.5 GD&T on CNC-machined and metal-stamped parts, with in-house dimensional inspection and DFM feedback. Upload your CAD file and get a free quote within 24 hours.