GD&T Symbol Guide

The 14 GD&T symbols, grouped by what they control

Every symbol in ASME Y14.5 belongs to one of five families. Knowing the family tells you immediately whether the callout needs a datum reference and whether a bonus tolerance can apply.

• Form (no datums): straightness, flatness, circularity, cylindricity
• Profile: profile of a line, profile of a surface — the most versatile controls in the standard
• Orientation (datums required): angularity, perpendicularity, parallelism
• Location (datums required): position, concentricity, symmetry
• Runout (datums required): circular runout, total runout

How to read a feature control frame

A feature control frame reads left to right in four blocks: the geometric characteristic, the tolerance zone shape and size, any material condition modifier, and the datum references in order of precedence.

A callout such as position, diameter 0.2 at maximum material condition, referenced to A, B, C, means the axis of the feature must fall inside a 0.2 mm cylindrical zone located from the datum reference frame — and that zone grows as the feature departs from its maximum material condition.

• Block 1 — the symbol: what geometric property is controlled
• Block 2 — the zone: diameter symbol means a cylindrical zone, no symbol means a width zone
• Block 3 — modifiers: MMC, LMC, projected tolerance zone, free state
• Block 4 — datums: primary, secondary, tertiary, always in that order

Datums and the 3-2-1 rule

Datums are not arbitrary letters. The primary datum removes three degrees of freedom, the secondary removes two, and the tertiary removes the last one — the 3-2-1 rule. Choose them to match how the part is located in its assembly, not how it is convenient to hold in a fixture.

The most common cause of parts failing inspection while fitting perfectly in assembly is a datum scheme that does not reflect the real mating condition.

MMC, LMC and bonus tolerance

Maximum material condition is the state where the feature contains the most material — the largest pin or smallest hole. When a position callout carries the MMC modifier, any departure from that condition is added to the tolerance zone as bonus tolerance.

This is why functional gauges work, and why a hole that measures at the loose end of its size limits can legally sit further off position without hurting assembly.

Where GD&T goes wrong in inspection

In our metrology work the same handful of mistakes appear again and again: profile tolerances applied without a datum reference frame when the surface is location-critical, concentricity used where runout or position was meant, and CMM alignments built on the wrong datum precedence so every reported deviation is shifted.

• Using concentricity instead of runout or position — expensive and rarely necessary
• Datum features too small or too rough to establish a stable reference
• Missing the diameter symbol on a positional tolerance, halving the usable zone
• Inspecting to the CAD model when the drawing carries a different tolerance scheme

Common questions