Geometric Dimensioning and Tolerancing (GD&T) Guide

GD&T (Geometric Dimensioning and Tolerancing) is a symbolic language defined by ASME Y14.5 that controls form, orientation, location, and runout of features. Unlike ± linear dimensions, GD&T defines tolerance zones (e.g., a cylinder for position, two parallel planes for flatness) and uses datums to establish a repeatable measurement reference. Use GD&T when parts must assemble with functional requirements—bearing bores, mounting holes, sealing faces—because it communicates design intent clearly and often allows larger tolerance zones than stacked ± dimensions, reducing cost while maintaining fit.

ASME Y14.5 (US) and ISO 1101 (international) both define GD&T symbols and rules, but differ in default interpretations and modifiers. ASME Y14.5-2018 uses inches as the default unit and has specific rules for datum precedence, material condition (MMC/LMC/RFS), and composite position. ISO 1101 defaults to metric and uses slightly different symbolism in places (e.g., envelope requirement). For US-based manufacturing and supply chains, specify ASME Y14.5 on the drawing; for global programs, note the standard in the title block to avoid ambiguity.

Use position (true position) when you need to control the location of a feature (hole, pin, boss) relative to datums in a way that allows maximum tolerance zone. Position uses a cylindrical or spherical zone; ± X and ± Y on a hole effectively create a square tolerance zone, which is smaller than the inscribed circle. Position also clarifies that the tolerance applies at MMC or LMC when material condition is specified, enabling functional gaging and often reducing inspection cost. Reserve ± dimensions for non-critical features or when no datum relationship is needed.

GD&T can reduce cost when it replaces over-tight ± tolerances with a single position or profile callout that gives the shop a larger usable zone. It can increase cost when it requires CMM or functional gage inspection, multiple datums, or very tight form/orientation controls. Typical impact: position callouts with MMC can allow functional pin gaging (fast); flatness and perpendicularity on large surfaces may require CMM or surface plate inspection. Specify only the controls that affect fit and function, and call out inspection method in the drawing notes when it matters.

Datums are theoretically exact points, axes, or planes derived from datum features (physical part surfaces) that establish the reference frame for measurement. Choose datums that match how the part mounts or functions: a mounting face as primary datum A, a locating hole or edge as B, and a secondary locating feature as C. The order matters—A establishes the first plane, B the second, C the third. Use features that are stable, accessible for inspection, and representative of the part's function. Avoid datums on small or flexible features when a larger, stiffer surface can serve.

MMC (Maximum Material Condition) means the feature is at its maximum material limit—largest pin, smallest hole. LMC (Least Material Condition) is the opposite—smallest pin, largest hole. RFS (Regardless of Feature Size) means the tolerance applies at any size. When you specify position at MMC, the position tolerance can increase (bonus tolerance) as the feature departs from MMC, which often allows easier assembly and enables fixed-size functional gages. RFS is the default in ASME Y14.5-2018 when no modifier is shown. Specify MMC or LMC when functional gaging or bonus tolerance is desired.