GD&T (Geometric Dimensioning and Tolerancing) is a symbolic language for controlling a feature's shape, orientation and location relative to named reference surfaces called datums, instead of just its size. Where a plus-minus tolerance controls one dimension between two limits, GD&T controls how a feature actually needs to behave, using a boxed callout called a feature control frame. It is defined by the ASME Y14.5 standard in the US and ISO 1101 internationally.

What does GD&T stand for?

Geometric Dimensioning and Tolerancing. It is a standardized system of symbols, defined by ASME Y14.5 in the US and ISO 1101 internationally, for specifying and controlling the allowable variation in a part's form, orientation and location.

Is GD&T the same as a plus-minus tolerance?

No. A plus-minus tolerance limits one dimension between two numbers. GD&T controls a feature's geometry relative to datums, for example how close a hole's true position has to be to a reference edge, which is a different and often more forgiving kind of control.

Do all technical drawings need GD&T?

No. Most dimensions on most parts are fine with a general tolerance and a few plus-minus callouts. GD&T earns its place on the features whose geometric relationship to another feature decides whether the part actually fits or works.

What is GD&T? (Geometric Dimensioning & Tolerancing)

A language for controlling geometry, not just size

GD&T stands for Geometric Dimensioning and Tolerancing. It is a standardized set of symbols that specify how much a feature is allowed to vary in shape, orientation or position, measured against named reference surfaces called datums, rather than just how much a single length or diameter can vary. The rules are defined by ASME Y14.5 in the US and the ISO 1101 family internationally.

Why it is different from a plus-minus tolerance

A plus-minus tolerance, like 20 ±0.1 mm, controls one dimension between two limits. That works fine for a length, but it breaks down once two features have to line up with each other, such as a hole that must mate with a hole in another part. GD&T solves that by tolerancing the relationship: how true a hole's position is to two reference surfaces, not just its X and Y coordinates independently.

Family	What it controls
Form	How flat, straight, round or cylindrical a surface is
Orientation	Perpendicularity, angularity, parallelism to a datum
Location	Position, concentricity and symmetry relative to datums
Runout	How much a surface wobbles as the part rotates
Profile	The allowed envelope around a complex surface or line

The feature control frame

Every GD&T requirement lives inside a boxed callout called a feature control frame: the geometric symbol, then the tolerance value, then the datums it is measured against, in priority order. Learn to read that one box and the rest of GD&T follows.

You do not need all fourteen symbols to read most drawings. Recognise the feature control frame and the datum symbols first, and add the individual characteristics as you meet them.

Where to go deeper

For the full chart of all 14 GD&T characteristics with examples, see our engineering drawing symbols and GD&T guide. If you are deciding whether your part needs tolerances at all, start with do I need tolerances on a drawing, and for the plus-minus basics see how to read tolerances on a drawing.