GD&T (Geometric Dimensioning and Tolerancing) is a symbolic language on drawings that controls a feature’s geometry — not just its size, but its form, orientation, and position — so parts fit and function as intended. It communicates design intent far more precisely than plus/minus dimensions alone.
The main categories of control
| Category | Controls | Example symbols |
|---|---|---|
| Form | Shape of a single feature | Flatness, straightness, circularity |
| Orientation | Angle to a datum | Perpendicularity, parallelism |
| Location | Position relative to datums | Position, concentricity |
| Runout | Variation as a part rotates | Circular & total runout |
| Profile | Overall surface shape | Profile of a line/surface |
GD&T relies on datums — reference surfaces that establish a coordinate system — and feature control frames, the boxed callouts pairing a symbol, a tolerance, and the datums it references. For controlled drawings, follow the governing standard (such as ASME Y14.5).
Frequently asked questions
What does GD&T do that dimensions don’t? It controls geometry — form, orientation, and position — not just size.
What is a datum? A reference surface that defines the coordinate system for measurements.
Is GD&T standardized? Yes — commonly to ASME Y14.5 (or ISO GPS).