The GD&T Guide
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Foundation 05

Basic Dimensions & Tolerance Zones

The exact targets and allowable spaces that turn drawing intent into an inspectable requirement.

A basic dimension is theoretically exact. It identifies the perfect target location, orientation, or profile; the associated GD&T control defines the allowable variation around that target with a tolerance zone.

Basic does not mean measured to zero variation

A basic dimension is usually enclosed in a rectangular box. It is the exact design target used to construct the tolerance zone. The permitted variation comes from the feature control frame, not from a separate plus/minus tolerance attached to that basic number.

Think in zones, not coordinate limits

GD&T asks whether the controlled feature stays inside its allowed zone. For example, a position tolerance can create a cylindrical zone around the basic location of a hole axis. The axis can move anywhere inside that cylinder.

Why this helps assembly

Basic dimensions and a geometric tolerance can communicate functional freedom more directly than a collection of independent plus/minus dimensions. The design says exactly what needs to line up and how much variation is acceptable.

Example: locating a hole

25.0040.00⌖⌀0.20AB

The boxed 25.00 and 40.00 dimensions establish the exact target. The position frame supplies the 0.20 mm diameter zone around that target.

Common mix-up

A boxed basic dimension is not a normal size tolerance. Do not add plus/minus limits to it unless the drawing explicitly defines a separate requirement.

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