A GD&T tolerance zone is the allowable space within which a controlled feature must lie. Instead of thinking only in plus/minus values, picture the zone around the perfect design target. The feature passes when it stays inside that zone.
Two parallel planes
Flatness, parallelism, perpendicularity, angularity, and surface straightness often use two parallel planes separated by the tolerance value. The controlled surface or line element must remain between them. The planes may be oriented to a datum when the control is an orientation requirement.
Cylindrical tolerance zones
Position, axis straightness, and some orientation controls use a cylindrical zone when a diameter symbol appears before the tolerance. A hole or shaft axis can move within that cylinder, centered on the ideal location or orientation defined by the drawing.
Profile tolerance zones
Profile of a line creates a two-dimensional band around the true profile in a cross-section. Profile of a surface creates a three-dimensional shell around the true surface. With datum references, a profile zone can also control location and orientation.
How to read the diameter symbol
The diameter symbol before 0.20 means the controlled hole axis must stay within a cylindrical zone 0.20 in diameter. Compare this to the flatness symbol, which uses two parallel planes.
Frequently asked questions
GD&T Tolerance Zones Explained: Cylindrical, Planar, and Profile Zones FAQ
What is a tolerance zone?
It is the defined space within which a controlled surface, line, axis, or center plane is allowed to exist.
Why does a position tolerance use a diameter symbol?
The diameter symbol specifies a cylindrical tolerance zone around the true position of a round feature’s axis.
Do all GD&T symbols use the same tolerance zone?
No. The zone depends on the geometric characteristic, the feature being controlled, and the symbols and modifiers in the frame.
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