Position is the GD&T control that locates a feature of size, such as a hole, pin or slot, relative to datums. Its axis must fall inside a zone, usually a cylinder, centred on the true position set by basic dimensions. It is the most used GD&T symbol, and with MMC it gives bonus tolerance as the feature size moves away from MMC.
Position replaces the rectangular plus-or-minus box that coordinate dimensions create with a round zone around the exact location. A round zone matches how a round bolt fits a round hole: what matters is how far off-centre the hole is, in any direction. That round zone is about 57% larger in area than the square that fits inside it, so more good parts pass.
The true position comes from basic (boxed) dimensions measured from the datums in the frame. The datums tell inspection how to hold the part: seat on A, push against B, then C. See datums and the datum reference frame.
Position also controls orientation of the axis within its zone, so a hole positioned to datum A is also limited in perpendicularity to A. Use the true position calculator to turn measured X and Y deviations into a position result.
With a diameter symbol the zone is a cylinder of the stated diameter, centred on the true position and running through the depth of the feature, square to the primary datum. Without Ø (for slots and tabs) it is two parallel planes centred on the true centre plane.
The measured result, often called "true position", is twice the radial distance from the true position to the actual axis: 2 × √(ΔX² + ΔY²).
Position almost always references datums, typically a primary face and one or two locating features. The exception is a pattern of features positioned only to each other, or coaxial features controlled to each other.
| Modifier | With position |
|---|---|
| MMC | The most common case for clearance holes and pins. Bonus tolerance equals the departure from MMC, and a functional gauge can be used. |
| LMC | Used to protect minimum wall thickness or edge distance. |
| RFS (no modifier) | The tolerance is fixed whatever the size, for precision locations such as bearing bores. |
| Projected tolerance zone (P) | For threaded and press-fit holes: the zone is projected above the part over the height of the mating part. |
| Datum modifiers (MMB) | A datum feature of size referenced at MMB can shift, giving extra movement to the whole pattern. |
Example: a Ø8.0–8.2 mm clearance hole at basic X 30, Y 20 from datums B and C, with this frame
Result: Ø0.30 ≤ Ø0.35, so the hole passes. Without the MMC bonus it would fail the base Ø0.25.
Every GD&T symbol page: GD&T symbols chart
True position is the exact location set by basic dimensions. Position is the GD&T control that limits how far the feature may be from it. Shops also say "true position" for the measured result.
Find the X and Y deviations from the basic location and compute 2 × √(ΔX² + ΔY²). Compare it with the tolerance plus any MMC bonus.
Almost always. Without datums a position tolerance can only relate features to each other, such as holes within a pattern.
Extra position tolerance earned when a feature controlled at MMC is made away from MMC. For a hole, bonus = actual size − smallest allowed size.
A position zone that extends above the part over the height of the mating part, used on threaded or press-fit holes so a tilted hole cannot push the bolt out of position.
Yes. For slots, tabs and other width features the zone is two parallel planes, and no Ø is shown.
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