True Position Calculator
True position is the most common GD&T (Geometric Dimensioning and Tolerancing) callout for locating holes and features. Enter the X and Y deviations from the nominal position, and this calculator tells you the diametrical true position and whether the feature passes its tolerance.
True Position Formula
True position uses a circular (diametrical) tolerance zone instead of the square zone you get with traditional plus/minus tolerancing. The formula is:
True Position = 2 x sqrt(X deviation² + Y deviation²)
The factor of 2 converts the radial distance to a diameter, matching the diametrical tolerance callout on the drawing.
Example Calculation
A hole is supposed to be at X = 2.000, Y = 3.000. You measure it at X = 2.003, Y = 3.004. The drawing calls out a position tolerance of 0.014 diameter.
- X deviation = 0.003
- Y deviation = 0.004
- Radial deviation = sqrt(0.003² + 0.004²) = 0.005
- True position = 2 x 0.005 = 0.010
- Result: PASS (0.010 is within the 0.014 tolerance zone)
MMC Bonus Tolerance
When a position callout includes a circled M (MMC modifier), you get extra tolerance as the feature departs from Maximum Material Condition. For a hole, the bonus equals the actual hole diameter minus the MMC (smallest) hole diameter. This can significantly loosen the position requirement for oversized holes.
Why Diametrical vs. Coordinate Tolerancing?
A 0.014 diameter tolerance zone gives you 57% more area than a 0.010 x 0.010 square zone from coordinate tolerancing. The circular zone better represents functional fit since mating parts (like a bolt through a hole) only care about the radial distance from true position.