True Position Calculator

True position is calculated as TP = 2 × √(ΔX² + ΔY²), where ΔX is the deviation in X (measured minus nominal), and ΔY is the deviation in Y. The result is the diameter of the circle centered at the nominal position that contains the actual feature center. For GD&T per ASME Y14.5, true position must be within the specified tolerance diameter. A measured deviation of X=0.005" and Y=0.003" gives TP = 2 × √(0.005² + 0.003²) = 2 × √(0.000034) = 2 × 0.00583 = 0.01167". If the tolerance is 0.014", this passes.

Bonus tolerance applies when using MMC (Maximum Material Condition) modifier. It allows additional positional tolerance equal to the difference between the actual feature size and the MMC size. For a hole at MMC (smallest hole = tightest position required), as the hole gets larger (LMC = Least Material Condition), you get bonus tolerance. Bonus = Actual Size − MMC Size for internal features, or MMC Size − Actual Size for external features. This can significantly increase the allowable position error without affecting function.

The ⌀ (diameter) symbol before the tolerance value in GD&T indicates that the tolerance zone is a cylinder (for a hole/pin axis) or a circle (for a point). Without the ⌀ symbol, the tolerance zone is square (X and Y tolerances applied independently). A ⌀0.014" tolerance zone means the feature axis must fall within a cylinder of diameter 0.014". This is approximately 57% larger area than a ±0.005" square tolerance (±0.005 gives 0.010" square → ⌀0.014" is equivalent area). Always use true position for round features.

True position controls the location of a feature axis relative to a datum reference frame. It is the most common GD&T control for hole locations, boss positions, and pattern features. Concentricity (rarely used) controls the median points of a feature relative to a datum axis — it is much stricter and harder to measure. True position with RFS (Regardless of Feature Size) is preferred over concentricity in nearly all cases. ASME Y14.5-2018 deprecates concentricity in favor of true position or runout controls.