Quick Answer: How Do You Calculate True Position?
Calculate true position with the ASME Y14.5 formula TP = 2 × √(dX² + dY²), where dX and dY are how far the feature center deviates from its exact (true) location in the X and Y directions. The result is a diameter because the tolerance zone is a cylinder centered on the true location.
This calculator uses the ASME Y14.5 true position formula to compute: true position diameter (mm or inches), MMC or LMC bonus tolerance, total allowed tolerance, and a pass or fail result with percent of tolerance used. Inputs include X and Y deviations (or nominal and measured coordinates), the stated geometric position tolerance, the material condition modifier, feature type (hole or shaft), MMC size and actual feature size.
Worked example: dX = 0.1, dY = 0.1 → TP = 0.2828 | Hole MMC = 10.0, actual = 10.2, geometric tol = 0.1 → bonus = 0.2, total allowed = 0.3 → PASS, about 94.3% used.
How True Position Works
True position controls where a feature sits. The drawing gives an exact location, called the true position. The real part is never perfect, so the feature center lands a little off. True position measures how far off it is and compares that to the allowed tolerance zone.
The True Position Formula
First find how far the center moved in X and Y. These are dX and dY. Then use the formula TP = 2 × √(dX² + dY²). The square root finds the straight-line distance from the true location. Multiplying by 2 turns that radius into a diameter, because the tolerance zone is a cylinder.
From Measured Coordinates
If you measured actual X and Y coordinates instead of deviations, subtract first. dX = X measured − X nominal and dY = Y measured − Y nominal. Then put dX and dY into the formula. This calculator does both steps for you when you switch to coordinate mode.
MMC Bonus Tolerance
When the drawing shows an M circle modifier, the feature earns bonus tolerance. For a hole, the bonus equals the actual size − the MMC (smallest) size. For a shaft or pin, the bonus equals the MMC (largest) size − the actual size. The bonus is never negative. Add the bonus to the stated geometric tolerance to get the total allowed tolerance.
Pro tip: Bonus tolerance is free real estate. A hole drilled larger than MMC gives you more position tolerance. On a tight bolt pattern, calling out MMC instead of RFS can turn a reject into a pass without changing the machine setup.
Pass, Fail and Percent Used
A feature passes when the true position diameter is at or below the total allowed tolerance. Percent of tolerance used is the true position divided by the total allowed, times 100. Below 100% passes with margin. Above 100% fails. Keeping this number under 80% leaves room for measurement uncertainty and machine drift.
Worked Example
A hole is located with a 0.1 mm position tolerance at MMC. Its MMC size is 10.0 mm and it was made at 10.2 mm, so the bonus is 0.2 mm. Total allowed is 0.1 + 0.2 = 0.3 mm. The hole center sits 0.1 mm off in X and 0.1 mm off in Y, so true position is 2 × √(0.1² + 0.1²) = 0.2828 mm. Since 0.2828 is less than 0.3, the hole passes and uses about 94.3% of the allowed tolerance.