Engineering Reference

True Position Calculator

Enter the measured X and Y deviation. Get the true position, the MMC bonus if your callout carries one, and a pass or fail against the drawing.

By Atishay Jain, founder of Mavlon · Updated July 2026

Measured true position
Formula: true position = 2 × √(ΔX² + ΔY²): the common planar X-Y case with a diametral zone (ASME Y14.5). Projected zones, composite frames, and 3D axis evaluations need their own treatment.

The formula, and why the 2 is there

true position = 2 × √(ΔX² + ΔY²)

ΔX and ΔY are how far the actual axis of the feature landed from its nominal location, the position defined by basic dimensions. The square root gives the straight-line radial distance from nominal. The tolerance zone on the drawing is a diameter, not a radius, so the radial distance is doubled before you compare it to the callout. This covers the common planar X-Y evaluation; projected tolerance zones, composite frames, and full 3D axis evaluations are separate cases with their own math.

Worked example

A hole is measured 0.05 mm off in X and 0.03 mm off in Y.

√(0.05² + 0.03²) = √0.0034 = 0.0583 radial

True position = 2 × 0.0583 = 0.117 mm. Against a Ø0.25 callout, this passes with room to spare.

If the feature control frame carries an MMC modifier, the feature earns bonus tolerance as it departs from maximum material condition: a hole that runs large, or a pin that runs small, gets the departure added to the stated tolerance. Tick the MMC box in the calculator and it handles the bonus and the verdict for you.

Calculating one hole is easy. A drawing has two hundred.

Mavlon reads every feature control frame on your drawing, datums and modifiers included, and builds the AS9102 Form 3 with tolerances already interpreted.

Try with One Drawing

First drawing is free · No signup

Frequently asked questions

What is the true position formula?
True position = 2 × √(ΔX² + ΔY²), where ΔX and ΔY are the measured deviations of the feature axis from its nominal (basic) location. The result is the diameter of the smallest cylindrical zone, centered on the true position, that contains the actual axis.
Why is true position multiplied by 2?
The square root term gives the radial distance from nominal to the actual axis. The drawing specifies the tolerance zone as a diameter, so the radial deviation is doubled to compare against the callout.
How does MMC bonus tolerance work?
With an MMC modifier, the stated tolerance applies at maximum material condition. As the feature departs from MMC (a hole gets bigger, a pin gets smaller), the departure is added as bonus: total allowed = stated tolerance + |actual size − MMC size|.
Can a measured value larger than the callout still pass?
Yes, with an MMC modifier. Example: callout Ø0.25 at MMC, hole MMC 6.35, actual hole 6.42. Bonus is 0.07, total allowed is 0.32, so a measured 0.30 passes.
What does a true position of Ø0.25 mean?
The feature axis must lie inside a cylindrical zone 0.25 in diameter (0.125 radius) centered on the nominal position. It does not mean the feature may sit 0.25 away from nominal; the maximum radial deviation is half of that.

Keep reading

Glossary: True Position Glossary: Bonus Tolerance Glossary: Composite Position True Position Calculation for Aerospace FAI MMC Bonus Tolerance: Worked Calculations Positional Tolerance on Hole Patterns ISO 2768 Tolerance Calculator Surface Finish Converter Hardness Converter FAI Software: The Buyer Guide