True Position Calculator (GD&T Position Tolerance)
Convert measured X and Y coordinates into a true position deviation and compare it with the position tolerance from the feature control frame. True position is the diametral zone, twice the radial distance from the basic location, exactly as defined in ASME Y14.5 and ISO 1101.
How to use this calculator
- Enter the basic X and Y dimensions as they appear in the boxed dimensions on the drawing.
- Enter the measured X and Y coordinates of the feature axis.
- Enter the position tolerance value from the feature control frame.
- Read the diametral true position deviation, the tolerance consumed and whether the part passes.
Frequently asked questions
What is the true position formula?
True position = 2 x sqrt((measured X - basic X)^2 + (measured Y - basic Y)^2). The factor of two is what turns a radial distance into a diametral tolerance zone, which is how position tolerance is specified on a drawing.
Why does a small radial error use up so much tolerance?
Position tolerance is applied as a diameter, so the diametral deviation is twice the radial error. A feature only 0.14 mm away from its basic location already consumes 0.28 mm of diametral tolerance, which fails a 0.25 mm callout.
What about bonus tolerance at maximum material condition?
When the position callout carries the maximum material condition modifier, the allowable position tolerance grows by however far the feature departs from MMC, up to the size tolerance. That bonus is not applied here, so enter the tolerance exactly as it appears in the frame.