What Is a True Position Calculator?
A true position calculator computes the GD&T (Geometric Dimensioning and Tolerancing) true position of a feature from its measured deviations in the X and Y directions. True position describes how far a feature's actual location is from its intended position, defining a circular (or cylindrical) tolerance zone. Enter the deviations and the calculator returns the true position value.
How to Use the Calculator
- Enter the X deviation from the intended position.
- Enter the Y deviation.
- Calculate — see the true position value.
The True Position Formula
True Position = 2 × √(X² + Y²)
The deviations X and Y are how far the feature's center is off in each direction. The radial error is √(X² + Y²), and doubling it gives the diameter of the tolerance zone — the true position value. For example, X = 0.003 and Y = 0.004 gives 2 × √(0.000009 + 0.000016) = 2 × 0.005 = 0.010.
Why It's Doubled
| Quantity | Meaning |
|---|---|
| √(X² + Y²) | Radial distance from true position (radius) |
| 2 × √(X² + Y²) | Diameter of the tolerance zone (true position) |
GD&T position tolerance is specified as a diameter zone, so the true position is twice the radial error.
Pass or Fail
If the calculated true position is less than or equal to the allowed position tolerance on the drawing, the feature passes. If it exceeds the tolerance, the feature is out of position. Bonus tolerance from material condition modifiers (like MMC) can increase the allowable value.
Where True Position Is Used
- Manufacturing: verifying hole and feature locations.
- Quality control: inspecting parts against GD&T drawings.
- Assembly: ensuring features align for proper fit.
Frequently Asked Questions
What is true position in GD&T?
True position describes how far a feature's actual location deviates from its intended position, defining a circular tolerance zone around the ideal location.
How do you calculate true position?
Use True Position = 2 × √(X² + Y²), where X and Y are the deviations from the intended location. The result is the diameter of the tolerance zone.
Why is the result multiplied by 2?
Because GD&T position tolerance is specified as a diameter, the true position is twice the radial deviation from the ideal location.
How do I know if a part passes?
Compare the calculated true position to the position tolerance on the drawing. If it is within the tolerance (plus any bonus tolerance), the feature passes.
Is this true position calculator free?
Yes — it is completely free, requires no signup, and computes GD&T true position.