Geometric dimensioning and tolerancing (GD&T) controls the location of a feature with a cylindrical tolerance zone around its exact (true) position. The measured centre may lie anywhere inside that zone, so the positional error is twice the radial distance from true position. When the tolerance is applied at MMC, the zone grows as the feature departs from its maximum material size.
How to use the True Position Calculator
- X deviation
- Y deviation
- Position tolerance (diameter)
- Material condition
- Actual feature size
- MMC size
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the X and Y deviations from true position, the position tolerance and, for MMC, the feature type, its actual size and its MMC size.
Inputs explained
| Field | What to enter |
|---|---|
| X deviation | Units: mm, in |
| Y deviation | Units: mm, in |
| Position tolerance (diameter) | Units: mm, in |
| Material condition | Choose one: RFS (no bonus), Hole at MMC, Pin / shaft at MMC |
| Actual feature size | Units: mm, in |
| MMC size | Units: mm, in |
What each result means
- True position (diametral) = 2×sqrt([X deviation]^2 + [Y deviation]^2)
- Total allowed tolerance = [Position tolerance (diameter)] + [Bonus tolerance]
- Margin (positive = pass) = [Total allowed tolerance] – [True position (diametral)]
Example values
How the main result changes when X deviation changes and everything else stays at the example values above:
| X deviation (mm) | True position (diametral) (mm) |
|---|---|
| 0.04 | 0.1442 |
| 0.06 | 0.1697 |
| 0.08 | 0.2 |
| 0.1 | 0.2332 |
| 0.12 | 0.2683 |
| 0.16 | 0.3418 |
Formula
True position = 2 × √(Δx² + Δy²)
Bonus (hole at MMC) = actual size − MMC size; (pin) = MMC size − actual size
Allowed = tolerance + bonus; margin = allowed − true position
Worked example
Inputs: X deviation: 0.08 mm; Y deviation: 0.06 mm; Position tolerance (diameter): 0.2 mm; Material condition: Hole at MMC; Actual feature size: 10.05 mm; MMC size: 10 mm.
Results: True position (diametral) = 0.2 mm; Bonus tolerance = 0.05 mm; Total allowed tolerance = 0.25 mm; Margin (positive = pass) = 0.05 mm; Pass (1) or fail (0) = 1.
Key terms
- MMC
- Maximum material condition: the smallest hole or the largest pin within the size limits.
Frequently asked questions
Why is true position twice the radial deviation?
The tolerance is a diameter, while the deviation √(Δx² + Δy²) is a radius.
What is bonus tolerance?
Extra positional tolerance allowed at MMC as a hole gets larger (or a pin smaller) than its maximum-material size.
What does RFS mean?
Regardless of feature size, so no bonus tolerance applies.