# True Position Calculator

Calculate GD&T true position from X and Y deviations. Check pass/fail against tolerance zones with optional MMC bonus tolerance.

## What this calculates

True position is the most common GD&T (Geometric Dimensioning and Tolerancing) callout for locating holes and features. Enter the X and Y deviations from the nominal position, and this calculator tells you the diametrical true position and whether the feature passes its tolerance.

## Inputs

- **X Deviation from Nominal** (in) — min -10, max 10 — Measured position minus nominal X position
- **Y Deviation from Nominal** (in) — min -10, max 10 — Measured position minus nominal Y position
- **Tolerance Zone Diameter** (in) — min 0, max 10 — Diametrical tolerance from the drawing callout
- **Actual Feature Size (for MMC bonus)** (in) — min 0, max 100 — Enter actual hole size for MMC bonus tolerance (0 = RFS)
- **MMC Size (Smallest Hole / Largest Pin)** (in) — min 0, max 100 — Maximum Material Condition size (0 = no bonus)

## Outputs

- **True Position** (in) — Diametrical true position value
- **Radial Deviation** (in) — Distance from nominal center
- **MMC Bonus Tolerance** (in)
- **Total Allowable Tolerance** (in)
- **Result** — formatted as text — Pass or Fail based on tolerance zone

## Details

## True Position Formula

True position uses a circular (diametrical) tolerance zone instead of the square zone you get with traditional plus/minus tolerancing. The formula is:

**True Position = 2 x sqrt(X deviation² + Y deviation²)**

The factor of 2 converts the radial distance to a diameter, matching the diametrical tolerance callout on the drawing.

### Example Calculation

A hole is supposed to be at X = 2.000, Y = 3.000. You measure it at X = 2.003, Y = 3.004. The drawing calls out a position tolerance of 0.014 diameter.

- X deviation = 0.003
- Y deviation = 0.004
- Radial deviation = sqrt(0.003² + 0.004²) = 0.005
- True position = 2 x 0.005 = 0.010
- Result: **PASS** (0.010 is within the 0.014 tolerance zone)

### MMC Bonus Tolerance

When a position callout includes a circled M (MMC modifier), you get extra tolerance as the feature departs from Maximum Material Condition. For a hole, the bonus equals the actual hole diameter minus the MMC (smallest) hole diameter. This can significantly loosen the position requirement for oversized holes.

### Why Diametrical vs. Coordinate Tolerancing?

A 0.014 diameter tolerance zone gives you 57% more area than a 0.010 x 0.010 square zone from coordinate tolerancing. The circular zone better represents functional fit since mating parts (like a bolt through a hole) only care about the radial distance from true position.

## Frequently Asked Questions

**Q: How do you calculate true position?**

A: True position equals 2 times the square root of (X deviation squared plus Y deviation squared). Measure the actual feature location, subtract the nominal X and Y positions to get the deviations, then plug them into the formula. The result is a diametrical value that you compare against the tolerance on the drawing.

**Q: What is the difference between true position and concentricity?**

A: True position locates a feature relative to datums using a fixed tolerance zone. Concentricity controls how well the median points of a feature align with a datum axis. True position is much easier to measure and is preferred in most cases. Concentricity is rarely used in modern GD&T practice.

**Q: What does MMC mean in a position callout?**

A: MMC (Maximum Material Condition) is a modifier that gives you bonus tolerance as the feature departs from its tightest material condition. For a hole, MMC is the smallest allowable diameter. If the actual hole is larger than MMC, the difference gets added to the position tolerance, making it easier to pass.

**Q: Can true position be zero?**

A: Yes. A true position of zero means the feature is exactly at its nominal location with no deviation in X or Y. In practice this is nearly impossible to achieve, which is why drawings always specify a tolerance zone. Even very precise CNC machining typically produces true position values of 0.001-0.005 inches.

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Source: https://vastcalc.com/calculators/construction/true-position
Category: Construction
Last updated: 2026-04-08