How to Use the True Position Calculator

A True Position Calculator is a measurement tool that helps engineers, machinists, and quality inspectors determine whether a manufactured part feature (like a hole or pin) is within the allowable tolerance specified on a technical drawing using Geometric Dimensioning and Tolerancing (GD&T).

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Why True Position Matters

True position is one of the most widely used GD&T callouts in manufacturing. It defines the exact location of a feature relative to its nominal (intended) position. Unlike simple linear dimensions, true position accounts for both X and Y (or 3D coordinates), ensuring the feature’s location is within a specified tolerance zone.

This matters because:

• Misaligned holes may cause assembly failures.
• Parts may not fit together if true position tolerances are exceeded.
• Proper tolerance management reduces rework and rejects.

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How to Use the True Position Calculator

1. Enter the nominal X and Y values (from the engineering drawing).
   Example: Nominal hole location = X: 50.000 mm, Y: 25.000 mm
2. Enter the measured X and Y values (from a CMM, caliper, or other inspection tool).
   Example: Measured hole location = X: 50.120 mm, Y: 24.890 mm
3. Enter the positional tolerance as specified on the drawing.
   Example: Tolerance = 0.500 mm
4. Click "Calculate True Position."

The calculator applies the formula:TP=2×(Xmeasured−Xnominal)2+(Ymeasured−Ynominal)2TP=2×(Xmeasured​−Xnominal​)2+(Ymeasured​−Ynominal​)2​

5. Review Results:
   • If the computed true position is less than or equal to the tolerance, the feature passes.
   • If it is greater than the tolerance, the feature fails inspection.

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Example Calculation

• Nominal Position: (50.000, 25.000)
• Measured Position: (50.120, 24.890)
• Tolerance: 0.500 mm

Step 1: ΔX = 50.120 – 50.000 = 0.120
Step 2: ΔY = 24.890 – 25.000 = -0.110
Step 3: √(ΔX² + ΔY²) = √(0.120² + (-0.110)²) = 0.163
Step 4: True Position = 2 × 0.163 = 0.326 mm

Since 0.326 mm ≤ 0.500 mm, the feature is within tolerance.

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Benefits of Using a True Position Calculator

• Accuracy: Removes manual calculation errors.
• Speed: Saves time during inspections.
• Consistency: Ensures standardized checks across parts.
• Quality Control: Improves product reliability and reduces waste.

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True Position Calculator FAQ

Q1: Why is the true position formula multiplied by 2?
A: The multiplier accounts for the fact that the true position defines the diameter of the tolerance zone, not just the radial distance from the nominal point.

Q2: Can this calculator handle 3D true position (X, Y, Z)?
A: This version is for 2D true position (holes and planar features). The formula can be extended into 3D by adding Z deviation.

Q3: Does this account for MMC (Maximum Material Condition) modifiers?
A: No. This basic version checks the location only. MMC adjustments require additional allowances depending on feature size.

Q4: What units does the calculator use?
A: It works in millimeters (mm) by default, but you can input inches if your tolerance values are in inches (as long as all values use the same unit system).

Q5: Can I use this for circular pattern holes?
A: Yes, as long as you input the nominal and measured coordinates for each feature individually.

Q6: Is this calculator industry-standard?
A: It uses the standard GD&T definition of true position. However, for critical inspections, use certified CMM software for formal measurement reports.