True Position Calculator
ConstructionCalculate GD&T true position deviation from measured X and Y coordinate offsets. Free tool for machinists and inspectors checking feature position tolerance.
True Position
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What is a True Position?
A True Position Calculator computes the GD&T (geometric dimensioning and tolerancing) true position deviation of a feature — such as a drilled or tapped hole — based on how far its measured center deviates from the exact nominal location in the X and Y directions. The result is the diametrical tolerance zone value used to check conformance against a feature control frame on an engineering drawing.
Pair this calculator with the Clearance Hole Calculator when verifying that a hole both fits its intended fastener and sits within tolerance, or the Thread Pitch Calculator when the feature being checked is a tapped hole.
How to use this True Position calculator
- Measure the feature's actual center location using a CMM, height gauge, or optical comparator, and compare it to the exact nominal coordinates from the drawing.
- Enter the absolute X Deviation in inches — the offset from nominal along the X axis.
- Enter the absolute Y Deviation in inches — the offset from nominal along the Y axis.
- Review the True Position result and compare it against the tolerance value specified in the feature control frame.
Formula & Methodology
The calculator applies the standard GD&T true position formula, doubling the radial deviation to express it as a diameter: True Position = 2 × √(X Deviation² + Y Deviation²) Worked example: For a hole measured with an X deviation of 0.005 in and a Y deviation of 0.003 in: True Position = 2 × √(0.005² + 0.003²) True Position = 2 × √(0.000025 + 0.000009) = 2 × √0.000034 True Position = 2 × 0.00583 ≈ 0.01166 in
Frequently Asked Questions
True position is a geometric dimensioning and tolerancing (GD&T) control that defines a tolerance zone around the theoretically exact (nominal) location of a feature, such as a hole, and measures how far the actual feature's center deviates from that exact location. It's expressed as a single diameter value representing a circular (or cylindrical, for depth) tolerance zone.
True position is calculated as two times the square root of the sum of the squared X deviation and squared Y deviation: 2 × √(X² + Y²). The factor of 2 converts the radial deviation from nominal into the diametrical tolerance zone value used on engineering drawings.
True position tolerance zones are specified as a diameter, not a radius, so the raw radial distance from nominal position (found via the Pythagorean theorem) must be doubled to express the deviation in the same diametrical terms as the drawing's position tolerance callout. This lets you directly compare your calculated value against the tolerance specified in the feature control frame.
Coordinate tolerancing applies separate plus/minus tolerances to the X and Y dimensions independently, which creates a square tolerance zone and can be overly restrictive at the corners, while true position defines a circular tolerance zone around the exact nominal location, providing about 57% more usable tolerance area for the same numeric value and a single pass/fail number.
Measure the actual center location of the feature (using a CMM, height gauge, or optical comparator) and compare it against the exact nominal coordinates from the drawing — the difference in each axis is your X deviation and Y deviation, which should be entered as absolute (positive) values regardless of direction.
No — this calculator computes the raw true position value from measured deviation only. If the feature control frame specifies Maximum Material Condition (MMC), you may be entitled to additional bonus tolerance based on how far the feature's actual size departs from MMC, which must be added separately when evaluating conformance.
If your calculated true position value exceeds the tolerance specified in the feature control frame, the part is out of tolerance for that position callout and should be flagged as nonconforming, unless applicable bonus tolerance (at MMC or LMC) brings it back within limits — consult the full feature control frame and any material condition modifiers before making a final accept/reject decision.
The calculation method is the same for each individual hole, but a pattern of holes typically references a datum feature and may share a single position tolerance applied to every hole in the pattern — each hole in the pattern still needs its own X and Y deviation measured and evaluated independently against that shared tolerance.
Yes, the same X-Y deviation formula applies to any feature located by two coordinate dimensions, such as pins, bosses, or slots, not just holes — as long as you have the measured deviation from nominal position in both the X and Y directions.
Enter X and Y deviation in inches for a result in inches; the calculator does not perform unit conversion, so make sure your measured deviation values and the tolerance you're comparing against use the same unit system as your drawing.
Use this True Position Calculator to verify hole location during quality inspection, then check the [Clearance Hole Calculator](/clearance-hole-calculator/) if you need to confirm the hole diameter itself accommodates the intended fastener, or the [Thread Pitch Calculator](/thread-pitch-calculator/) if the feature in question is a tapped hole.
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