Precision & Quality Control
True Position Calculator
Verify that anchor bolts, drilled holes, and embed plates are within GD&T position tolerance. Enter the nominal (designed) coordinates from your drawing and the actual (measured) coordinates from your survey or CMM report. The calculator returns the True Position diameter and a clear Pass / Fail result for each feature — in metric (mm) or imperial (inches).
True Position Calculator
GD&T position tolerance — check actual vs nominal coordinates
| Feature | Nominal (mm) | Actual (mm) | Tol ⌀ (mm) | |||
|---|---|---|---|---|---|---|
| X | Y | X | Y | |||
Enter nominal & actual coordinates, then click Calculate
Tolerance Zone Diagram
The true position tolerance zone is a cylinder (shown here as a circle in plan view) centred on the nominal coordinate. The actual feature must lie within this zone.
Step-by-Step Formulas
1. True Position Formula (2D)
ΔX and ΔY are the deviations between the actual measured coordinate and the nominal (designed) coordinate. The factor of 2 converts the radial error into a diameter, because the position tolerance zone is always specified as a diameter (⌀) in GD&T — matching the cylindrical zone a bolt or hole must fall within.
2. True Position Formula (3D)
For features where the axial deviation also matters — such as the height of an embed plate stud or a through-bolt in a deep foundation — the Z-axis deviation is included. This is less common on structural drawings but standard in precision machining (ASME Y14.5).
3. Tolerance Zone
The tolerance zone is a cylinder of the specified diameter centred on the true (nominal) position. The actual feature axis or centre point must lie entirely within this cylinder. Common structural tolerances: anchor bolts ±3 mm (1/8 in) for standard connections; ±1.5 mm (1/16 in) for precision moment connections per AISC Code of Standard Practice Section 7.
4. Bonus Tolerance (MMC)
When a feature of size (hole or pin) is produced away from its Maximum Material Condition (MMC — smallest hole or largest pin), the position tolerance may be increased by the bonus amount. For example, a 20 mm hole drilled at 20.5 mm earns 0.5 mm extra position tolerance. This is the most common modifier in machined part GD&T but less frequently applied in structural construction.
Worked Example — Anchor Bolt Inspection
Column base plate: 4 anchor bolts at corners of a 200 × 200 mm pattern. Tolerance ⌀3.0 mm.
💡 Pro tip
When a bolt fails true position, the first remedy is enlarging the base plate holes (slot or oversize). Check the AISC Code of Standard Practice and the structural engineer's tolerance notes before deciding whether to re-set the bolt, use a shim plate, or accept with a documented NCR.
Frequently Asked Questions
- What is True Position in GD&T?
- True Position is a GD&T (Geometric Dimensioning and Tolerancing) control that defines the exact theoretical location of a feature relative to datum references. The actual feature centre must fall within a cylindrical tolerance zone (defined by diameter ⌀) centred on that true position.
- How is True Position calculated?
- For 2D: TP = 2 × √(ΔX² + ΔY²), where ΔX and ΔY are the differences between actual and nominal coordinates. The factor of 2 converts from radius to diameter. For 3D: TP = 2 × √(ΔX² + ΔY² + ΔZ²).
- What are typical anchor bolt tolerances in construction?
- Per AISC Code of Standard Practice (Section 7.5): standard anchor bolt groups must be within ±3 mm (1/8 in) of their true position for column base plate connections. Critical moment frames or exposed column connections often specify ±1.5 mm (1/16 in). Always confirm with the project structural engineer.
- What units should I use?
- Use millimetres (mm) for metric projects — all coordinates and tolerances in the same unit. Use inches (in) for imperial projects. The formula works for any consistent unit system. Do not mix units in the same calculation.
- Can I check multiple bolts or holes at once?
- Yes. The calculator accepts a table of features. Add as many rows as needed — each gets its own ΔX, ΔY, True Position result, tolerance comparison, and Pass/Fail status. This matches a typical anchor bolt survey report or CMM inspection table.