GD&T True Position Calculator

Formula

• Radial error = √(ΔX² + ΔY² [+ ΔZ²])
• True position = 2 × radial error
• Pass when true position ≤ position tolerance

About this calculator

Part of our GD&T & Metrology Calculators collection. True position, fits, ISO tolerances, and surface roughness.

How it works

True position equals twice the radial deviation from the basic location. In 3D mode the radial term includes Z deviation. Compare the result to your position tolerance zone. This follows the standard ASME Y14.5 true position rule for features of size.

Enter your values in the inputs above and click CALCULATE. Results appear on the right without a page reload. No login and no server upload.

Worked example

Feature offset ΔX=0.02 mm, ΔY=−0.01 mm, tolerance 0.1 mm (2D).

1. Radial error = √(0.02² + 0.01²) = 0.0224 mm
2. True position = 2 × 0.0224 = 0.0447 mm
3. 0.0447 < 0.1 → within tolerance

Result: True position 0.045 mm, passes 0.1 mm zone.

When to use

• CMM or inspection report true position check
• Verifying hole or pin location against ASME Y14.5
• 2D or 3D feature location deviation

Limitations

• Does not apply material condition bonus tolerance
• Assumes deviations are from basic true position
• No datum reference frame shift calculation

FAQ

Why multiply by 2?

ASME Y14.5 true position tolerance is a diameter zone. Measured radial deviation is doubled to compare to the diametral tolerance.

2D vs 3D mode?

Use 2D for flat pattern holes. Enable 3D when Z deviation contributes to location error on angled features.