True Position Calculator
Diametric position tolerance with bonus tolerance from MMC or LMC, per ASME Y14.5. Done entirely in your browser.
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True position calculator
Diametric position tolerance with bonus tolerance from a material condition modifier.
Modifier
Stated position tol. (mm)
MMC size (mm)
Actual measured size (mm)
Measured X deviation (mm)
Measured Y deviation (mm)
Bonus tolerance
0.150 mm
Total position tol. (⌀)
⌀0.350 mm
Position error (⌀) vs tol.
⌀0.200 mm · PASS
Diametric zone per ASME Y14.5: position error = 2×√(X²+Y²). Done client-side, nothing sent anywhere. For datum reference frame validation and full drawing review, run it in the full workspace.
Full drawing review, freeNo confusion, no setup
Exactly what happens, in order
Enter the stated tolerance and modifier
Position tolerance from the drawing, plus MMC, LMC, or RFS. MMC and LMC unlock bonus tolerance from the produced size.
Enter measured deviations
X and Y deviation from true position, from your CMM report or inspection data.
Get position error and PASS/FAIL
Diametric position error (2×√(X²+Y²)) compared against total tolerance (stated + bonus), per ASME Y14.5.
Need the full drawing reviewed, not just one callout?
Upload a drawing or CAD file and the GD&T drawing checker reviews datum structure, tolerances, and callouts against the standard, not just one position calculation.
Try the GD&T drawing checkerFrequently asked questions
How do you calculate true position?
Position error is the diametric distance from true position: 2 × √(Xdev² + Ydev²), where Xdev and Ydev are the measured deviations from the theoretical exact position. That error is compared against the total position tolerance (stated tolerance plus any bonus tolerance earned from a material condition modifier).
What is bonus tolerance in GD&T?
When a feature of size callout uses MMC (Maximum Material Condition, Ⓜ) or LMC (Least Material Condition, Ⓛ), the actual produced size can differ from the limit size in a way that loosens the position tolerance. That extra allowance is bonus tolerance, calculated as the difference between the limit size and the actual measured size. RFS (Regardless of Feature Size) gets no bonus.
Why is position error calculated as diametric, not radial?
ASME Y14.5 defines position tolerance as a cylindrical (diametric) zone, not a radial one. The formula 2×√(X²+Y²) accounts for this, a common manual-calculation mistake is forgetting the factor of 2 and reporting a radial value instead.
Does this handle datum reference frames?
No, this calculator assumes your X/Y deviations are already measured relative to the correct datum reference frame from the drawing. It doesn't validate datum setup, feature-to-datum relationships, or bonus tolerance on the datums themselves. For a full drawing review including datum structure, use the GD&T drawing checker.
Does ForgePilot store what I enter here?
No. This calculator runs entirely in your browser, nothing you enter is sent to ForgePilot's servers.