GD&T True Position Calculator
Instantly check position tolerance, bonus tolerance, datum shift allowance, and pass/fail
(MMC, LMC, RFS, Datum Shift)
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True Position Datum Shift Calculator (MMC, LMC, RFS)
This GD&T True Position Datum Shift Calculator helps calculate true position tolerance, feature bonus tolerance, datum shift allowance, and pass/fail results based on ASME Y14.5 GD&T standards.
The calculator supports MMC (Maximum Material Condition), LMC (Least Material Condition), and RFS (Regardless of Feature Size) for both controlled features and datum features.
True position controls how far a feature such as a hole or pin can deviate from its exact theoretical location. Instead of separate X and Y tolerances, GD&T combines both directions into a single diametrical tolerance zone.
When MMC or LMC modifiers are applied, additional positional tolerance may be gained as the feature departs from its material condition boundary. Datum features referenced at MMC or LMC may also create additional datum shift allowance during assembly and inspection.
What This Datum Shift Calculator Does
This calculator automatically calculates:
• Radial deviation from nominal position
• True position (diametrical value)
• Feature bonus tolerance
• Datum shift allowance
• Total allowed tolerance
• Pass or fail result
The tool is designed for machinists, inspectors, CNC programmers, manufacturing engineers, and quality engineers requiring fast and accurate GD&T calculations.
How To Use This Tool
Enter the nominal/basic X and Y location
Enter the measured/actual X and Y location
Enter the base position tolerance
Select hole or pin feature type
Select MMC, LMC, or RFS modifier
Enter feature sizes if applicable
Enable datum shift if required
Enter datum feature sizes
Click Calculate
The calculator instantly determines true position, bonus tolerance, datum shift allowance, total tolerance, and pass/fail status.
Understanding MMC, LMC, and RFS
MMC — Maximum Material Condition
MMC allows bonus tolerance as the feature departs from its maximum material condition.
For holes, bonus tolerance increases as the hole becomes larger than MMC.
For pins, bonus tolerance increases as the pin becomes smaller than MMC.
LMC — Least Material Condition
LMC protects minimum wall thickness while still allowing positional variation.
RFS — Regardless of Feature Size
RFS means no bonus tolerance is applied regardless of actual feature size.
Datum Shift Allowance
Datum shift occurs when datum features are referenced at MMC or LMC and the actual datum feature departs from its material boundary size.
This additional freedom allows more assembly variation while still maintaining proper functional fit.
Datum shift is commonly used in aerospace, defense, automotive, and precision machined assemblies where mating components require additional assembly clearance.
True Position Formula
Radial Deviation = √[(X actual − X nominal)² + (Y actual − Y nominal)²]
True Position = 2 × Radial Deviation
Feature Bonus Tolerance = Actual Size − MMC Size
Total Allowed Tolerance = Base Position + Feature Bonus + Datum Shift
Example (MMC With Datum Shift)
Basic Position: (2.000, 1.500)
Actual Position: (2.018, 1.514)
Position Tolerance: Ø0.010
Hole Size at MMC: Ø0.500
Actual Hole Size: Ø0.506
Datum Pin at MMC: Ø0.750
Actual Datum Pin Size: Ø0.745
Result:
True Position = Ø0.0456
Feature Bonus = Ø0.0060
Datum Shift Allowance = Ø0.0050
Total Allowed = Ø0.0210
Status = FAIL
Common Applications
• Aerospace hole patterns
• Dowel pin alignment
• Bolt circle inspection
• Functional gaging
• CNC machined assemblies
• Precision machined components
• GD&T inspection reports
• Assembly fit verification
Frequently Asked Questions
Why is true position shown as a diameter?
True position uses a cylindrical tolerance zone instead of separate X and Y tolerances.
What is bonus tolerance?
Bonus tolerance is additional positional tolerance gained as a feature departs from MMC or LMC.
What causes datum shift?
Datum shift occurs when datum features referenced at MMC or LMC vary from their material boundary size.
Does RFS allow bonus tolerance?
No. RFS maintains a constant tolerance regardless of actual feature size.
Related Machining Calculators
Use these related machining calculators to improve accuracy and save time:
True Position Pro (MMC)
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