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True Position Calculator

True position tolerancing locates feature axes relative to datum references in geometric dimensioning and tolerancing (GD&T). Manufacturing parts can never achieve exact positions, so engineers specify tolerance zones to define acceptable deviations. This calculator compares actual measured positions against allowable limits, accounting for material conditions and bonus tolerances that increase acceptance windows when features deviate from their size boundaries.

Last updated:

Creators Tom Wright, MSc Construction Management
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Understanding True Position in GD&T

True position defines the ideal theoretical location of a feature axis—the bore's or shaft's center—as specified in engineering drawings using basic dimensions from secondary and tertiary datums. Because perfect positioning is impossible in production, tolerance zones create three-dimensional cylinders or spheres around these ideal locations.

The actual feature's axis can drift within this zone and still pass inspection. When measuring, you record offsets in two directions (typically from datums B and C). The radial distance from true to measured position determines whether the part accepts or rejects.

Position tolerancing proves more efficient than coordinate tolerancing for multi-hole patterns. Instead of independent tolerances on X and Y dimensions, a single position tolerance creates a larger acceptance zone, letting manufacturers produce more conforming parts without tightening other specifications.

Position Variation Calculation

True position deviation is the radial distance from the theoretical axis to the actual measured location, doubled because the tolerance is specified as a diameter. Calculate offsets in each datum direction first, then apply the distance formula.

Offset (B) = Measured (B) − True (B)

Offset (C) = Measured (C) − True (C)

Position Variation = 2 × √[Offset (B)² + Offset (C)²]

  • Offset (B) — Displacement from datum B reference in X-direction
  • Offset (C) — Displacement from datum C reference in Y-direction
  • Position Variation — Diametrical tolerance zone requirement (compared against total allowable tolerance)

Material Conditions and Bonus Tolerance

Maximum Material Condition (MMC) occurs when an internal feature (hole) is smallest or an external feature (shaft) is largest—the part contains the most material. Least Material Condition (LMC) is the opposite: largest hole or smallest shaft.

When a feature deviates from its MMC size, bonus tolerance becomes available. For holes under MMC, if the measured diameter exceeds the MMC size, the position tolerance relaxes by that difference. For shafts under MMC, if the measured diameter falls below the MMC size, similar relaxation applies.

Bonus Tolerance Formula (MMC for holes): Actual Diameter − MMC Diameter

This bonus combines with the stated position tolerance, creating a total tolerance window. Rigid Perpendicularity (RFS—regardless of feature size) ignores bonus tolerance entirely; the stated position tolerance applies without modification.

Selecting Datums for Position Control

Datum selection anchors the tolerance coordinate system and determines measurement reference planes. For holes, the mounting or structural surface (the face into which the hole drills) typically serves as the primary datum. This orientation ensures holes remain perpendicular or at the specified angle to the mounting face.

Long holes may require different datum selection. If functional performance demands parallelism to a side rather than perpendicularity to a face, designate that side as primary. Secondary and tertiary datums (B and C) locate holes within the feature plane, establishing an X-Y grid for position measurements.

Consistent datum interpretation across design, manufacturing, and inspection teams is critical. Ambiguous datum schemes cause disputes about conformance; clear documentation prevents coordinate system mismatches.

Common True Position Pitfalls

Avoid these mistakes when applying and measuring true position tolerance.

  1. Confusing RFS with MMC — Rigid Perpendicularity allows no bonus tolerance; the stated tolerance is absolute. Material conditions (MMC/LMC) unlock bonus tolerance windows. Verify the drawing symbol—no material condition symbol means RFS applies, and bonus tolerance is zero.
  2. Measuring from wrong datums — Position requires careful datum plane identification. Holes must be measured from the secondary and tertiary datums shown in the feature control frame. Using wrong reference surfaces invalidates the inspection result even if math is correct.
  3. Ignoring feature size effects — Under MMC, larger features gain tolerance cushion. A hole 0.010 inches oversized gains 0.010 inches of bonus tolerance. Conversely, smaller holes lose that window. Always measure actual feature size before calculating bonus tolerance.
  4. Forgetting the factor of two — Position variation is diametrical—the radial distance between axes must be doubled before comparison. Many reject parts unnecessarily by comparing radius instead of diameter to the stated tolerance.

Frequently Asked Questions

What is the difference between position tolerance and coordinate tolerance?

Position tolerance establishes a cylindrical or spherical acceptance zone centered on the true position, typically allowing a larger combined tolerance window than coordinate tolerances. Coordinate tolerance requires separate X and Y dimension limits, each creating a square zone. Position tolerancing is more generous: a 0.100-inch position tolerance creates a 0.100-inch diameter zone, whereas coordinate tolerances of 0.050 X and 0.050 Y create a 0.100 × 0.100 square, rejecting features near the square's corners.

How do I find bonus tolerance when the hole is larger than its MMC size?

Measure the actual hole diameter on the part and subtract the MMC size specified on the drawing. For example, if the drawing shows a 0.500-inch MMC hole and you measure 0.508 inches, the bonus tolerance is 0.508 − 0.500 = 0.008 inches. Add this bonus to the stated position tolerance to find the total allowable tolerance for pass/fail judgment.

Can position tolerance apply to external features like shafts?

Yes. For shafts under MMC, the maximum material condition is the largest possible shaft diameter. If the actual shaft measures smaller than its MMC size, bonus tolerance is available. The bonus formula reverses: MMC diameter − actual diameter. Unlike holes, where oversizing grants tolerance, shafts tighten positionally at maximum size and relax as they shrink.

Why does the position formula multiply by two?

Position tolerance is expressed as a diameter specifying the full width of the cylindrical acceptance zone around the true position. The radial distance—the shortest straight-line distance from true to measured axis—equals half the diameter. Multiplying by two converts the radial distance to a diametrical value comparable to the stated tolerance limit.

What happens if I measure a feature at LMC instead of MMC?

LMC (least material condition) grants bonus tolerance when the feature is undersized relative to LMC. Holes at LMC are largest; shafts are smallest. The tolerance zone expands proportionally. Many designs use MMC for assembly-critical fits because it's stricter when parts approach functional limits, but LMC can apply if loose fits are acceptable for position.

How do I determine which datum is primary, secondary, and tertiary?

The feature control frame orders datums left to right. The leftmost datum is primary—it constrains most degrees of freedom and is typically the largest or most stable surface. Secondary and tertiary datums refine orientation and location. For hole position, the mounting face (primary) ensures perpendicularity; hole-plane references (secondary and tertiary) establish an X-Y coordinate grid for position measurement.

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