GD&T True Position Calculator Online

What is GD&T True Position?

True position is a geometric tolerance that defines the allowable variation in the location of a feature relative to its theoretically exact position. Unlike traditional coordinate tolerancing (which uses rectangular tolerance zones), true position creates a circular (diametrical) tolerance zone that more accurately reflects functional requirements. This makes it the preferred method for locating features such as holes, pins, and bosses in precision engineering and manufacturing.

In geometric dimensioning and tolerancing (GD&T), true position is specified using a feature control frame that includes a position symbol, a diametrical tolerance value, and datum references. The true position tolerance zone is a cylinder (for 2D) or a sphere (for 3D) centered at the feature's basic location. The actual feature axis or center must fall within this zone to be considered acceptable. For related GD&T tools, explore our Concentricity Calculator and Runout Calculator.

The True Position Formula

The true position calculation determines the actual positional deviation of a measured feature from its specified true position. The formula for position variation (PV) is:

PV = 2 × √((TB − MB)² + (TC − MC)²)

Where:

TB = True (basic) position from Datum B
MB = Measured position from Datum B
TC = True (basic) position from Datum C
MC = Measured position from Datum C

The result is the diametrical distance between the measured location and the true location. This value is then compared against the specified position tolerance to determine if the feature passes inspection.

Material Condition Modifiers

Material condition modifiers allow additional tolerance when the feature is not at its maximum material condition. Understanding these modifiers is essential for proper GD&T implementation:

RFS (Regardless of Feature Size) — No Bonus: The position tolerance applies regardless of the actual feature size. No bonus tolerance is added. This is the default condition when no modifier is specified.
MMC (Maximum Material Condition) — Bonus on Internal Features: For holes (internal features), MMC occurs at the smallest diameter (most material). If the hole is larger than MMC, bonus tolerance is added equal to the difference between actual size and MMC. For shafts (external features), MMC occurs at the largest diameter.
LMC (Least Material Condition) — Bonus on External Features: For holes, LMC occurs at the largest diameter (least material). For shafts, LMC occurs at the smallest diameter. Bonus tolerance is the absolute difference between LMC size and actual size.

How to Use the True Position Calculator

Our true position calculator simplifies GD&T inspection verification. Follow these steps:

Enter the Position Tolerance: Input the diametrical tolerance from the feature control frame (e.g., ø0.5 mm).
Select Material Condition: Choose RFS, MMC, or LMC based on your drawing specifications.
Define the Feature: Select hole or shaft, then enter the nominal size and upper/lower tolerance limits.
Input Actual Size: Enter the measured size of the feature (used for bonus tolerance calculation).
Enter True and Measured Positions: Provide the basic (true) dimensions from datums B and C, along with the actual measured coordinates.

The calculator instantly computes the position variation, bonus tolerance, total allowable tolerance, and whether the feature passes or fails. A step-by-step breakdown shows each calculation for educational purposes.

Bonus Tolerance Calculation

Bonus tolerance is additional positional tolerance that becomes available when a feature departs from its maximum material condition (MMC) or least material condition (LMC). The calculation depends on feature type and material condition:

Hole at MMC: MMC size = nominal − lower tolerance. Bonus = |MMC size − actual size|. The larger the hole, the more bonus tolerance.
Hole at LMC: LMC size = nominal + upper tolerance. Bonus = |LMC size − actual size|. The smaller the hole, the more bonus tolerance.
Shaft at MMC: MMC size = nominal + upper tolerance. Bonus = |MMC size − actual size|. The smaller the shaft, the more bonus tolerance.
Shaft at LMC: LMC size = nominal − lower tolerance. Bonus = |LMC size − actual size|. The larger the shaft, the more bonus tolerance.

Total tolerance equals the specified position tolerance plus any bonus tolerance. The feature passes if the position variation is less than or equal to the total tolerance.

Frequently Asked Questions

What is the difference between true position and concentricity?

True position controls the location of a feature's center axis relative to datums, while concentricity controls the median points of a feature relative to a datum axis. True position is a location tolerance; concentricity is a runout-related tolerance. True position is more commonly used and easier to inspect than concentricity. For concentricity calculations, use our Concentricity Calculator.

When should I use MMC vs RFS for position tolerance?

Use MMC (Maximum Material Condition) when you want to guarantee assembly fit — features at their worst-case (most material) condition must still assemble. This is the most common choice for positional tolerances on holes and pins. Use RFS (Regardless of Feature Size) when the position tolerance must be maintained regardless of size variation, typically for functional or alignment-critical features. LMC is rarely used but applies when minimum wall thickness or material coverage is critical.

What does the ø symbol mean in the position tolerance frame?

The ø (diameter) symbol before the tolerance value indicates that the tolerance zone is diametrical (circular). This means the feature's actual center can deviate in any direction within a circle of the specified diameter. Without the ø symbol, the tolerance zone would be square, which is less representative of functional requirements. All true position tolerances should use the diametrical symbol.

How do I convert coordinate tolerances to true position?

To convert traditional ±X and ±Y coordinate tolerances to a diametrical true position tolerance, use the formula: Position tolerance = 2 × √(ΔX² + ΔY²), where ΔX and ΔY are the coordinate tolerances. For example, ±0.1 mm in both X and Y gives a position tolerance of 2 × √(0.1² + 0.1²) ≈ 0.283 mm. This produces the same size tolerance zone while allowing more usable tolerance area.

Can true position be applied to patterns of features?

Yes, true position tolerances can be applied to patterns of features such as bolt hole patterns. In these cases, composite position tolerancing is often used, with a lower segment controlling the spacing between features within the pattern and an upper segment controlling the pattern's location relative to datums. Each feature in the pattern must fall within both tolerance zones simultaneously.

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True Position Calculator