GD&T True Position Guide: Tolerances

What Is True Position in GD&T?

True position is a geometric dimensioning and tolerancing (GD&T) control that locates a feature, such as a hole, pin, or slot, relative to a datum reference frame. The symbol is a circle with crosshairs (⌀), and it defines a circular or cylindrical tolerance zone around the exact, theoretically perfect location.

Unlike a rectangle defined by X and Y coordinate limits, a circular tolerance zone is more forgiving: a circular zone of diameter D accepts roughly 57% more area than a square whose edges span D. This extra positioning allowance is the core reason true position became the industry-standard way to locate holes and pins — it gives the machinist more usable tolerance while still guaranteeing fit.

Round zone vs. square zone:: identical dimension limits, but the circular zone covers more valid positions

Linked to datums:: position is defined from a datum reference frame, not from part edges by default

Great for clearance:: holes that must accept a fastener or mating pin benefit directly from the enlarged tolerance zone

Engineers choose true position when parts need to assemble reliably and interchange. Because it reflects how parts actually fit together — rather than how convenient it is to measure on a coordinate grid — true position reduces scrap and rework for mating features.

Anatomy of the Feature Control Frame

True position is always expressed inside a feature control frame — a rectangular box divided into compartments that the machinist reads left to right.

Compartment 1:: the geometric characteristic symbol (⌀ position)

Compartment 2:: the total tolerance value, with a ⌀ prefix if the zone is cylindrical

Compartment 3 (optional):: the material condition modifier (M, L, or S)

Compartments 4-6:: the datum references, read in priority order (primary, secondary, tertiary)

A frame reading `@0.5(M)|A|B|C` means the true position of the feature must fall within a cylindrical zone 0.5 mm in diameter, taken at maximum material condition, referenced to datums A (primary), B (secondary), and C (tertiary). The order of the datums matters: changing which datum is primary shifts both the theoretically exact location and the tolerance zone, so always call out datums in the order that reflects how the part is fixtured and used.

Material Condition Modifiers: MMC, LMC, RFS

Material condition modifiers tell the CMM operator whether the tolerance can grow as the feature departs from its worst-case size.

MMC (Maximum Material Condition, symbol M):: the feature is at its largest hole / smallest pin. The tolerance applies at this worst-fit condition; as the actual size departs from MMC, bonus tolerance becomes available

LMC (Least Material Condition, symbol L):: the feature is at its smallest hole / largest pin. The tolerance applies at this condition

RFS (Regardless of Feature Size, no symbol):: the tolerance applies regardless of the feature's actual size; no bonus tolerance

The most common choice for locating holes is **MMC**. The bonus tolerance equals the difference between the actual size and the MMC limit. For example, a 10 mm hole at MMC with a true position of 0.5 mm can receive up to 0.5 mm of bonus if the hole actually measures 10.5 mm, giving a total positional tolerance of 1.0 mm. This keeps mating fasteners and pins within a guaranteed fit while relaxing the machining requirement where it is safe.

Datums and the Datum Reference Frame

A datum is a theoretically exact plane, axis, or point used as the origin of measurement. Datums are established by contacting the physical datum features — faces, bores, or surfaces — with standard fixtures or the CMM probe. Together they form the datum reference frame, the coordinate system from which true position is measured.

For machined parts, the usual order mirrors how the part faces in production and in service: a primary planar datum establishes the first three degrees of freedom, a secondary datum the next two, and a tertiary datum the last one. Establishing the datum reference frame correctly on the CMM is critical because datum errors propagate to every feature measured from them. A CMM must first construct and verify the datum frame before evaluating true position on individual features.

Measuring and Inspecting True Position

True position can be verified three ways, depending on volume, feature size, and how strict the requirement is:

Functional (hard) gauging:: a GO/NO-GO pin or fixture checks assembly fit directly and fast — ideal for high-volume production of holes that receive a fastener

CMM (coordinate measuring machine):: the standard for true position, because a CMM can construct the datum reference frame, evaluate circular zones, and apply MMC/LMC bonus automatically. GD&T position requirements almost always require CMM measurement for full verification

Optical / vision inspection:: useful for thin parts and features a probe cannot reach, but less precise for deep or angled holes

Because the aim of true position is guaranteed assembly, functional gauging and CMM together give the strongest confidence: the gauge confirms fit, the CMM quantifies deviation and certifies the datum frame.

True Position vs. Coordinate (X/Y) Tolerancing

The classic alternative to true position is locating a hole with direct X and Y dimensions and individual ± tolerances. That approach creates a square tolerance zone and forces the designer to sum tolerances across the diagonal, which is neither the way parts fit nor how they should be inspected.

Square vs. circular zone:: ±X/±Y forms a square; the same dimensions expressed as true position form a more permissive circle

Bonus tolerance:: coordinate tolerancing has no MMC bonus; true position can grow at MMC

Tolerance accumulation:: coordinate tolerances stack feature to feature; true position references a single datum frame, limiting accumulation

Interchangeability:: true position guarantees that any part within tolerance will assemble, which is exactly what interchangeable manufacturing requires

For parts with multiple holes that must line up — such as mounting plates, housings, and connector brackets — true position consistently reduces the effective tolerance burden while guaranteeing fit.

True Position Symbol Reference and Free Resources

Beyond true position, the datum reference frame and modifier symbols above are part of the wider ASME Y14.5 and ISO GPS standards. For a compact engineering reference covering position, flatness, perpendicularity, runout, and the full GD&T symbol set, see our [machining standards resource](/en/resources/machining-docs).

Correctly understood tolerances are only half the equation — the other half is inspecting them. Our [inspection services](/en/inspection) use CMM and first-article inspection to verify true position, profile, and datum requirements on every production run, so your parts meet the drawing on the first batch.

True Position FAQ

What is the difference between true position, concentricity, and profile?

True position locates a feature's axis or center relative to datums. Concentricity measures how well the centers of two features coincide. Profile controls the entire surface form. Position is the most commonly used and the most practical of the three.

Does adding MMC make the tolerance tighter?

No. MMC gives the tolerance room to grow as the feature departs from worst-case size, so it generally relaxes the manufacturing requirement while still protecting assembly fit.

Does GD&T true position require a CMM?

For full verification, yes. Functional gauges confirm fit quickly, but a CMM is required to construct the datum frame and apply MMC/LMC bonus correctly. For critical features, pair a GO gauge with CMM measurement.

Conclusion

True position is the single most useful GD&T control for guaranteeing that holes, pins, and slots assemble and interchange. By replacing square coordinate zones with a circular zone, linking the callout to a proper datum reference frame, and using MMC bonus, you gain more manufacturing tolerance while tightening real-world fit.

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