How is true position calculated from X and Y deviation?

True position in diametral form is 2 x sqrt(dx^2 + dy^2), where dx and dy are the measured deviations from the basic dimensions. The factor of two converts the radial deviation into the diameter of the position zone that the feature centre must sit inside.

What is the difference between diametral and radial true position?

Diametral true position is the diameter of the tolerance zone, which is how position is normally specified on a drawing with the diameter symbol. Radial true position is the deviation from the true centre, equal to sqrt(dx^2 + dy^2). The diametral value is always twice the radial value.

What is worst-case versus RSS tolerance stack-up?

Worst-case assumes every dimension sits at its extreme at the same time, so the total is the sum of the individual tolerances. RSS (root-sum-square) assumes the deviations are random and independent, so the total is the square root of the sum of the squares. Worst-case is conservative; RSS is closer to real production output.

Does this calculator replace a CMM report or a drawing callout?

No. It is a check tool for engineers sizing tolerance zones and stack-ups. Acceptance decisions must use the drawing, the applicable standard and a calibrated inspection report.

GD&T True Position & Tolerance Stack-Up Calculator

Enter the basic dimension and the measured X/Y deviation to get true position in both diametral and radial form. Then run the same dimensions through worst-case and RSS stack-up. Everything computes in your browser.

Position tolerance check

X deviation

Y deviation

Stated position tolerance (diametral)

Actual feature size

MMC size of the feature

Feature type (for MMC bonus)

Calculated locally in your browser. Check values only — acceptance requires the drawing and a calibrated inspection report.

Result

True position (diametral)— True position (radial)— MMC bonus— Total allowed— Verdict—

TP(dia) = 2 × √(dx² + dy²). MMC bonus for a hole = actual size − MMC; for a shaft = MMC − actual size. Total allowed = stated position tolerance + bonus.

Copy-ready text

Stack-up

Tolerance stack-up — worst-case and RSS

Enter one nominal and one plus/minus tolerance per line. Both assemblies use the same numbers so the two methods are directly comparable.

One dimension per line as nominal +/- tolerance. Lines that cannot be read are ignored and counted in the note below.

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Basis & references: ASME Y14.5 — Dimensioning and Tolerancing · ISO 1101 — Geometrical product specifications (GPS) · NIST — Dimensional metrology

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