True position is one of the most common Geometric Dimensioning and Tolerancing (GD&T) controls on a mechanical drawing — it defines how far a hole, pin, or other feature of size can drift from its theoretical exact location before a part is rejected. This calculator turns a measured X/Y deviation into the true position value and a pass/fail verdict, and shows how MMC bonus tolerance can widen that zone.

How true position is calculated

True position is computed from the measured deviation of a feature's actual center from its theoretical exact ("basic") location, expressed as separate X and Y offsets. The formula TP = 2 × √(Δx² + Δy²) converts that radial deviation into a diameter, because ASME Y14.5 and ISO 1101 both specify positional tolerances as a circular (diametral) zone around the true position, not a linear distance or a radius.

In practice, Δx and Δy are read directly off a coordinate measuring machine (CMM) report, or measured with gauge pins and a height gauge against the part's datum reference frame. Whatever the measurement method, both deviations and the specified tolerance must be expressed in the same linear unit — inches or millimeters — for the result to be meaningful.

Reading pass/fail and the tolerance zone

A feature passes when its calculated true position is less than or equal to the specified positional tolerance from the feature control frame (e.g. ⌀0.010). The margin — specified tolerance minus true position — tells you how much room is left before the part would be rejected; a margin near zero is a signal to tighten process control even if the part currently passes.

Because the tolerance is diametral, a small increase in either Δx or Δy can use up margin faster than intuition suggests — doubling both deviations quadruples neither the position error nor the margin lost in a simple linear way, since the true position itself scales linearly with the combined (root-sum-square) deviation, then gets doubled again for the diametral conversion.

Bonus tolerance at MMC

When a feature control frame includes the Maximum Material Condition modifier (Ⓜ) after the positional tolerance value, the specified tolerance is no longer fixed — it grows as the feature's actual size departs from its MMC size. For a hole, MMC is the smallest permitted diameter, so any hole produced larger than that earns "bonus" positional tolerance equal to the difference (actual diameter − MMC diameter).

This bonus tolerance is added directly to the base specified tolerance to get the total allowable positional tolerance for that specific part. It is one of the most powerful cost-saving tools in GD&T: a part whose position measurement fails the base tolerance can still be accepted if it was produced away from MMC and the resulting bonus is large enough to cover the shortfall — no rework needed, provided the feature control frame actually carries the Ⓜ modifier.