True Position Calculator

What True Position Tells You

True position is the GD&T callout that controls where a feature is located relative to its nominal coordinates. Unlike basic +/- tolerancing that creates a square tolerance zone, true position defines a cylindrical zone — meaning the feature has the same allowable deviation in every direction. This is why a hole can be 0.005" off in X and 0.005" off in Y and still pass: the diagonal distance is what matters, not each axis independently.

• Diametral tolerance zone: The tolerance on the print is a diameter, not a radius. A 0.010 position tolerance means the feature center must fall within a circle 0.010" across — that's only 0.005" from nominal in any direction.
• MMC bonus: When the callout includes a circled M (MMC modifier), you get bonus tolerance as the feature departs from its maximum material condition. A hole at MMC gets zero bonus; as it opens up, you get extra position tolerance equal to the departure.
• Datum reference matters: True position is always measured from datums. If your datums aren't set up correctly on the CMM, your position results are meaningless — garbage in, garbage out.

Practical Tips

• When checking position on a CMM, make sure the datum reference frame matches the print exactly. Wrong datum order = wrong results, even if the part is perfect.
• If a hole is right on the edge of failing position, check if the callout has an MMC modifier. The bonus tolerance from an oversized hole might save it.
• For bolt hole patterns, tight position tolerance matters more than hole size. A pattern that's shifted 0.005" means every bolt fights the misalignment.
• Use this calculator to quickly check CMM results before rejecting parts. Sometimes a manual calculation catches a CMM setup error.
• Position tolerance of 0.014" or tighter on a milled hole usually means you need to ream or bore — a drill alone won't hold it reliably.

Related Tools