True Position Calculator (with MMC Bonus)
Enter nominal and measured coordinates and get the true position — plus the part most people trip on: bonus tolerance at MMC, calculated from the actual hole size and added to your budget automatically. Drawn on the tolerance circle so you can see how much margin is left. Reverse mode gives the per-axis budget for a stated tolerance.
Measured X minus nominal X. Negative is fine.
The Ø value in the feature control frame.
For a hole, MMC = its smallest permitted diameter.
True position and bonus tolerance, without the mystery
True position measures how far a feature’s axis is from where the drawing says it should be — as a diameter, which is why the formula doubles the radial miss: TP = 2 × √(ΔX² + ΔY²). A hole that is 0.05 mm off in X and 0.03 mm off in Y is 0.058 mm from nominal radially, which is a true position of 0.117 — inside a Ø0.2 zone with room to spare. The doubling catches people out constantly: your coordinate deviations can each be well under the tolerance number and the feature can still fail.
The Ⓜ symbol after the tolerance is where the free lunch lives. It means the stated tolerance applies when the hole is at maximum material condition — its smallest size. Every micron the actual hole is bigger than MMC gets added to the position budget as bonus tolerance, because a bigger hole will still assemble over a slightly misplaced pin. A Ø0.2 Ⓜ tolerance on a hole drilled 0.05 over its MMC size is really a Ø0.25 budget. On fastener clearance patterns this routinely rescues parts that a bare coordinate check would scrap.
That is also why it matters commercially: parts get rejected on position by people checking X and Y against a rectangle in their head, when the standard defines a circle — and often a growing one. The circular zone alone is 57% more area than the inscribed square, before any bonus. If a supplier or customer is scrapping parts on position, the first two questions are always “was it evaluated as a diameter?” and “was the bonus applied?”.
The reverse mode answers the planning question: with a Ø0.2 zone, how much can each axis drift? If the error splits evenly, each axis gets tolerance ÷ (2√2) — 0.071 mm here. That is the number to compare against your process capability before committing, and it is exactly the kind of thing we check in a free DFM review.
True position — FAQ
How do you calculate true position?
TP = 2 x the square root of (X deviation squared + Y deviation squared). It is expressed as a diameter, so the radial distance from nominal is doubled. A hole 0.05 mm off in X and 0.03 mm off in Y has a true position of 0.117 mm.
What is bonus tolerance?
When the position tolerance carries the MMC modifier (the circled M), the stated tolerance applies at the feature’s maximum material condition. For a hole, every unit the actual size is larger than its minimum permitted size is added to the position budget. Actual size minus MMC size = bonus.
Why is true position measured as a diameter?
Because the tolerance zone is a cylinder around the true (nominal) axis. Stating it as a diameter makes the zone size independent of direction — a miss of r in any direction uses 2r of the zone.
How much can each axis be off for a given true position tolerance?
If the error is split evenly between X and Y, each axis can deviate by the tolerance divided by 2 x sqrt(2) — about 35% of the tolerance value. A 0.2 mm zone allows roughly plus or minus 0.071 mm per axis.
Does datum shift add more tolerance?
It can — if a datum feature of size is referenced at MMC, movement of the datum itself can allow additional shift. That is separate from bonus tolerance and is not included in this calculator.
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Related: Limits & fits · Tolerance stack-up · Inspection · All tools