Engineering · Calculator

True position calculator.

Check planar position for hole patterns from basic and measured X/Y, MMC hole size, and actual size. Results cover position only, not full drawing acceptance: verify size limits, datum setup, axis orientation, and any composite requirements separately.

Pattern parameters

Hole-size limits and tolerance.

MMC hole size (in)Position tol at MMC (in)

Upper hole-size limit (in, optional)Apply MMC modifier (Ⓜ) — bonus tolerance from feature-size departure

Features

Add holes in the pattern.

Up to 8 features. Basic X/Y is the CAD-defined location; measured X/Y is the CMM result; actual size is the inspected hole diameter.

Pattern result

Pattern status

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All features must pass for the pattern to pass

Worst feature

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Tightest margin

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Actual position vs total allowable

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Actual position = 2 × √((Δx)² + (Δy)²). With MMC modifier active, total allowable = position tolerance + (actual size − MMC size). RFS (no modifier) means actual size doesn't add bonus.

Visual CMM Target

Active: Hole #1

Position Deviation Map

Theoretical CAD basic is at center (0,0). Solid ring is original tolerance zone. Dotted ring is expanded MMC zone. Click and drag the probe dot to visually inspect deviations.

Grid radial spacing: 0.0010"X: +0.0000 | Y: +0.0000

Per-feature breakdown

Click row to select active feature

#	Δx (in)	Δy (in)	Actual position	Bonus	Total allowable	Margin	Result
Add features to see results.

Understand it

What “true position” actually means.

A position callout doesn’t give the hole a little box to live in — it gives it a round zone centered on the theoretically exact (basic) location. Here’s the picture, the formula, and the conversion machinists actually use.

True position practical guide: an illustrated machined plate with a four-hole pattern and a caliper, an enlarged hole showing the round crimson tolerance zone centered on the true (basic) position with the actual hole center offset inside it, a feature control frame, and a spec panel listing tolerance zone diameter T, cylindrical zone shape, and datum reference A B C. — True position controls a feature’s *location* against datums A | B | C — the hole’s axis must lie inside a cylindrical zone standing on the basic (CAD-exact) location.↓ Download all 3 on one sheet

Round versus square: on a machined surface, a crimson round position tolerance zone circumscribes a smaller blue square plus-minus coordinate zone, touching its four corners; the extra crescent areas show the round zone gives more usable area than plus-minus. — Round beats ±: the position circle circumscribes the ± square, so it holds ~57% more area for the same worst-case corner.

MMC bonus: three holes increasing in diameter left to right, each with a wider crimson tolerance zone, and one fixed-diameter gauge pin clearing all three — a looser hole earns more position tolerance. — MMC bonus: as the hole opens up past MMC, the zone grows — a looser hole still clears the same fastener.

Reference

Coordinate ↔ true position conversion

For equal ± X/Y tolerances, this circle encloses the square (diameter = 2√2 × coordinate tolerance). It also accepts points outside the square: these are not interchangeable drawing requirements.

± Coordinate tol	Enclosing circle Ø	Coord zone (□)
± 0.0010″	Ø 0.0028″	0.0020″ sq
± 0.0020″	Ø 0.0057″	0.0040″ sq
± 0.0025″	Ø 0.0071″	0.0050″ sq
± 0.0050″	Ø 0.0141″	0.0100″ sq
± 0.0075″	Ø 0.0212″	0.0150″ sq
± 0.0100″	Ø 0.0283″	0.0200″ sq
± 0.0150″	Ø 0.0424″	0.0300″ sq
± 0.0200″	Ø 0.0566″	0.0400″ sq

Rules of thumb

A round position zone gives 57% more tolerance area than the ± square that fits inside it — same worst-case corner, more room everywhere else.
MMC bonus = actual size − MMC size. A looser hole earns extra position tolerance because it still clears the fastener.
Clearance-hole rule: position Ø ≈ hole Ø − fastener Ø (floating fastener — each part gets the full amount; fixed fasteners get (hole − fastener)/2 per part).

How it works

Position math, in plain English.

Actual position is a diameter, not a radius.

If your hole is 0.0015″ off in X and 0.002″ off in Y, the radial deviation is √(0.0015² + 0.002²) = 0.0025″. Position is 2× that = 0.0050″. The tolerance zone is a cylinder, so you compare against diameter.

MMC bonus = (actual size − MMC size).

If MMC is Ø 0.250″ and the hole came in at Ø 0.253″, you have 0.003″ of bonus. Total allowable position = original tolerance + bonus. The functional rationale: a bigger clearance hole tolerates more positional error.

RFS (no modifier) ignores actual size.

Default ASME behavior with no Ⓜ modifier: position tolerance is the position tolerance, regardless of how the size came out. Strictest.

Bonus only goes one way.

With MMC active, a hole below the entered MMC size is flagged BELOW MMC and cannot pass this check. Bonus is clamped at zero. An optional upper size limit can also flag oversize holes. Without it, upper size is not checked.

For the full GD&T background, see position (true position) reference.

Keep exploring

Related tools & references

Position tolerance (GD&T)→

The characteristic this calculator evaluates.

GD&T symbols reference→

MMC, datums, and feature control frame basics.

Tolerance stack-up calculator→

Combine position with the rest of the dimensional stack.

Tolerances reference→

Size limits that set MMC and bonus tolerance.

Standard fits reference→

Hole-and-pin fits behind positional callouts.

Need true position held on a hole pattern?

Tell us about your part and we'll get back to you promptly. Aerospace, medical, defense, and semiconductor production work welcome.Email CAD models and drawings to sales@digitalmachine.com, including ITAR, EAR, CUI and AS9100 work. Keep controlled technical data out of web forms.

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