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Offset tolerance zone UZ: where the zone goes

“2.5 UZ−0.5” means: the tolerance zone is 2.5 wide and sits 0.5 inside the material. What the sign means, how ISO 1101 constructs the zone, and why manufacturing and inspection need to know the offset.

ISO GPS

Christian Schärer · 2 September 2026

A profile tolerance without a modifier places the zone symmetrically about the theoretically exact feature, the TEF: half the tolerance outward, half into the material. For many functions that is the wrong split. A sealing face may sit low but must not stand proud. A casting needs machining allowance. In the past, designers shifted the nominal contour in the CAD model or described the tolerance with separate plus and minus portions. ISO 1101 resolves this directly in the tolerance indicator with the modifier UZ.

UZ stands for “unequally disposed zone”. ISO 1101 calls the indication the specified tolerance zone offset: the value after UZ carries a sign and shifts the entire zone relative to the TEF. The width of the zone stays what the tolerance says. What changes is its position.

Key point UZ moves the zone. Its width stays. A negative value pushes the zone into the material, a positive value out of it. The sign follows the material. Which side is up in the drawing view plays no part.

How to read 2.5 UZ−0.5

Tolerance t 2.5 Width of the zone. It stays the same whatever the offset.

Offset UZ −0.5 The reference feature lies 0.5 from the TEF, inside the material. The zone is centred on this line.

Zone boundaries +0.75 / −1.75 The zone extends 0.75 out of the material and 1.75 into it. A contour on nominal is therefore not in the middle of the zone.

ISO 1101 constructs the zone in two steps. First the reference feature is derived from the TEF: spheres with a diameter equal to the UZ value roll along the TEF, on the material side for a negative value and on the outside for a positive one. Their envelope is the offset feature. Then spheres with a diameter equal to the tolerance roll with their centres on this reference feature. Their envelopes form the two boundaries of the zone.

The spheres ensure that on curved contours the offset is always measured normal to the surface. At a convex point the outer boundary runs wider and the inner one tighter; at a corner a radius appears. A CAD offset does essentially the same: first offset the surface by UZ, then take half the tolerance either side.

Try it yourself

The sweep from UZ− to UZ+ starts by itself as soon as the graphic comes into view. Tolerance and offset can be adjusted by hand at any time. The graphic shows the TEF with the material below it, the reference feature as a dash-dot line and the zone as a blue band. The two spheres are the ones from the construction above.

Sign convention and special cases

The sign follows the material. A minus pushes the zone in, a plus pushes it out. On an external contour UZ+ points away from the part; in a bore it points toward the bore axis. Anyone who ties the sign to the drawing view instead of the material turns the zone around.

UZ = 0 Symmetrical Equivalent to a profile tolerance without UZ. An offset of zero is not written.

UZ = −t/2 Entirely in the material The outer boundary coincides with the TEF. The actual contour may only sit low, the case for machined and sealing faces. In the example: 2.5 UZ−1.25.

UZ = +t/2 Entirely outside The inner boundary lies on the TEF. The contour may only stand proud, for instance as allowance for later machining.

|UZ| > t/2 Zone clear of the TEF Formally permitted; the TEF then lies outside the permissible contour. In most such cases the nominal contour itself is in the wrong place.

OZ, short for offset zone, the unspecified offset. With OZ the zone may sit at any offset, constant over the whole feature. This checks the form of the contour; how far it lies from the TEF is left open. UZ fixes the amount and direction of the offset, OZ leaves both free.

Anyone working with drawings to ASME Y14.5 knows the principle from the unequally disposed profile modifier, the circled U. The notation is different there: after the symbol comes the portion of the tolerance that lies outside the material, without a sign. When transferring between the two systems, convert on paper first, before the value lands in the measuring program.

What this means for manufacturing and inspection

Manufacturing. The middle of the zone is no longer the nominal contour. Machining to nominal with 2.5 UZ−0.5 leaves 0.75 of room outward and 1.75 inward. The target contour belongs 0.5 into the material; then the full tolerance is available.

Inspection. The measuring software has to know the offset. A program that places the zone symmetrically about the CAD contour evaluates wrongly: it rejects parts that lie inside the shifted zone and accepts parts that already stand too far out. In the evaluation UZ appears as an offset of the nominal contour or as its own entry on the profile tolerance. If the software does not know the modifier, the zone is shifted by hand and the step documented in the report. In CMM programming we check this match before the first part is measured.

Drawing. UZ pays off wherever the function is one-sided. In a drawing review we check that the material side follows unambiguously from the drawing and that offset and tolerance fit together. An offset larger than half the tolerance points to a misplaced nominal contour.

In the inspection report

The conformity decision is made against the shifted zone. The report states which offset was applied so that the decision stays traceable. That holds for every dimensional inspection with profile tolerances.

In practice

Tie the sign to the material: minus into the material, plus out of it. The drawing view says nothing about it.

Work out the zone boundaries and carry them into the manufacturing instruction: outside UZ + t/2, inside UZ − t/2. The target contour lies at UZ.

Check the evaluated boundaries in the measuring program before the first part is measured. A glance at the two limit values is enough.

When switching between ISO and ASME drawings, convert the indication and cross-check it with a sample value.

UZ is one number in the tolerance indicator that shifts nominal contour, target contour and inspection zone against each other. Read it the way the standard means it, and all three line up again.

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