Tolerances for visible metalwork

At a glance

For non-structural interior metalwork. Status was read on 5 and 6 November 2026 from standards listings, not from the standards, except ISO 2768-1 (public preview) and the foreword of ISO 22081. Confirm each against the current edition.

One general tolerance, and which standard to name

A general tolerance is the default for every dimension that carries no tolerance of its own. EN ISO 13920:2023 says that manufacturing documents giving dimensions without individual tolerances are deemed incomplete if there is no, or an inadequate, reference to general tolerances (read from its listing; the standard itself was not read). That is the practical reason to name one, once, in the title block.

• Welded assemblies: EN ISO 13920:2023, published in November 2023 as a CEN edition of ISO 13920:2023. Its listing says it gives general tolerances for lengths and angles, and for shape and position, in four classes “based on customary workshop accuracy”, and that the class is chosen mainly by function. We did not read its class values, so none appear here.
• Machined parts, and parts formed from sheet, with no welding: ISO 2768-1:1989, which BSI publishes as BS EN 22768-1:1993. Four classes: f (fine), m (medium), c (coarse) and v (very coarse). Its own clause 5 gives “ISO 2768-m” as the example of what to write. ISO now lists it as “to be revised”, and a replacement ISO 2768 is shown as in development.
• Flatness and form on those parts: this was ISO 2768-2:1989, the Part 2 that old title blocks cite with a letter such as K. The foreword of ISO 22081:2021 says that edition cancels and replaces ISO 2768-2:1989. BSI lists BS EN ISO 22081:2021 as current. A drawing that still says “ISO 2768-mK” cites a cancelled part for the second letter.
• Cut edges: EN ISO 9013:2017 classifies the quality of flame, plasma and laser cuts (laser 0.5 to 32 mm). Name a cut quality for edges that show, after asking the partner what it achieves.
• Structural steelwork: tolerances differ and belong to the engineer: BS EN 1090 execution classes apply.

Our metalwork brief checklist puts the general tolerance at item 5 and the critical tolerances at item 6.

What the four ISO 2768-1 classes allow

This is an extract: three size ranges and one angular band from Tables 1 and 3 of ISO 2768-1:1989, read from a public preview of the standard. The full tables have eight size ranges, and the standard is under revision, so confirm the figures against the current edition before you rely on them.

• The class steps are large. On a dimension between 1,000 and 2,000 mm, class m permits ±1.2 mm and class c ±3 mm. A long visible reveal will not hold a millimetre under a general tolerance.
• Angles act at the tip. A ±20′ error on a 100 mm flange moves its tip by about 0.6 mm, and the error grows with the length of the flange (arithmetic ours, checked by script).
• Exceeding the tolerance is not automatic rejection. Clause 6 says that, unless otherwise stated, a workpiece outside the general tolerance is not rejected automatically provided its ability to function is not impaired. If appearance is the function, say so on the drawing.
• Choose the class by function. Clause 2 says to take the customary workshop accuracy into account, and to give a smaller tolerance beside the dimension where one is needed or a larger one is allowed and cheaper. No source we read says which class suits visible interior work, so ask the partner which class it works to.

Flatness is a separate statement

The tolerance on a sheet's size does not limit how flat it is. ISO 2768-1 says its angular tolerances control only the general orientation of lines, not their form deviations, and EN ISO 13920's listing says it follows the independency principle of ISO 8015, so size, flatness and straightness are tolerated separately. A visible panel can be in size and still show waves in raking light.

• Welded assemblies: EN ISO 13920 has its own tolerances for straightness, flatness and parallelism. We did not read its values.
• Other parts: ISO 22081:2021 replaced ISO 2768-2:1989 for general geometrical specifications. We read only its front pages, so we cannot say what it gives by default.
• Thin, wide sheet shows waves for reasons other than tolerance: see oil canning in sheet metal thickness and gauge conversion.
• Say how it will be judged. A flatness figure alone does not decide whether a panel looks flat. Name the sample, the light and the viewing distance, as in the brief checklist.

Where metal meets joinery: how tolerances add

At a joint, the gap you see is the sum of every part's tolerance. Take a 1,200 mm steel insert in a 1,210 mm joinery opening, so the clearance is 10 mm in all, 5 mm each side. The insert is made to ISO 2768-m: ±1.2 mm for a dimension between 1,000 and 2,000 mm. For the opening we use ±1 mm as a placeholder, because no source we read gives a joinery figure: ask the joiner for the real one.

• Worst case: add the tolerances: 1.2 + 1 = ±2.2 mm, so the total clearance can be anything from 7.8 to 12.2 mm, and each side, if the insert sits centred, from 3.9 to 6.1 mm.
• Root sum of squares: the square root of (1.2² + 1²) = ±1.56 mm, so the clearance is likely to fall between 8.4 and 11.6 mm. This assumes the two errors are independent and random, which a single run of two parts may not be.
• What it means for the look. If the design shows a 5 mm shadow gap, it will vary by more than a millimetre from one side to the other even when both parts are within tolerance. Either accept a deliberate variation, tighten the tolerance on the parts that matter, or move the tolerance into an adjustable joint, a scribe or a wider gap that hides it.

Walls, floors and stone add their own variation, usually larger than the metal's: see joinery tolerances and scribing, shadow gaps and flush trim, door sizes and tolerances and furniture dimensional tolerances. For welded visible joints see weld finish and visible welds.

General tolerances: welded assemblies to EN ISO 13920, class [  ] for lengths and angles and class [  ] for straightness, flatness and parallelism; parts machined or formed from sheet without welds to ISO 2768-1, class [m]; geometrical tolerances on those parts [as stated on the drawing / to ISO 22081 as agreed with the fabricator]. The fabricator is to confirm in writing the class it works to before pricing. Edition of each standard: [current edition at the date of the drawing].

Individual tolerances apply only at: [inlay gap to timber [  ] mm]; [metal insert to joinery opening, clearance [  ] mm, tolerance [  ] mm]; [door or panel in frame [  ] mm]; [edge to stone [  ] mm]. All other dimensions take the general tolerance. Dimensions are finished sizes from datum [  ]. Interfaces to joinery, stone and floors are to be set out from the surveyed opening, not the drawing.

Visible flat panels [  ] mm by [  ] mm are to be flat to [  ] mm over their length, and to show no waves in [raking light] at [  ] m, judged against approved sample [ref. __]. Flatness is tolerated separately from thickness and size.

Parts outside the general tolerance are to be [rejected / referred to [name] for a decision on function and appearance]. Dimensions are to be checked against the shop drawing at [  ] hold points, and the results kept with the approved sample.

Standards and sources

• EN ISO 13920:2023, welding: general tolerances for welded constructions: listing read: published 15 Nov 2023, replaces EN ISO 13920:1996. Text not read; BSI page not found.
• ISO 2768-1:1989 (BS EN 22768-1:1993): text read from a public preview; ISO stage 90.92 'to be revised' (4 Oct 2022); BSI listing: current, under review.
• ISO 22081:2021 (BS EN ISO 22081:2021): front pages read; it cancels and replaces ISO 2768-2:1989; BSI listing: current, published 28 Feb 2021.
• EN ISO 9013:2017, thermal cutting quality: listing read: published 2017, amendment A1:2024 listed; text not read.
• ISO 8015, EN 1090-2: named only; status and text not checked.

Questions we are asked

What tolerance class should I put on a metalwork drawing?

Name one general tolerance for every dimension that has no tolerance of its own, and add individual tolerances only where parts must fit or line up. For welded assemblies the reference is EN ISO 13920 and a class; for machined or sheet-formed parts without welds it is ISO 2768-1 and a class f, m, c or v. No source we read says which class suits visible interior work, so ask the partner which it works to and put that in the title block.

What does ISO 2768-m mean?

It means the medium class of ISO 2768-1:1989, which gives permissible deviations for lengths and angles that have no individual tolerance, in four classes: f (fine), m (medium), c (coarse) and v (very coarse). For a dimension between 1,000 and 2,000 mm, class m allows plus or minus 1.2 mm. ISO lists the standard as to be revised, so confirm the current edition.

Is ISO 2768-mK still valid?

The second letter belongs to ISO 2768-2:1989, which ISO 22081:2021 says it cancels and replaces, so a title block that reads ISO 2768-mK cites a cancelled part for K. ISO 2768-1, which supplies the m, is still listed, as a standard to be revised. Ask the partner how it reads old drawings, and say what you want on new ones.

What is the difference between ISO 13920 and ISO 2768?

EN ISO 13920 is for welded constructions and covers shape and position as well as lengths and angles. ISO 2768-1 is for parts made by metal removal or formed from sheet metal, and covers lengths and angles only. A welded fabrication that also has machined features can need both, so ask the fabricator how it will read the drawing.

How flat should a visible metal panel be?

We found no source for a figure. Flatness is a separate tolerance from size, thin wide sheet waves for reasons that are not tolerance, and a figure does not say how the panel will look. Name the sample, the light and the viewing distance as well as any number, and see the sheet metal thickness sheet for oil canning.

Do structural tolerances differ?

Yes. Structural steelwork is built to different standards and the engineer sets its tolerances. This sheet is for non-structural interior metalwork. See the BS EN 1090 sheet.