Most dimensions on a drawing carry no individual tolerance. The title block names a general tolerance, and everything not separately toleranced falls under it. On the drawings we receive, that default is almost always ISO 2768-m, and it is often written without anyone checking what the four classes actually contain. This page is the reference we use when we read it, and it explains which class belongs on a formed part.
What is ISO 2768 general tolerance?
ISO 2768 is a two-part standard for general tolerances: the tolerances that apply to dimensions on a drawing which carry no individual tolerance of their own. Part 1 covers linear and angular dimensions and defines four classes, f for fine, m for medium, c for coarse and v for very coarse; the class is named in the title block and its table then fixes the permissible deviation according to the size of the dimension. Part 2 covered general geometrical tolerances in classes H, K and L, and was withdrawn and replaced by ISO 22081 in 2021, which sets out how general geometrical specifications are written. ISO 2768 is a general, or title-block, standard only, and it is not a statement of what a process can achieve. It is not a substitute for functional tolerances: any dimension the assembly depends on should carry its own tolerance regardless of the general class.
What ISO 2768 actually governs
A general tolerance standard does one job: it tells the shop what to do with dimensions the designer did not tolerance individually. That keeps a drawing readable, because only the dimensions that matter carry a number. What it does not do is make a part easier to inspect, or turn an untoleranced dimension into a critical one. When the title block says ISO 2768-m, every untoleranced dimension on the sheet is governed by the m column, and the achievable class on a formed part is exactly what the general tolerance was invented to express.
ISO 2768-1: the linear tolerance table
The table is indexed by the nominal size of the dimension, not by the process, the material or the tolerance class of the part. Pick the row for the dimension's nominal length and the column for the class named in the title block. All values are plus or minus, in millimetres.
| Nominal size range (mm) | f (fine) | m (medium) | c (coarse) | v (very coarse) |
|---|---|---|---|---|
| 0.5 up to 3 | 0.05 | 0.1 | 0.2 | - |
| over 3 up to 6 | 0.05 | 0.1 | 0.3 | 0.5 |
| over 6 up to 30 | 0.1 | 0.2 | 0.5 | 1.0 |
| over 30 up to 120 | 0.15 | 0.3 | 0.8 | 1.5 |
| over 120 up to 400 | 0.2 | 0.5 | 1.2 | 2.5 |
| over 400 up to 1000 | 0.3 | 0.8 | 2.0 | 4.0 |
| over 1000 up to 2000 | 0.5 | 1.2 | 3.0 | 6.0 |
| over 2000 up to 4000 | - | 2.0 | 4.0 | 8.0 |
Angular dimensions, and the radii and chamfer heights, have their own tables in the same standard, indexed the same way: by the class in the title block and, for angles, by the length of the shorter side of the angle. The pattern is the one above, with f and m the tight columns and c and v the loose ones. If a drawing calls up a general tolerance class, the shop is expected to hold all three tables, not only the linear one. The standard itself remains the authority; the table above is reproduced so that a buyer reading a title block knows what the class implies.
ISO 2768-2, and why it was replaced by ISO 22081
Part 1 deals with size. Form and position are a separate question, and Part 2 was written to answer it, with classes H, K and L covering general straightness or flatness, perpendicularity, symmetry and circular run-out. Part 2 was withdrawn and replaced by ISO 22081 in 2021, which sets out how general geometrical specifications should be written. The practical consequence for a buyer is simple: if a drawing from 2015 says ISO 2768-mK in the title block, the K refers to a standard that no longer exists, and the geometry it was meant to govern should be confirmed rather than assumed. On a drawn part this matters, because concentricity and roundness are exactly the characteristics a general geometrical class was intended to cover.
Which class belongs on a drawn or stamped part
- m is the right default for formed parts, and it is the class we apply where a drawing is silent. It is loose enough not to over-constrain a drawn wall and tight enough to sit sensibly alongside the machining that follows.
- f belongs on machined features rather than formed ones. Asking a drawn wall to meet a fine class is one of the most reliable ways to raise tooling cost without improving the part.
- c and v are reasonable where nothing downstream touches the dimension, for example on an unmeasured flange edge or a clearance outline.
- No general class is a substitute for a functional tolerance. Sealing faces, bearing seats, threads and locating diameters should carry their own callouts, however tight or loose the title block is.
When a general tolerance is the wrong tool
A general tolerance is a default, and defaults are wrong wherever function is at stake. Three cases come up constantly on drawn housings. First, a sealing face: the general class cannot express a flatness or roughness requirement, and the face usually has to be coined or machined after drawing anyway. Second, concentricity between a bore and an outside diameter: this is a tooling-axis property, and leaving it to a general geometrical class means nobody has designed for it. Third, wall thickness: the standard tolerates a dimension, but a wall that varies around the circumference needs a measurement point before the tolerance means anything. Our deep drawing tolerances page sets out how each of those is handled in practice.
The three mistakes we see most often
- A title block with a class but no inspection method, so a dimension is toleranceable but not verifiable. Say whether the acceptance method is a drawing dimension, a gauge, a report or a functional check.
- A general class so tight that it silently covers hundreds of dimensions. The remedy is not to loosen the class but to tolerance the functional dimensions individually and leave the rest general.
- A general tolerance used to avoid deciding. If a dimension matters, it deserves a number; if it does not, the general class is exactly right.
Key point
ISO 2768-m is a sensible default for a formed part and the class Balford applies where a drawing is silent. Use it for everything the assembly does not depend on, and put individual tolerances on the features that it does. Note that Part 2 has been replaced by ISO 22081, so a K or L in an older title block should be confirmed rather than assumed.
Frequently asked questions
What does ISO 2768-m mean on a drawing?
It names the medium class of general tolerances. Any dimension on the sheet without its own tolerance is governed by the m column of the ISO 2768-1 table, according to the nominal size of that dimension. For a dimension between 30 and 120 mm, for example, the permissible deviation is plus or minus 0.3 mm.
Is ISO 2768 still current?
Part 1, which covers linear and angular dimensions, remains current and is still the standard most title blocks reference. Part 2, which covered general geometrical tolerances in classes H, K and L, was withdrawn and replaced by ISO 22081 in 2021.
Should I use ISO 2768-f for a precision metal part?
Only for the features that are machined, not for the ones that are formed. A drawn wall is produced by material flowing, so a fine class on a formed surface raises cost without adding function. Put the fine band on the machined features and leave the formed geometry on m.
What if my drawing has no general tolerance at all?
Then every untoleranced dimension is ambiguous, and it is worth fixing before the part is quoted. Where a drawing states nothing, our default is ISO 2768-m, and we will flag the ambiguity during the DFM review rather than leave it to first article.