CNC Machining Tolerance Chart: ISO 2768 General Tolerances Explained

Révisé par l'équipe d'ingénieurs de Rapidcision | Dernière mise à jour : juin 2026

A CNC machining tolerance chart tells you the permissible variation allowed on a dimension that does not carry its own explicit tolerance. On most engineering drawings, that default comes from ISO 2768, an international standard you invoke with a single note in the title block, such as ISO 2768-mK. That one callout sets the allowed deviation for every untoleranced length, diameter, angle, and form feature on the part, which keeps a drawing clean and removes the guesswork that would otherwise leave each shop to interpret loose dimensions differently.

The tables below give the full ISO 2768 values for linear dimensions, radii and chamfers, angles, and geometrical features, in millimeters. Use them to read a drawing, to pick the right class for a new design, and to avoid the expensive habit of tightening tolerances that do not need to be tight. We apply these defaults across our Services d'usinage CNC, and a clear title-block callout makes your quote and your parts more predictable.

What ISO 2768 Covers

ISO 2768 is published by the International Organization for Standardization and is the dominant general tolerance standard in Europe and Asia, recognized by CNC suppliers worldwide. It is also published as DIN 7168 in Germany. The standard comes in two parts that work together:

•             ISO 2768-1 sets general tolerances for linear and angular dimensions, including lengths, diameters, step sizes, external radii, chamfer heights, and angles. It offers four classes: fine (f), medium (m), coarse (c), and very coarse (v).
•             ISO 2768-2 sets general geometrical tolerances for straightness, flatness, perpendicularity, symmetry, and circular run-out. It offers three classes: H, K, and L, with H the most precise.

It applies to parts made by material removal such as milling, turning, drilling, and grinding, and to sheet metal parts. It does not govern thread tolerances, surface roughness, or any dimension that already has its own explicit tolerance, since an explicit callout always takes precedence.

ISO 2768-1: Linear Dimensions

Permissible deviations in millimeters.

For nominal sizes below 0.5 mm, deviations are indicated next to the relevant dimension.

ISO 2768-1: External Radii and Chamfer Heights

ISO 2768-1: Angular Dimensions

Permissible deviation by the length of the shorter side of the angle.

ISO 2768-2: Geometrical Tolerances

These control form and position. Values are in millimeters. Note that ISO 2768-2 does not cover parallelism, cylindricity, concentricity, profile, or true position, which all require explicit GD&T callouts.

Straightness and flatness (by nominal length)

Perpendicularity (by length of the shorter side)

Symmetry

Circular run-out

How to Read ISO 2768-mK

A combined callout pairs one class from each part. In ISO 2768-mK, the lowercase m is the medium linear class from Part 1, and the uppercase K is the medium geometrical class from Part 2. So an untoleranced 25 mm dimension on an ISO 2768-mK drawing carries ±0.2 mm, because 25 mm falls in the over-6-to-30 range of the medium column. A flatness callout on a 200 mm face would allow 0.4 mm under the K class.

ISO 2768-mK is the most common general tolerance on Usinage CNC and sheet metal drawings, because the medium classes match what a capable machine holds without special effort.

Choosing the Right Class

Pick the loosest class that still lets the part function. Tighter is not better, it is more expensive.

•             Fine (f) and H for precision features, mating surfaces, and parts with tight fits.
•             Medium (m) and K for general machined and sheet metal parts. This is the default for most work.
•             Coarse (c) and L for non-critical parts such as covers, brackets, and enclosures where fit is not demanding.
•             Very coarse (v) for rough parts, flame-cut pieces, and castings, rarely used for CNC work.

Specifying tighter than necessary inflates machining time and cost, while specifying looser than the part needs causes assembly failures. The right call is to tighten only the features that matter and let the general tolerance handle the rest.

ISO 2768 vs Explicit GD&T

ISO 2768 is a safety net for the dimensions you did not individually tolerance. It is not a substitute for GD&T where function demands control. Any dimension with its own explicit tolerance overrides the general default, and features that need position, cylindricity, concentricity, or profile control must be called out explicitly, since ISO 2768-2 does not cover them. The practical pattern on a good drawing is a single ISO 2768 note in the title block, with explicit tolerances and GD&T applied only to the critical features.

Le rôle de Rapidcision

When you upload a drawing for an instant quote, the ISO 2768 class in your title block tells us how to treat every untoleranced feature, which makes pricing and production predictable. If your part has critical features, call them out explicitly and let the general tolerance cover the rest, and our DFM feedback will flag anything that looks tighter than it needs to be. Review capability on our Services d'usinage CNC, see how finish ties in on our surface roughness chart, or Téléchargez votre fichier CAO pour obtenir un devis.

Questions fréquemment posées

What is ISO 2768? ISO 2768 is an international standard that defines general tolerances for dimensions on a drawing that do not have their own explicit tolerance. A single title-block note, such as ISO 2768-mK, sets the default permissible variation for all untoleranced features.

What does ISO 2768-mK mean? It pairs the medium linear class (m) from ISO 2768-1 with the medium geometrical class (K) from ISO 2768-2. It is the most common general tolerance callout for CNC machined and sheet metal parts.

What tolerance does ISO 2768 medium give for a 25 mm dimension? Under the medium (m) class, a 25 mm dimension carries ±0.2 mm, because it falls in the over-6-to-30 mm range. The deviation depends on which size range the dimension falls into.

Which ISO 2768 class should I use for CNC parts? Medium (m) with K geometrical tolerances suits most CNC work, since it matches what a capable machine holds economically. Use fine (f) and H for precision features, and coarse (c) or L for non-critical parts.

Does ISO 2768 cover all geometric tolerances? No. ISO 2768-2 covers straightness, flatness, perpendicularity, symmetry, and circular run-out. Parallelism, cylindricity, concentricity, profile, and true position require explicit GD&T callouts.

Does an explicit tolerance override ISO 2768? Yes. Any dimension with its own stated tolerance takes precedence over the general ISO 2768 default. The general tolerance only applies to features without an individual callout.

Using the Tolerance Chart

ISO 2768 keeps drawings clean and unambiguous by setting sensible default tolerances with a single callout, most often ISO 2768-mK. Read your dimension’s size range against the right class, tighten only the features that need it, and use explicit GD&T where function demands. That approach holds quality where it matters and keeps cost down everywhere else.

If you have a part to make, upload your drawing for an instant quote. We will apply your ISO 2768 class to the general dimensions and confirm the critical tolerances with DFM feedback.