General tolerance is a way to keep drawings clean without giving up quality. Instead of writing a specific tolerance next to every single dimension, you declare one set of allowable deviations that applies to all unspecified dimensions at once. The result is a faster, clearer workflow and parts that still meet requirements.
In this article we explain what general tolerance is, how it differs from individually specified dimensional tolerances, and how the ISO 2768 tolerance classes work in practice for custom machined and sheet metal parts.
Dimensional tolerances vs. general tolerances
A dimensional tolerance is applied to a single dimension that needs specific precision, written explicitly on the drawing — for example 100 ±0.1. You choose the value yourself to communicate exactly how much deviation that feature can accept, which makes the designer's intent unambiguous.
A general tolerance works the other way around. Instead of annotating each dimension, you specify one tolerance range that covers every dimension not individually toleranced. This cuts drawing effort dramatically and keeps the drawing readable, while still guaranteeing that unmarked features stay within a defined limit. In practice you reserve explicit dimensional tolerances for the few critical features and let the general tolerance handle the rest.
General tolerance grades in ISO 2768
Once you decide to use general tolerances, the grade you pick matters. For linear and angular dimensions, ISO 2768-1 defines four tolerance classes, from tightest to loosest:
- f — fine: tightest general tolerance, for precise components.
- m — medium: the most common general-purpose class.
- c — coarse: for less demanding features.
- v — very coarse: loosest class, for rough or non-critical dimensions.
The important point is that a single grade does not mean a single tolerance value. The permitted deviation scales with the size of the dimension. A tolerance of ±0.1 on a 1 mm feature and on a 100 mm feature covers the same absolute range, but relative to length the precision differs by a factor of 100. To keep precision consistent, ISO 2768 assigns tolerance values per nominal size range rather than one flat number.
ISO 2768-1 general tolerances for linear dimensions (mm)
| Nominal size range | 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 |
Reading the table is straightforward. For a 20 mm dimension in the fine class, the range 6–30 mm applies and the tolerance is automatically ±0.1 — no annotation needed on the drawing. To apply a general tolerance, you simply state the standard and the class in the title block, for example "ISO 2768-m", and it governs every unmarked dimension on the part.
When to use each tolerance grade
The right grade depends on the function and material of the part. As a general orientation:
- Fine: precision metal parts for accurate machinery and assemblies. Well suited to CNC turning where tight diametral control matters.
- Medium: common metal parts and the default choice for most CNC milling and sheet metal fabrication work.
- Coarse / very coarse: common plastic parts and non-critical features where a wider deviation is acceptable.
Summary
General tolerances let you keep drawings lean while still controlling quality — you annotate only the critical dimensions and let a declared class handle everything else. For parts sourced internationally, aligning on a shared standard such as ISO 2768 removes guesswork: a supplier reads the class in the title block and knows exactly what deviations are permitted, with no assumptions on either side. That shared understanding of dimensional and geometric tolerances is what makes procuring custom parts reliable across borders.
