ISO 2768 general tolerances

The general tolerances according to the ISO 2768 standard allow controlling dimensional and geometric variations of mechanical parts without needing to dimension each value on the drawing. They are divided into two parts: ISO 2768-1 for linear and angular tolerances, and ISO 2768-2 for geometric tolerances. The standard defines four accuracy classes: f (fine), m (medium), c (coarse) and v (very coarse). This guide includes tables of permissible deviations, machining processes capable of achieving each class, real application examples, and a comparison with the American system ANSI B4.1.

Linear tolerances (ISO 2768-1)

Permissible deviations for linear dimensions range from ±0,05 mm (±0.002 in) in the finest class to ±8 mm (±0.315 in) in the coarsest, depending on the nominal range of the dimension.

Nominal range (mm)	Class f (fine)	Class m (medium)	Class c (coarse)	Class v (very coarse)
0,5 – 3	±0,05 mm / ±0.002 in	±0,1 mm / ±0.004 in	±0,2 mm / ±0.008 in	±0,5 mm / ±0.02 in
3 – 6	±0,05 mm / ±0.002 in	±0,1 mm / ±0.004 in	±0,3 mm / ±0.012 in	±0,5 mm / ±0.02 in
6 – 30	±0,1 mm / ±0.004 in	±0,2 mm / ±0.008 in	±0,5 mm / ±0.02 in	±1,0 mm / ±0.039 in
30 – 120	±0,15 mm / ±0.006 in	±0,3 mm / ±0.012 in	±0,8 mm / ±0.031 in	±1,5 mm / ±0.059 in
120 – 400	±0,2 mm / ±0.008 in	±0,5 mm / ±0.02 in	±1,2 mm / ±0.047 in	±2,5 mm / ±0.098 in
400 – 1000	±0,3 mm / ±0.012 in	±0,8 mm / ±0.031 in	±2,0 mm / ±0.079 in	±4,0 mm / ±0.157 in
1000 – 2000	±0,5 mm / ±0.02 in	±1,2 mm / ±0.047 in	±3,0 mm / ±0.118 in	±6,0 mm / ±0.236 in
2000 – 4000	—	±2,0 mm / ±0.079 in	±4,0 mm / ±0.157 in	±8,0 mm / ±0.315 in

Angular tolerances (ISO 2768-1)

For angles without tolerance indication, the permissible deviation ranges from ±1° to ±0°5’ in class f and from ±3° to ±0°30’ in class v, depending on the length of the shortest side of the angle.

Length of shortest side, L (mm)	Class f (fine)	Class m (medium)	Class c (coarse)	Class v (very coarse)
L ≤ 10	±1°	±1°30’	±3°	±3°
10 < L ≤ 50	±0°30’	±1°	±2°	±4°
50 < L ≤ 120	±0°20’	±0°30’	±1°	±2°
120 < L ≤ 400	±0°10’	±0°15’	±0°30’	±1°
L > 400	±0°5’	±0°10’	±0°15’	±0°30’

The equivalent linear deviation over 100 mm of length can be estimated using:

ΔL ≈ 100 mm × tan(α)

where α is the angular tolerance in degrees. For example, ±1° is approximately equivalent to ±1,75 mm / ±0.069 in per 100 mm / 3.94 in.

Geometric tolerances (ISO 2768-2)

Part 2 of the standard establishes general tolerances for form and position in three classes: H (high precision), K (medium precision) and L (low precision), with values starting from 0,02 mm (0.0008 in) for straightness of a short element in class H.

Straightness and flatness

Nominal range (mm)	Class H	Class K	Class L
≤ 10	0,02 mm / 0.0008 in	0,05 mm / 0.002 in	0,1 mm / 0.004 in
>10 – 30	0,05 mm / 0.002 in	0,1 mm / 0.004 in	0,2 mm / 0.008 in
>30 – 100	0,1 mm / 0.004 in	0,2 mm / 0.008 in	0,4 mm / 0.016 in
>100 – 300	0,2 mm / 0.008 in	0,4 mm / 0.016 in	0,8 mm / 0.031 in
>300 – 1000	0,3 mm / 0.012 in	0,6 mm / 0.024 in	1,2 mm / 0.047 in
>1000 – 3000	0,4 mm / 0.016 in	0,8 mm / 0.031 in	1,6 mm / 0.063 in

Perpendicularity and symmetry

Nominal range (mm)	Class H	Class K	Class L
≤ 100	0,2 mm / 0.008 in	0,4 mm / 0.016 in	0,6 mm / 0.024 in
>100 – 300	0,3 mm / 0.012 in	0,6 mm / 0.024 in	1,0 mm / 0.039 in
>300 – 1000	0,4 mm / 0.016 in	0,8 mm / 0.031 in	1,5 mm / 0.059 in
>1000 – 3000	0,5 mm / 0.02 in	1,0 mm / 0.039 in	2,0 mm / 0.079 in

Circular runout

Nominal range (mm)	Class H	Class K	Class L
≤ 30	0,1 mm / 0.004 in	0,2 mm / 0.008 in	0,5 mm / 0.02 in
>30 – 100	0,15 mm / 0.006 in	0,3 mm / 0.012 in	0,6 mm / 0.024 in
>100 – 300	0,2 mm / 0.008 in	0,4 mm / 0.016 in	0,8 mm / 0.031 in
>300 – 1000	0,3 mm / 0.012 in	0,6 mm / 0.024 in	1,2 mm / 0.047 in
>1000 – 3000	0,4 mm / 0.016 in	0,8 mm / 0.031 in	1,6 mm / 0.063 in

Machining processes and tolerance classes

Each machining process can typically achieve a range of IT tolerance grades, allowing selection of the most suitable ISO 2768 class according to the shop’s capability. The following table relates processes with IT grades and the linear tolerance class they can normally guarantee.

Machining process	Typical IT grades	Recommended ISO 2768-1 class
Lapping and honing	IT4 – IT6	f
Cylindrical grinding	IT5 – IT7	f / m
Surface grinding	IT6 – IT7	f / m
Diamond turning	IT5 – IT7	f / m
Diamond boring	IT5 – IT7	f / m
Broaching	IT6 – IT9	m
Reaming	IT7 – IT9	m
Turning	IT8 – IT12	c
Boring	IT9 – IT12	c / v
Milling	IT10 – IT13	v
Planing and shaping	IT11 – IT13	v
Drilling	IT10 – IT12	v

For reference, an IT8 grade on a 30 mm diameter corresponds to a total tolerance of 0,033 mm (±0,0165 mm / ±0.0007 in), while at 120 mm the IT8 tolerance is 0,054 mm (±0,027 mm / ±0.0011 in). Processes capable of maintaining IT6-IT8 fit with classes f or m, while processes such as milling or drilling are more associated with class v.

Comparison with ANSI B4.1

The American standard ANSI B4.1 defines general tolerances for dimensions without explicit tolerance, but does not use the same f/m/c/v classes. The closest equivalent to ISO 2768 class m (medium) is the “Normal” grade of ANSI, which for dimensions between 30 mm and 120 mm (1.2 in – 4.7 in) typically allows a variation of approximately ±0,4 mm (±0.015 in), compared to ±0,3 mm (±0.012 in) for class m.

Nominal range	ANSI B4.1 (grade “Normal”)	ISO 2768 class m
30 – 120 mm (1.2 – 4.7 in)	±0,4 mm / ±0.015 in (approx.)	±0,3 mm / ±0.012 in
120 – 400 mm (4.7 – 15.7 in)	±0,75 mm / ±0.030 in (approx.)	±0,5 mm / ±0.02 in

The comparison is indicative because the ANSI-B4.1 system uses a different rounding approach and does not distinguish between angular and general geometric tolerances as ISO 2768 does. In projects that combine American and international standards, it is advisable to explicitly indicate the applicable standard to avoid interpretation conflicts.

Application example

A bracket needs to be manufactured whose dimensions carry no individual tolerance. The total length is 200 mm and the title block indicates “ISO 2768-m”. The linear tolerance table for the 120 – 400 mm range gives a permissible deviation of ±0,5 mm (±0.02 in). Therefore the 200 mm dimension must be within the interval 199,5 mm – 200,5 mm (7.85 in – 7.89 in).

This margin is wide for processes such as milling, which at 200 mm can routinely achieve IT10 (total tolerance ≈ 0.185 mm, i.e., ±0,0925 mm / ±0.0036 in). Even a turning IT12 (total tolerance ≈ 0.46 mm, ±0,23 mm / ±0.009 in) easily meets it. Class m is appropriate because it does not raise manufacturing cost and ensures part interchangeability.

Interpretation on drawings

The indication on a technical drawing is made in the title block, using the standardized nomenclature:

ISO 2768-m

If general geometric tolerances are also applied:

ISO 2768-mK

where the letter K indicates the geometric accuracy class according to ISO 2768-2. Lowercase letters (f, m, c, v) correspond to dimensional tolerances, and uppercase letters (H, K, L) to geometric tolerances. This designation exempts individually dimensioning the tolerances for all dimensions not expressly tolerated.

Different classes should not be mixed on the same drawing within a single designation (for example “ISO 2768-f” together with “ISO 2768-c”); the standard assumes that all untoleranced dimensions inherit the same general class.

Frequently Asked Questions (FAQ)

What do the letters f, m, c and v mean in ISO 2768?

They represent the four accuracy classes for linear and angular dimensional tolerances: f = fine, m = medium, c = coarse, v = very coarse. More demanding measures (f) correspond to smaller deviations and vice versa.

When should I apply class f instead of m?

Class f is used when manufacturing processes are capable of maintaining tighter tolerances (grinding, lapping) and the design requires it, for example in sliding fits or high-precision parts. Class m covers most general mechanical applications.

Does ISO 2768 cover geometric tolerances such as parallelism or coaxiality?

Yes, part ISO 2768-2 defines general tolerances for straightness, flatness, perpendicularity, symmetry, and circular runout. Parallelism is implicitly covered by the combination of form and position tolerances, although the standard does not give a specific value for isolated parallelism.

How is an ISO 2768 general tolerance indicated on a drawing?

In the title block or in a note, write for example “ISO 2768-mK”. This applies the linear/angular deviations of class m and the geometric deviations of class K to all dimensions without explicit tolerance indication.

What is the difference between ISO 2768 and ISO 286?

ISO 286 (ISO fit system) defines tolerances for shaft-hole fits using IT grades and tolerance positions, while ISO 2768 specifies general tolerances for dimensions that carry no individual tolerance. ISO 2768 applies to complete parts and simplifies the drawing; ISO 286 is used when a specific fit is required (e.g., H7/g6).

Is it possible to convert an IT grade to the ISO 2768 class?

Approximately, yes. Class f is associated with IT6–IT8 processes, class m with IT9–IT10, class c with IT11–IT12 and class v with IT12–IT13. However, the equivalence is not normative and must be verified with the specific tables of each standard.

Sources Consulted

• engineeringtoolbox.com: https://www.engineeringtoolbox.com/machine-processes-tolerance-grades-d_1367.html
• roymech.org: https://roymech.org/