ISO 2768-mK Explained: General Tolerances on a Real Drawing
"ISO 2768-mK" in a title block is a default tolerance for every feature the drawing does not tolerance individually. The m is the medium class of ISO 2768-1 for linear and angular dimensions; the K is the middle class of ISO 2768-2 for general geometric tolerances. On a part around 100 mm in size, mK means lengths are ±0.3 mm, a 12 mm thickness is ±0.2 mm and flatness over 30–100 mm is 0.2 mm — whether or not anyone chose those numbers on purpose.
The video above walks through the tables on a real bracket in about six minutes. This page has the same tables in full, the worked example, and one thing the video does not cover: the geometric half of the callout has been replaced in the standard itself.
What the two letters control
- m — ISO 2768-1, linear and angular. Lengths, diameters, distances, steps, external radii, chamfer heights and angles. Classes are f (fine), m (medium), c (coarse) and v (very coarse).
- K — ISO 2768-2, geometric. Straightness, flatness, perpendicularity, symmetry and circular run-out. Classes are H, K and L, from fine to coarse.
- Only where nothing else is written. Any tolerance on the feature itself overrides the general note. Below 0.5 mm, ISO 2768-1 gives no value: each such dimension needs its own tolerance.
The linear table (ISO 2768-1, Table 1)
Permissible deviations in ±mm for classes f (fine), m (medium), c (coarse) and v (very coarse). Bands run from “over” the lower value up to and including the upper value.
| Size (mm) | f | m | c | v |
|---|---|---|---|---|
| 0.5 – 3 | 0.05 | 0.1 | 0.2 | — |
| over 3 – 6 | 0.05 | 0.1 | 0.3 | 0.5 |
| over 6 – 30 | 0.1 | 0.2 | 0.5 | 1 |
| over 30 – 120 | 0.15 | 0.3 | 0.8 | 1.5 |
| over 120 – 400 | 0.2 | 0.5 | 1.2 | 2.5 |
| over 400 – 1000 | 0.3 | 0.8 | 2 | 4 |
| over 1000 – 2000 | 0.5 | 1.2 | 3 | 6 |
| over 2000 – 4000 | — | 2 | 4 | 8 |
Worked example: a bracket under mK
A plate bracket 80 × 40 × 12 mm with two Ø10 holes on a 60 mm spacing, and nothing toleranced except the title-block note:
| Feature | Nominal (mm) | Band (mm) | Under mK |
|---|---|---|---|
| Length | 80 | over 30 – 120 | ±0.3 |
| Width | 40 | over 30 – 120 | ±0.3 |
| Hole spacing | 60 | over 30 – 120 | ±0.3 |
| Thickness | 12 | over 6 – 30 | ±0.2 |
| Hole diameter | Ø10 | over 6 – 30 | ±0.2 |
Change only the class letter and the same 80 mm length becomes ±0.15 (f) or ±0.8 (c). More than a factor of five between f and c, on one dimension, from one letter: that is the whole cost argument of the note.
Radii, chamfers and angles
External radii and chamfer heights (ISO 2768-1, Table 2), ±mm:
| Nominal (mm) | f and m | c and v |
|---|---|---|
| 0.5 – 3 | 0.2 | 0.4 |
| over 3 – 6 | 0.5 | 1 |
| over 6 | 1 | 2 |
Angles are toleranced by the length of the shorter leg, so a longer leg gets a tighter angle. For classes f and m:
| Shorter leg (mm) | Angle tolerance |
|---|---|
| up to 10 | ±1° |
| over 10 – 50 | ±0°30′ |
| over 50 – 120 | ±0°20′ |
| over 120 – 400 | ±0°10′ |
| over 400 | ±0°5′ |
The K: geometric classes (ISO 2768-2)
Straightness and flatness, tolerance zone width in mm by nominal length:
| Nominal length (mm) | H | K | L |
|---|---|---|---|
| up to 10 | 0.02 | 0.05 | 0.1 |
| over 10 – 30 | 0.05 | 0.1 | 0.2 |
| over 30 – 100 | 0.1 | 0.2 | 0.4 |
| over 100 – 300 | 0.2 | 0.4 | 0.8 |
| over 300 – 1000 | 0.3 | 0.6 | 1.2 |
| over 1000 – 3000 | 0.4 | 0.8 | 1.6 |
Class K also sets perpendicularity at 0.4 mm for a shorter side up to 100 mm (0.6 over 100–300, 0.8 over 300–1000) and circular run-out at 0.2 mm. On the bracket, the 80 mm face may be out of flat by 0.2 mm under K.
ISO 2768-2 has been withdrawn. ISO 2768-2:1989 was withdrawn in February 2021 and replaced by ISO 22081:2021, which defines general geometrical specifications without the H, K and L classes. Existing drawings that say “mK” still mean the values above and you will be reading them for years. For new drawings, state general geometric specifications under ISO 22081, or put explicit GD&T on the features that matter.
f, m or c: choose on purpose
- Finer costs more. Up to 120 mm, class f is exactly half of class m. Each step tighter usually means slower machining and more inspection for every untoleranced feature on the part.
- Coarser is cheaper and fine for cosmetic edges, outside profiles of fabrications and generous clearances.
- Functional features get their own number. Fits, bearing seats, sealing faces and anything in a critical stack-up should never lean on a general tolerance. Hole and shaft fits belong to ISO 286, not ISO 2768.
General tolerances still drive stack-ups
An untoleranced dimension is not a perfect dimension. On the bracket, the 60 mm spacing at ±0.3 and each Ø10 hole at ±0.2 feed any tolerance stack-up that runs through them, and they are the easiest contributors to forget because there is no number on the drawing to remind you. SuperNX applies the part’s general-tolerance class to contributors that carry no individual tolerance, so a stack that runs through them is checked with the general tolerance included rather than treated as exact.
The linear and radius tables are also on the free standards reference and as machine-readable data at /data/iso-2768.json.
The short version
- m = ISO 2768-1 medium class for sizes, radii, chamfers and angles: ±0.3 mm on anything over 30 up to 120 mm.
- K = the ISO 2768-2 geometric class: 0.2 mm flatness over 30–100 mm, 0.2 mm run-out. That part of the standard is now ISO 22081.
- The note is a default, not a design decision. Anything functional gets its own tolerance.
Video chapters and transcript
- 0:00 Intro
- 0:26 The title-block problem
- 0:57 What m and K mean
- 1:35 Reading the linear table
- 2:38 Radii and chamfers
- 3:03 Angles
- 3:26 K-class flatness
- 4:08 f, m or c?
- 4:53 Stack-ups
- 5:30 Doing it automatically
Read the full transcript
0:00 Intro
Almost every drawing has a line in the title block that reads ISO 2768-mK. Most engineers know it means general tolerances, but few can say what it actually allows on a given feature. In the next few minutes we'll decode it, read the tables on a real bracket, and see where it quietly goes wrong.
0:26 The title-block problem
Look at any bracket drawing and count the dimensions with no tolerance written beside them. The length, the hole spacing, the hole diameters. They can still be measured, and they can still be rejected. One note in the title block covers all of them: ISO 2768-mK. That note is a default, and a default is a design decision. If nobody chose the class on purpose, habit chose it.
0:57 What m and K mean
Read the note in two halves. The small m is the linear and angular class from part one of the standard: medium. It covers sizes, radii, chamfers and angles. The capital K is the geometric class from part two. The classes there are H, K and L, and K covers straightness, flatness, perpendicularity and run-out. And both apply only where the drawing gives no individual tolerance. Write one on the feature, and the general note steps aside.
1:35 Reading the linear table
Take a bracket eighty by forty by twelve millimetres, with two holes sixty millimetres apart. The holes are ten millimetres in diameter, and none of it is toleranced. Here is the linear table, in plus or minus millimetres, one row per size range. Eighty, sixty and forty all sit in the thirty-to-one-twenty row. In column m, that's plus or minus zero point three. The length, the width and the hole spacing each float by six tenths in total. The twelve-millimetre thickness and the ten-millimetre hole diameter fall in the six-to-thirty row: plus or minus zero point two. For comparison, class f would hold the eighty-millimetre length to plus or minus zero point one five, half as much. So the title block just handed the shop six tenths of freedom on the hole spacing, and nobody wrote it down.
2:38 Radii and chamfers
Radii and chamfers have their own small table, the same for classes f and m. Up to three millimetres, plus or minus zero point two. Above three and up to six, plus or minus zero point five. Above six millimetres, plus or minus one. An eight-millimetre radius can legally land anywhere from seven to nine.
3:03 Angles
Angles are given by the length of the shorter leg, not the longer one. Take a leg of forty millimetres: plus or minus half a degree. The longer that leg, the tighter the angle, down to five minutes of arc above four hundred millimetres. A leg of ten millimetres or less gets a full degree.
3:26 K-class flatness
Now the K. Straightness and flatness are set by the nominal length, and the table has three columns: H, K and L. The eighty-millimetre face of our bracket sits in the thirty-to-hundred row. Under K it must be flat within zero point two, a zone zero point two wide. On a plate five hundred millimetres long, the same class allows zero point six. K also covers perpendicularity, zero point four for a shorter side up to a hundred millimetres, and circular run-out of zero point two.
4:08 f, m or c?
So which class? Finer costs more. Class f holds our eighty-millimetre length to plus or minus zero point one five instead of zero point three, and each step tighter usually means slower machining and more inspection. Coarser is cheaper. Class c allows plus or minus zero point eight on the same length, which is fine for cosmetic edges and loose clearances. And functional features get an explicit tolerance. Fits, bearing seats, and anything that feeds a stack-up should never rely on a general tolerance. Put the number on the drawing. To look up any of these values, there's a free calculator at nxtolerance.com slash standards.
4:53 Stack-ups
Here's why this matters beyond one part. General tolerances don't disappear in an assembly. An un-toleranced dimension is still a contributor. The hole spacing at plus or minus zero point three, and the hole diameter at plus or minus zero point two, both feed any stack that runs through them. They're also the easiest to forget, because there's no number on the drawing to remind you. SuperNX applies the general-tolerance class to un-toleranced contributors in a stack, so they're counted, not skipped.
5:30 Doing it automatically
Doing this by hand means reading the table for every contributor in every stack. SuperNX reads the geometry and PMI from your Siemens NX part, applies the general-tolerance class wherever no tolerance is given, and runs worst case and RSS for every key characteristic. When a stack fails, it shows the fixes that would reach the spec. There's a free demo part on nxtolerance.com, plus a free ISO 286 and ISO 2768 calculator. No sign-up needed. Links are in the description.
FAQ
- What does ISO 2768-mK mean on a drawing?
- It sets default tolerances for every feature the drawing leaves without its own tolerance. "m" is the medium class of ISO 2768-1 for linear and angular dimensions (an 80 mm length is ±0.3 mm); "K" is the middle geometric class of ISO 2768-2 for straightness, flatness, perpendicularity, symmetry and run-out.
- What is the ISO 2768-m tolerance for 100 mm?
- ±0.3 mm. 100 mm falls in the over 30 up to 120 mm band, which is ±0.3 in class m, ±0.15 in class f and ±0.8 in class c.
- Does ISO 2768 apply to hole diameters?
- Yes. ISO 2768-1 covers linear sizes, including diameters, distances and steps, when no individual tolerance is given. A hole that is part of a fit should not rely on it: give it an ISO 286 fit tolerance such as H7 instead.
- Is ISO 2768-2 still valid?
- No. ISO 2768-2:1989 was withdrawn in February 2021 and replaced by ISO 22081:2021, which does not use the H, K and L classes. Drawings that say "mK" are still everywhere and the K values still define what those drawings meant, but new drawings should state general geometric specifications under ISO 22081 or with explicit GD&T.