General tolerances
Not every dimension on a drawing needs its own tolerance. Instead, a general tolerance is given in the title block — for example “ISO 2768-mK” — and applies to every dimension without an explicit tolerance.
What is a general tolerance?
A general tolerance is the allowed deviation range for dimensions that don't carry an individual tolerance. It keeps drawings clean, sets a realistic manufacturing accuracy and prevents tolerances from being forgotten. The tolerance grows with the dimension — a 1000 mm length is harder to hit exactly than a 10 mm one.
ISO 2768-1 — General tolerances for linear and angular dimensions
Mainly used for machined parts (turning, milling). Four accuracy classes, from finest (f) to coarsest (v).
Linear dimensions (mm)
Table values are ± tolerance in mm.
| Range (mm) | f | m | c | v |
|---|---|---|---|---|
| 0.5 – 3 | ±0.05 | ±0.1 | ±0.2 | — |
| > 3 – 6 | ±0.05 | ±0.1 | ±0.3 | ±0.5 |
| > 6 – 30 | ±0.1 | ±0.2 | ±0.5 | ±1.0 |
| > 30 – 120 | ±0.15 | ±0.3 | ±0.8 | ±1.5 |
| > 120 – 400 | ±0.2 | ±0.5 | ±1.2 | ±2.5 |
| > 400 – 1000 | ±0.3 | ±0.8 | ±2.0 | ±4.0 |
| > 1000 – 2000 | ±0.5 | ±1.2 | ±3.0 | ±6.0 |
| > 2000 – 4000 | — | ±2.0 | ±4.0 | ±8.0 |
Angular dimensions
Tolerance in degrees and arc-minutes ( ′ ).
| Length of shorter side (mm) | f | m | c | v |
|---|---|---|---|---|
| ≤ 10 | ±1° | ±1° | ±1°30′ | ±3° |
| > 10 – 50 | ±0°30′ | ±0°30′ | ±1° | ±2° |
| > 50 – 120 | ±0°20′ | ±0°20′ | ±0°30′ | ±1° |
| > 120 – 400 | ±0°10′ | ±0°10′ | ±0°15′ | ±0°30′ |
| > 400 | ±0°5′ | ±0°5′ | ±0°10′ | ±0°20′ |
ISO 13920 — General tolerances for welded structures
Accounts for heat distortion in welded parts. Four classes: A (fine) to D (coarse). Written e.g. “ISO 13920-BF” where B = linear class and F = form/position.
Linear and angular dimensions (class A–D)
Table values are ± tolerance in mm.
| Nominal size (mm) | A | B | C | D |
|---|---|---|---|---|
| 2 – 30 | ±1 | ±2 | ±3 | ±4 |
| > 30 – 120 | ±1 | ±2 | ±4 | ±7 |
| > 120 – 400 | ±1 | ±2 | ±6 | ±9 |
| > 400 – 1000 | ±2 | ±3 | ±8 | ±12 |
| > 1000 – 2000 | ±3 | ±4 | ±11 | ±16 |
| > 2000 – 4000 | ±4 | ±6 | ±14 | ±21 |
| > 4000 – 8000 | ±5 | ±8 | ±18 | ±27 |
| > 8000 – 12000 | ±6 | ±10 | ±21 | ±32 |
Angular tolerance (mm per m of side length)
| Nominal size (mm) | A | B | C | D |
|---|---|---|---|---|
| ≤ 400 | ±20′ | ±45′ | ±1° | ±1°30′ |
| > 400 – 1000 | ±15′ | ±30′ | ±45′ | ±1° |
| > 1000 | ±10′ | ±20′ | ±30′ | ±45′ |
Straightness, flatness and parallelism (class E–H)
Table values are ± tolerance in mm.
| Nominal size (mm) | E | F | G | H |
|---|---|---|---|---|
| 30 – 400 | ±2 | ±3 | ±4.5 | ±6 |
| > 400 – 1000 | ±3 | ±4.5 | ±6 | ±8 |
| > 1000 – 2000 | ±4.5 | ±6 | ±8 | ±11 |
| > 2000 – 4000 | ±5 | ±8 | ±11 | ±14 |
| > 4000 – 8000 | ±6 | ±10 | ±14 | ±18 |
| > 8000 – 12000 | ±7 | ±12 | ±16 | ±21 |
ISO fits (H7, h6, etc.)
Designations like H7, h6, g6, k6 are ISO fits per ISO 286. They describe how tightly two parts mate — e.g. a shaft in a hole. The letter sets the position of the tolerance zone, the number sets its size (IT grade).
How to read the designation
- UPPERCASE letter (H, G, K…)Hole — internal feature
- lowercase letter (h, g, k…)Shaft — external feature
- Number (6, 7, 8…)IT grade — lower number = tighter tolerance
- H = hole-basis referenceHole's lower limit sits on the nominal size (most common “hole-basis” system)
- h = shaft-basis referenceShaft's upper limit sits on the nominal size (used in “shaft-basis” system)
IT grades (common range)
IT6 = very tight (precision machining). IT7 = standard for fit holes. IT8–IT9 = normal workshop tolerance. IT11+ = coarse dimensions, sheet metal and weldments.
Common fit types
Example values for a nominal diameter of ⌀20 mm.
| Type | Example | Use |
|---|---|---|
| Clearance | H7 / g6 | Free-running shaft, oil-lubricated — e.g. plain bearing |
| Clearance | H7 / f7 | Rotating shaft with noticeable clearance |
| Transition | H7 / k6 | Light press fit — can be removed with hand tools |
| Transition | H7 / n6 | Tight fit — requires light press force |
| Interference | H7 / p6 | Press fit — e.g. ball bearing inner race on shaft |
| Interference | H7 / s6 | Heavy press fit — shrink or press assembly |
Example: ⌀20 H7 / g6
Hole ⌀20 H7 has an upper limit of +0.021 mm and a lower limit of 0 — i.e. 20.000–20.021 mm. Shaft ⌀20 g6 has an upper limit of −0.007 mm and a lower limit of −0.020 mm — i.e. 19.980–19.993 mm. This always gives a clearance between 0.007 and 0.041 mm.
Drawing example
In the title block you'd see:
General tolerance: ISO 2768-mK / ISO 13920-BF
This means: machined dimensions follow ISO 2768 class m (linear) and K (form/position). Welded dimensions follow ISO 13920 class B (linear) and F (form & position). Only dimensions with their own tolerance (e.g. ⌀20 H7) deviate from this.
Good to know
- Don't pick a finer class than the function requires — tighter tolerances cost more.
- Welded structures need ISO 13920 — ISO 2768 doesn't account for heat distortion.
- Tolerance grows with the dimension — a 500 mm size has more allowed deviation than a 50 mm one.
- Always stated in the title block, never beside each dimension.