General Tolerances for CNC Parts
ISO 2768 general tolerances in four precision classes, IT grades IT01 to IT18, and what each CNC process holds — with the class to put on your drawing.
ISO 2768-1 general tolerances for linear dimensions (mm)
| Nominal size range (mm) | ISO 2768-f | ISO 2768-m | ISO 2768-c | ISO 2768-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 |
How to read this table. Chosen from the size range of the dimension itself, not the part. Standard designations: ISO 2768-f (fine), ISO 2768-m (medium), ISO 2768-c (coarse), ISO 2768-v (very coarse).
Which Precision Class to Specify
ISO 2768 gives four general tolerance classes. Pick one and put it in the title block or a general note — never leave the drawing without it.
| Class | Precision level | Typical use | Cost impact |
|---|---|---|---|
| f (fine) | Highest general | Precision parts, mating surfaces not requiring a specific fit, tight cosmetic requirements | +15–30% vs medium |
| m (medium) | Standard CNC | General CNC machined parts. The most widely used class, suitable for the majority of machined components. | Baseline |
| c (coarse) | Relaxed | Sheet metal parts, structural weldments, non-critical castings, large assemblies | −10–20% vs medium |
| v (very coarse) | Most relaxed | Welded structures, rough castings, non-precision fabricated parts, very large dimensions | −20–35% vs medium |
IT Grades — What a Tolerance Number Means
An IT grade is the number of tolerance steps between the two ends of the size range. IT1 is the finest, IT18 the coarsest. The table below gives the actual tolerance in micrometres for each size band.
| IT grade | Practical meaning | 1–3 mm | 6–10 mm | 18–30 mm | 50–80 mm | 120–180 mm | 250–315 mm |
|---|---|---|---|---|---|---|---|
| IT01 | Gauge block reference | 0.3 | 0.4 | 0.6 | 0.8 | 1.0 | 1.2 |
| IT0 | Reference standard | 0.5 | 0.6 | 0.9 | 1.2 | 1.5 | 2.0 |
| IT1 | Precision gauge | 0.8 | 1.0 | 1.5 | 2.0 | 2.5 | 3.0 |
| IT2 | High-precision gauge | 1.2 | 1.5 | 2.5 | 3.0 | 4.0 | 5.0 |
| IT3 | Ultra-precision work | 2.0 | 2.5 | 4.0 | 5.0 | 6.0 | 8.0 |
| IT4 | Precision grinding / wire EDM | 3 | 4 | 6 | 8 | 10 | 13 |
| IT5 | Gauge manufacturing | 4 | 6 | 9 | 13 | 16 | 20 |
| IT6 | Precision machining | 6 | 9 | 13 | 19 | 25 | 32 |
| IT7 | Precision fit (bearings, shafts) | 10 | 15 | 21 | 30 | 40 | 52 |
| IT8 | General precision machining | 14 | 22 | 33 | 46 | 63 | 81 |
| IT9 | General machining (ISO 2768-m equivalent) | 25 | 36 | 52 | 74 | 100 | 130 |
| IT10 | Medium precision | 40 | 58 | 84 | 120 | 160 | 210 |
| IT11 | Loose machining | 60 | 90 | 130 | 190 | 250 | 320 |
| IT12 | Very rough machining | 100 | 150 | 210 | 300 | 400 | 500 |
| IT13 | Pattern and rough work | 140 | 220 | 310 | 440 | 580 | 740 |
| IT14 | Coarse, low-grade work | 250 | 370 | 520 | 740 | 1000 | 1250 |
| IT15 | Very coarse | 400 | 600 | 840 | 1200 | 1600 | 2000 |
| IT16 | Approximate / unspecified | 600 | 1000 | 1400 | 2000 | 2600 | 3200 |
| IT17 | Very approximate | 1000 | 1600 | 2400 | 3300 | 4300 | 5300 |
| IT18 | General fabricated work | 1400 | 2500 | 3500 | 4800 | 6300 | 7600 |
What Each CNC Process Actually Holds
The tolerance a shop can hold depends on the process, the material, the size and the tolerance stack. These are the tolerances we hold as standard practice — the precision column needs a finishing operation, the ultra-precision column needs a grinding or EDM budget.
| Process | Standard (typical) | Precision (extra cost) | Ultra-precision (high cost) | IT equivalent |
|---|---|---|---|---|
| CNC Milling (3-axis) | ±0.025mm | ±0.005mm | ±0.002mm | IT8 → IT5 → IT3 |
| CNC Milling (5-axis) | ±0.010mm | ±0.005mm | ±0.002mm | IT7 → IT5 → IT3 |
| CNC Turning | ±0.025mm | ±0.005mm | ±0.002mm | IT8 → IT5 → IT3 |
| Swiss-Type Turning | ±0.010mm | ±0.005mm | ±0.002mm | IT7 → IT5 → IT3 |
| Surface Grinding | ±0.005mm | ±0.002mm | ±0.001mm | IT5 → IT3 → IT2 |
| Jig Boring | ±0.010mm | ±0.005mm | ±0.002mm | IT7 → IT5 → IT3 |
| Wire EDM | ±0.010mm | ±0.003mm | ±0.001mm | IT7 → IT4 → IT2 |
| Sinker EDM | ±0.015mm | ±0.005mm | ±0.002mm | IT8 → IT5 → IT3 |
How to Use These Numbers
- State the class on the drawing, in the title block or a general note — “ALL DIMENSIONS PER ISO 2768-m UNLESS OTHERWISE SPECIFIED”. A drawing with no general tolerance is a drawing with dozens of unanswered questions.
- Use ISO 2768 only on dimensions with no functional fit requirement. Bearing bores, shaft seats, seal grooves and locating dowels need individual tolerances — ISO 2768-m at 50 mm is ±0.3 mm, while a bearing seat needs ±0.01 mm or tighter.
- The size range comes from the dimension being toleranced, not from the overall part size. A 2 mm hole on a 400 mm plate still falls in the 0.5–3 mm band.
- Adding a general tolerance does not remove the need to tolerance critical features. GD&T and general tolerances work together: the general tolerance covers everything not otherwise dimensioned.
- Tolerance stacks add up. Five features each held to ±0.1 mm can stack to ±0.5 mm at the assembly level, which is why a stack-up calculation belongs in the drawing package for close-tolerance assemblies.
Frequently Asked Questions
Should I specify ISO 2768-f, -m or -c?
ISO 2768-m is the default for CNC machined parts and is what most shops expect. Use -f only when the part genuinely needs tighter general tolerances and you have accepted the cost, and -c for non-critical structural work. -v is for weldments and rough fabricated parts.
ISO 2768-m is the default for CNC machined parts and is what most shops expect. Use -f only when the part genuinely needs tighter general tolerances and you have accepted the cost, and -c for non-critical structural work. -v is for weldments and rough fabricated parts.
Does ISO 2768 apply to radii, chamfers and angles?
No. ISO 2768-1 covers linear dimensions only. Radii and chamfers fall under ISO 2768-2, and angular dimensions under ISO 2768-3. If your drawing mixes them, name the parts separately, for example “LINEAR DIMENSIONS PER ISO 2768-m”. This is one of the most common reasons a drawing gets questioned in a quote.
No. ISO 2768-1 covers linear dimensions only. Radii and chamfers fall under ISO 2768-2, and angular dimensions under ISO 2768-3. If your drawing mixes them, name the parts separately, for example “LINEAR DIMENSIONS PER ISO 2768-m”. This is one of the most common reasons a drawing gets questioned in a quote.
Can a CNC shop hold ±0.01 mm on a general tolerance?
Not as a general tolerance. ±0.01 mm is roughly IT4–IT5 territory and needs grinding, wire EDM or a finishing operation, priced accordingly. ISO 2768-f tops out at ±0.05 mm for sizes up to 3 mm and ±0.2 mm at 120–400 mm.
Not as a general tolerance. ±0.01 mm is roughly IT4–IT5 territory and needs grinding, wire EDM or a finishing operation, priced accordingly. ISO 2768-f tops out at ±0.05 mm for sizes up to 3 mm and ±0.2 mm at 120–400 mm.
What happens if my drawing has no general tolerance?
Every untoleranced dimension becomes a question, and most shops default to their own house standard. That usually slows the quote down and does not give you the tolerance you intended. Always state the class.
Every untoleranced dimension becomes a question, and most shops default to their own house standard. That usually slows the quote down and does not give you the tolerance you intended. Always state the class.
Is ISO 2768 the same as ISO 2768-2m?
No. ISO 2768-2 covers radii and chamfers, with its own m class and its own value table. Writing “ISO 2768-m” without a part number is ambiguous and should be split into “ISO 2768-1-m (linear) and ISO 2768-2-m (radii/chamfers)”.
No. ISO 2768-2 covers radii and chamfers, with its own m class and its own value table. Writing “ISO 2768-m” without a part number is ambiguous and should be split into “ISO 2768-1-m (linear) and ISO 2768-2-m (radii/chamfers)”.
Standards & Sources
- ISO 2768-1:1989 — General tolerances — Part 1: Tolerances for linear dimensions (f, m, c, v classes)
- ISO 286-1 — ISO code system for tolerances on linear dimensions (IT grade definitions)
- Site page: /wiki/tolerances/iso-2768/ — the long-form explanation of each class and its cost impact
Related Pages
- /wiki/tolerances/iso-2768/ — Full explanation of the four ISO 2768 classes, when to use each, and the cost impact
- /wiki/tolerances/gdt/ — How general tolerances and GD&T fit together on the same drawing
- /wiki/tolerances/fit-types/ — When you need an individual fit (H7/k6) instead of a general tolerance
- /resources/tolerance-table/ — Tolerance and precision tables across CNC, molding, powder metallurgy and 3D printing