ISO 286 is the ISO code system for tolerances on the size of cylinders and of two parallel opposite surfaces: a number, the IT grade, sets the width of the tolerance zone, and a letter sets where the zone sits relative to the nominal size. A fit such as H7/g6 pairs a hole class with a shaft class, and the difference between their limits gives the clearance or interference of the assembly.
What is ISO 286?
ISO 286 has two parts. Part 1 (ISO 286-1:2010) defines the terms, the tolerance grades and the principle of fits. Part 2 (ISO 286-2:2010) tabulates the limit deviations for the tolerance classes in common use. Between them they answer the question a general tolerance cannot: how far apart may a bore and a shaft be, and how loose or tight is the result.
A drawing uses ISO 286 for features that mate, locate or seal. Everything else stays on the general tolerance of ISO 2768, which sets ±0.2 mm in class m for sizes over 6 up to 30 mm. A Ø25 H7 bore has 0.021 mm of tolerance instead, about one nineteenth of the 0.4 mm band that class m leaves open at that size.
How do IT grades set the width of a tolerance?
An IT grade (international tolerance grade) is a number that fixes the width of the tolerance zone. The width depends on the nominal size: for the same grade, a larger size gets a wider zone. A higher grade number means a wider zone.
Table 1. Standard tolerance grades IT5 to IT11 in µm, ISO 286-1
| Nominal size, mm | IT5 | IT6 | IT7 | IT8 | IT9 | IT11 |
|---|---|---|---|---|---|---|
| over 3 to 6 | 5 | 8 | 12 | 18 | 30 | 75 |
| over 6 to 10 | 6 | 9 | 15 | 22 | 36 | 90 |
| over 10 to 18 | 8 | 11 | 18 | 27 | 43 | 110 |
| over 18 to 30 | 9 | 13 | 21 | 33 | 52 | 130 |
| over 30 to 50 | 11 | 16 | 25 | 39 | 62 | 160 |
| over 50 to 80 | 13 | 19 | 30 | 46 | 74 | 190 |
| over 80 to 120 | 15 | 22 | 35 | 54 | 87 | 220 |
| over 120 to 180 | 18 | 25 | 40 | 63 | 100 | 250 |
Source: ISO 286-1:2010, Table 1, values as reproduced on Wikipedia. The full table continues up to IT18 and to sizes up to 3150 mm.
An IT grade describes a zone, not a process. Whether a given machine, material and feature can hold IT6 or IT7 is a question for the supplier, feature by feature.
How do the letter and number in H7 or g6 work together?
A tolerance class is a letter followed by an IT grade. The number is the width from Table 1. The letter is the position of the zone: capital letters are for holes, lower case for shafts.
H is the reference position for holes. The zone starts at the nominal size and runs upward, so a Ø25 H7 hole measures 25.000 to 25.021 mm. The shaft counterpart h puts the zone below the nominal size, ending at it: h6 at 25 mm is 0 to −13 µm. Other letters move the zone away from zero. For a Ø25 shaft, g6 sits at −7 to −20 µm, k6 at +2 to +15 µm and p6 at +22 to +35 µm.
Two principles decide which part carries the H. In the hole-basis system the hole is H and the shaft class changes to give the fit; in the shaft-basis system the shaft is h and the hole class changes. ISO 286-1 explains both principles, and Table 2 gives each fit in both forms.
What are clearance, transition and interference fits?
The three families differ in what happens when the parts are at the ends of their tolerance zones.
- Clearance fit: the hole is always larger than the shaft, so the parts slide or turn after assembly.
- Transition fit: the hole may be slightly larger or slightly smaller than the shaft, so assembly may need light force.
- Interference fit: the hole is always smaller than the shaft, so assembly needs force or heat.
Table 2. Commonly used fits, hole-basis and the equivalent shaft-basis combination
| Type | Hole basis | Shaft basis | Typical use |
|---|---|---|---|
| Clearance | H11/c11 | C11/h11 | Loose running, wide tolerances |
| Clearance | H9/d9 | D9/h9 | Free running where accuracy is not essential |
| Clearance | H8/f7 | F8/h7 | Close running, moderate speeds |
| Clearance | H7/g6 | G7/h6 | Sliding fit that locates accurately |
| Clearance | H7/h6 | H7/h6 | Snug location, easy to assemble and take apart |
| Transition | H7/k6 | K7/h6 | Accurate location, compromise between clearance and interference |
| Transition | H7/n6 | N7/h6 | More accurate location, some interference allowed |
| Interference | H7/p6 | P7/h6 | Location with rigidity and alignment |
| Interference | H7/s6 | S7/h6 | Medium drive fit for ordinary steel parts |
| Interference | H7/u6 | U7/h6 | Force fit for highly stressed parts |
Source: preferred fits as tabulated by AmesWeb from ISO 286-1:2010 and ANSI/ASME B4.2-1978; descriptions shortened. Check the combination in the standard before using it.
How do you calculate a fit, for example Ø25 H7/g6?
Take the limit deviations from ISO 286-2 for the size range, add them to the nominal size, and compare the limits. A positive result is clearance, a negative one is interference. All values below are for the range over 18 up to 30 mm.
Table 3. Fits on a Ø25 hole, limits and result in µm (+ clearance, − interference)
| Fit | Hole limits | Shaft limits | Result: tightest to loosest | Type |
|---|---|---|---|---|
| H8/f7 | 0 to +33 | −41 to −20 | +20 to +74 | Clearance |
| H7/g6 | 0 to +21 | −20 to −7 | +7 to +41 | Clearance |
| H7/h6 | 0 to +21 | −13 to 0 | 0 to +34 | Clearance |
| H7/k6 | 0 to +21 | +2 to +15 | −15 to +19 | Transition |
| H7/n6 | 0 to +21 | +15 to +28 | −28 to +6 | Transition |
| H7/p6 | 0 to +21 | +22 to +35 | −35 to −1 | Interference |
Source: ISO 286-2 limit deviations, 18 to 30 mm, as tabulated by Roy Mech and CADforum. Results calculated from the limits.
For H7/g6 the hole measures 25.000 to 25.021 mm and the shaft 24.980 to 24.993 mm. The minimum clearance is the smallest hole minus the largest shaft: 25.000 − 24.993 = 0.007 mm. The maximum clearance is the largest hole minus the smallest shaft: 25.021 − 24.980 = 0.041 mm. Every pair of parts that passes inspection assembles with clearance between 7 and 41 µm.
Read the transition rows the same way. H7/k6 gives a worst-case clearance of 19 µm (21 − 2) and a worst-case interference of 15 µm (15 − 0). Which one a particular pair gets depends on where each part falls in its zone.
Is ISO 286 still current in 2026?
Yes. As of November 2026, ISO lists ISO 286-1:2010 and ISO 286-2:2010 as published; the latest systematic reviews confirmed Part 1 in June 2026 and Part 2 in October 2026. The 2010 editions replaced ISO 286-1:1988 and ISO 286-2:1988, and ISO 286-1:2010 also replaced ISO 1829:1975, "Selection of tolerance zones for general purposes", which was withdrawn in April 2010. A Technical Corrigendum from 2013 applies to both parts.
In Germany the current adoptions are DIN EN ISO 286-1:2019-09 and DIN EN ISO 286-2:2019-09, both based on the 2010 editions with the 2013 corrigendum. The 2019-09 issues are corrected versions of the 2019-02 issues, and the main correction is terminology: DIN replaced "Abweichung" with "Abmaß" throughout.
How do you put a fit on a drawing?
Write the nominal size with the class: "Ø25 H7" on the bore, "Ø25 g6" on the shaft. On an assembly drawing the fit is written "Ø25 H7/g6", hole first. A tolerance written as limits, such as Ø25 +0.021/0, is equivalent and often clearer for inspection.
Check three points before a drawing goes out for quotation:
- Each part drawing carries its own class. A bore marked H7 does not tell the supplier which shaft it meets, and the fit result depends on both parts.
- Form is controlled or not, on purpose. Under ISO 8015 the default is the independence principle: a size tolerance limits the size only, not form. If the fit needs a perfect-form boundary at maximum material size, the drawing has to invoke the envelope requirement, marked Ⓔ.
- The class matches what the feature needs. A tighter grade costs more to machine and to inspect, so choose the loosest grade and fit that still works. See the CNC machining tolerances handbook for how fits, general tolerances and geometrical tolerances stack on one part.
Our network works to ISO 2768-m as standard on CNC turning and CNC milling, and tighter where the drawing specifies it. The technical drawing checklist lists the other entries a supplier needs.
Frequently asked questions
- What does H7/g6 mean?
- H7 is the tolerance class of the hole and g6 the class of the shaft. For a 25 mm size, the hole measures 25.000 to 25.021 mm and the shaft 24.980 to 24.993 mm, so the assembly always has clearance, between 0.007 and 0.041 mm.
- What is the tolerance of an H7 hole?
- It depends on the nominal size, because H7 is IT7 placed above the nominal size. Over 18 up to 30 mm it is 0 to +21 µm, over 30 up to 50 mm it is 0 to +25 µm.
- What is the difference between IT7 and H7?
- IT7 is only the width of the tolerance zone for a size range, for example 21 µm over 18 up to 30 mm. H7 adds the position: the zone starts at the nominal size and runs upward. The same IT7 width is used for classes such as g7, k7 or P7.
- What is the difference between a transition fit and an interference fit?
- An interference fit always has the shaft larger than the hole, so assembly needs force or heat. A transition fit can end up with either a small clearance or a small interference, depending on where the two parts fall within their tolerances.
- Is ISO 286 still valid?
- Yes. As of November 2026, ISO lists ISO 286-1:2010 and ISO 286-2:2010 as published and confirmed in 2026. They replaced the 1988 editions. In Germany the current adoptions are DIN EN ISO 286-1:2019-09 and DIN EN ISO 286-2:2019-09.
Sources
- ISO 286-1:2010, ISO code system for tolerances on linear sizes, Part 1 (ISO catalogue, status 90.93, confirmed 2026-06-19)
- ISO 286-2:2010, Part 2, tables of standard tolerance classes and limit deviations (ISO catalogue, status 90.93, confirmed 2026-10-05)
- ISO 1829:1975, Selection of tolerance zones for general purposes (ISO catalogue, withdrawn 2010-04-16, revised by ISO 286-1:2010)
- DIN EN ISO 286-1:2019-09 (DIN Media)
- DIN EN ISO 286-2:2019-09 (DIN Media)
- ISO 286-1:2010 standard tolerance grades IT5 to IT11 (table reproduced on Wikipedia)
- ISO 286-2 shaft limit deviations f7, g6, h6, k6, n6, p6 (Roy Mech tables)
- ISO shaft and hole tables, 18 to 30 mm: f7, h11, H7, H8, H11 (CADforum)
- Preferred fits, hole and shaft basis, from ISO 286-1:2010 and ANSI/ASME B4.2-1978 (AmesWeb)
- Engineering fit: clearance, transition and interference definitions (Wikipedia)
- ISO 8015:2011, GPS fundamentals, concepts, principles and rules (ISO catalogue, confirmed 2021-12-10)
- Envelope requirement and the ISO independency default (GD&T Basics)
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