ISO 286 fits and tolerances explained.
ISO 286 is the system behind callouts like ∅25 H7/g6. It turns a plain diameter into a precise fit between a hole and a shaft. This guide explains the hole and shaft system, IT grades, the three families of fits, and how to read a fit callout without reaching for the table every time.
Why ISO 286 exists
Without a standard, "make this shaft fit that hole" is an argument waiting to happen. ISO 286 (the ISO system of limits and fits) replaces guesswork with a shared language: a couple of letters and numbers that tell the machinist exactly how much clearance or interference the designer wants, and tell the inspector exactly what limits to check. It is used worldwide and is the metric backbone of nearly every drawing coming through an Indian or global machine shop.
How the system works
Every fit combines two tolerances: one on the hole and one on the shaft. Each tolerance has two parts — a fundamental deviation (a letter) that fixes where the tolerance band sits relative to the nominal size, and an IT grade (a number) that fixes how wide the band is. Capital letters are holes, lowercase letters are shafts. So in ∅25 H7/g6, both parts share the 25 mm nominal, the hole is H7, and the shaft is g6.
Letters and grades
The letter positions the band. For holes, letters A to H sit progressively closer to nominal from above, with H placing the lower limit exactly on the nominal size. For shafts, a to h sit below nominal, with h placing the upper limit on nominal; letters past h (j, k, n, p, s, u) push the shaft into and past the nominal for interference.
The IT grade sets the band width:
| IT grade | Typical use |
|---|---|
| IT01 – IT4 | Gauges, high-precision instruments |
| IT5 – IT7 | Precision fits (bearings, spindles) |
| IT8 – IT11 | General machining |
| IT12 – IT16 | Rough machining, as-cast, as-forged |
A key point: the same IT grade is a wider absolute tolerance on a bigger part. IT7 on a 10 mm feature is about 15 um; on a 100 mm feature it is about 35 um. The standard scales the band with size so precision stays proportionate.
Hole basis vs shaft basis
You set the fit by holding one member constant and varying the other.
- Hole basis (preferred): the hole is always H, and you change the shaft letter to set clearance or interference. Holes are made with standard reamers and checked with standard plug gauges, so keeping the hole constant is the cheaper route.
- Shaft basis: the shaft is always h, and you change the hole letter. Used where a single shaft (such as a length of ground bar) runs through several different fits.
The three fit types
Every combination falls into one of three families:
| Fit type | Behaviour | Example |
|---|---|---|
| Clearance | Always a gap; parts move freely | H7/g6, H8/f7 |
| Transition | Slight clearance or slight interference | H7/k6, H7/n6 |
| Interference | Shaft always larger; press or shrink fit | H7/p6, H7/s6 |
Common fits you will actually use
| Callout | Type | Where it is used |
|---|---|---|
| H7/g6 | Close running clearance | Precision spindles, guide pins |
| H8/f7 | Running clearance | General rotating shafts, bushings |
| H11/c11 | Loose clearance | Pivots, agricultural pins |
| H7/k6 | Transition | Located gears, pulleys with key |
| H7/p6 | Light interference | Dowel pins, bushes |
| H7/s6 | Medium interference | Press-fit bushings, collars |
Reading H7/g6
Take ∅25 H7/g6. For the 25 mm size band, H7 gives the hole limits 25.000 to 25.021 mm (lower limit on nominal, IT7 band of 21 um). The shaft g6 gives 24.993 to 24.980 mm (g offsets the band 7 um below nominal, IT6 band of 13 um). The maximum clearance is 25.021 − 24.980 = 0.041 mm; the minimum clearance is 25.000 − 24.993 = 0.007 mm. The joint always has a gap between 7 and 41 um — a clean running fit. Rather than look these limits up by hand, the ISO 286 fits calculator returns the hole and shaft limits and the resulting clearance for any size and fit instantly.
Those computed limits become inspection characteristics on the part. CadNexa's auto-ballooning tool numbers each toleranced diameter and exports the limits, so the fit you designed is the fit the inspector verifies.
Common ISO 286 mistakes
- Mixing case. H is a hole, h is a shaft. Writing the wrong case inverts the entire fit.
- Assuming the IT number is the tolerance in microns. It is a grade; the actual band depends on the size band too.
- Choosing interference without checking stress. A press fit on a thin hub can crack it — check hoop stress before specifying H7/s6.
- Ignoring temperature. Clearance fits on dissimilar metals can close or open with heat; allow for it on engines and pumps.
- Specifying tighter than needed. Going from IT7 to IT5 can multiply cost for no functional benefit. Match the grade to the job.