EngiRef / Guides
Tolerances and Fits Explained: Reading H7/g6 and Choosing a Fit
Nothing is made to its nominal size. A drawing calling out a 25 mm shaft is really calling out a range of acceptable sizes, and what matters is how wide that range is and where it sits relative to nominal. When two parts work together, those ranges decide whether the assembly slides, locates, or has to be pressed together, and a designation such as H7/g6 is the shorthand for all of it.
This guide covers what the letters and numbers in an ISO 286 designation mean, why hole basis dominates in practice, the three families of fit and what each is for, and how to compute the actual clearance or interference from a pair of limits.
What a Tolerance Actually Specifies
A dimensional tolerance defines two limits of size, an upper and a lower, between which a feature is acceptable. The difference between them is the tolerance width; where that band sits relative to nominal is its position. Those are independent choices. Two shafts can share an identical band width while one is centred on nominal and the other sits entirely below it, and they behave completely differently in the same hole.
ISO 286 encodes exactly those two things. The letter gives the fundamental deviation, the position of the band relative to nominal; the number gives the international tolerance grade, or IT grade, which sets the width. So in 25 H7, the 25 is the nominal size in millimetres, H is the position, and 7 is the width. A complete fit is written as hole over shaft, such as 25 H7/g6.
Case carries real meaning and trips up every newcomer: uppercase letters always describe internal features, meaning holes and bores, while lowercase letters describe external features, meaning shafts and pins. H7 is a hole and h7 is a shaft. H specifically means a hole whose lower limit sits at nominal, so an H hole is never smaller than stated; h means a shaft whose upper limit sits at nominal.
- Letter: fundamental deviation, the position of the tolerance band relative to nominal.
- Number: IT grade, the width of the tolerance band.
- UPPERCASE letter: internal feature, a hole or bore. lowercase: external feature, a shaft or pin.
A tolerance has two independent properties: how wide the band is (the IT number) and where it sits (the letter). Changing either one changes the fit.
IT Grades and Why They Widen With Size
The IT grades run from the extremely fine, used for gauge blocks and metrology equipment, through the middle range used for ordinary machined fits, out to the coarse grades used for castings and structural work. Lower numbers mean tighter bands. The grades seen most often on machined mating features sit around IT6 to IT8, with IT6 typically on shafts and IT7 on the mating holes.
The important structural feature of the system is that an IT grade is not a fixed number of micrometres. For any given grade the permitted band widens as nominal size increases, because holding a given absolute tolerance gets harder on larger parts. This is why the standard is a table indexed by both size range and grade, and why you must look up the limits for your specific nominal size rather than carrying a remembered number from a previous job.
The hole usually gets the looser grade of the pair for a practical reason. Holes are typically produced with a fixed-size tool such as a reamer, and are harder to measure and adjust than an external diameter that can be taken down in successive passes. Giving the harder feature the wider band puts the burden where it is cheaper to carry.
Hole Basis, and Why It Won
There are two ways to build a system of fits. In hole basis the hole is fixed at H and the character of the fit is varied entirely by the shaft's letter, so H7/g6, H7/k6, and H7/p6 all share the same hole. In shaft basis the shaft is fixed at h and the hole's letter varies instead. Both are defined in the standard.
Hole basis dominates general machine design for a manufacturing reason. Holes are commonly made with tooling that comes in standard sizes, and a reamer or plug gauge for a given nominal is a stock item. Fixing the hole at H means one reamer and one gauge serve every fit at that size, while the shaft, easy to turn and easy to measure with a micrometer, absorbs all the variation.
Shaft basis is not obsolete. It suits a single continuous shaft carrying several components with different fits along its length, or work built around drawn bar or precision ground stock that already comes at an h tolerance. Rolling element bearings are the case where the convention is imposed on you: the bearing arrives made to the bearing standard, so the shaft and housing are toleranced to suit it.
Hole basis fixes the hole at H and varies the shaft. It wins because hole size is set by tooling while shaft size is set by a machine setting.
The Three Families of Fit
A clearance fit guarantees a gap: the smallest hole is still larger than the largest shaft, so every conforming pair assembles freely. This is the family for anything that rotates, slides, or has to come apart. The hole-basis designation H7/g6 falls here, giving a small but guaranteed clearance suited to precise sliding and locating, and looser shaft letters open the gap further for running fits that need room for lubricant and thermal growth.
An interference fit guarantees the opposite: the largest hole is still smaller than the smallest shaft, so the parts always overlap and assembly requires heat, chilling, or a press. The result transmits torque through friction alone and needs no key or pin. H7/p6 is a typical light press fit. These joints are strong and compact but effectively permanent, and they put a hoop stress into the outer part.
A transition fit sits between the two, with bands that overlap so a conforming pair might end up slightly loose or slightly tight depending on where each part falls. H7/k6 is the classic example. That sounds like a defect but it is the point: transition fits give excellent concentricity while remaining assemblable with a light press or a mallet.
- Clearance, for example H7/g6: always a gap. Rotation, sliding, easy service.
- Transition, for example H7/k6: may be slightly loose or slightly tight. Accurate location, assembly still practical.
- Interference, for example H7/p6: always an overlap. Torque by friction, essentially permanent.
Computing the Actual Fit
Once you have looked up the four limits, the arithmetic is short and worth doing explicitly rather than trusting the designation's name. Maximum clearance is the largest hole minus the smallest shaft; minimum clearance is the smallest hole minus the largest shaft. A positive minimum means a clearance fit, a negative maximum means interference, and a negative minimum with a positive maximum means the bands overlap, which is a transition fit.
Suppose a drawing gives a hole as 25.000 to 25.021 mm and the mating shaft as 24.980 to 24.993 mm. Maximum clearance is 25.021 − 24.980 = 0.041 mm and minimum clearance is 25.000 − 24.993 = 0.007 mm. Both are positive, so this is a clearance fit varying by about a factor of six between its loosest and tightest conforming cases. That spread, not the nominal, is the real design information.
Two effects routinely undo a fit that looked correct on paper. Temperature is the first: a fit computed at room temperature can change substantially in service, especially between dissimilar materials such as a steel shaft in an aluminum housing, and the differential expansion can be comparable to the whole tolerance band, as the companion guide on thermal expansion and clearances works through. Form is the second: ISO 286 controls size, not roundness or cylindricity, so a bore within size limits but out of round will not behave as calculated.
Finally, tolerance costs money in a strongly non-linear way. Each step to a finer IT grade pushes the part toward a slower process, an extra operation such as grinding, more inspection, and a higher scrap rate. Specify the loosest fit that does the job and leave the rest at a general tolerance.
Max clearance = largest hole − smallest shaft. Min clearance = smallest hole − largest shaft. A negative value means interference.
Frequently asked questions
What does H7/g6 actually mean?
It is a hole-basis fit at some nominal size. H7 is the hole: the letter H puts its lower limit at nominal, and the 7 sets the band width from the IT grade table. g6 is the shaft, positioned slightly below nominal by the letter g with a narrower IT6 band. Together they produce a guaranteed small clearance suited to precise sliding and locating.
Why is the hole usually given a looser tolerance grade than the shaft?
Because holes are harder and more expensive to control. They are often produced with a fixed-size tool such as a reamer and are more awkward to measure, while an external diameter can be taken down in successive passes and checked with a micrometer. Pairing an IT7 hole with an IT6 shaft puts the tighter requirement on the cheaper feature to control.
Should I use hole basis or shaft basis?
Hole basis for most general machine design, because one standard reamer and gauge then serve every fit at a given nominal size. Shaft basis suits a single continuous shaft carrying several parts with different fits, or work built around drawn or precision ground stock. With rolling element bearings the bearing's own tolerances decide, and the shaft and housing are toleranced to suit it.
Is a transition fit clearance or interference?
Either, depending on where the two parts actually land within their tolerance bands. The bands deliberately overlap, so one conforming pair may assemble with a slight gap and another with slight interference. That is the intended behaviour: it buys good concentricity and location while keeping assembly practical with a light press or a mallet.