ISO Fit & Tolerance Reference
What the common hole-basis fit callouts actually mean, and which one to reach for.
| Fit | Type | What it's for |
|---|---|---|
| H7/g6 | Clearance | Close running fit — accurate location with guaranteed clearance, needs lubrication to run |
| H7/h6 | Clearance | Sliding fit — parts locate accurately and assemble/disassemble by hand, minimal play |
| H7/k6 | Transition | Location fit — accurate location, light interference or clearance depending on actual size, assembled with hand pressure or a soft mallet |
| H7/n6 | Transition | Tighter location fit — more rigidity and alignment than k6, usually needs an arbor press to assemble |
| H7/p6 | Interference | Light press fit — permanent or semi-permanent assembly, requires real force to assemble |
| H7/s6 | Interference | Medium/heavy press fit — permanent assembly, often used with heat-shrinking the outer part or freezing the inner part |
Reading the letters and numbers
In this hole-basis system, the capital letter and number (H7) describe the hole's tolerance, and the lowercase letter and number (g6, h6, p6, and so on) describe the shaft's tolerance relative to that same basic size. "Hole basis" means the hole is held to one standard tolerance (H7 is extremely common) and the fit is adjusted entirely by changing the shaft's tolerance class — this is the more common convention in general machine design, since holes (especially reamed or bored ones) are often harder to adjust in small increments than a shaft turned to size.
Clearance, transition, and interference, in plain terms
A clearance fit always leaves the shaft slightly smaller than the hole, guaranteeing the parts slide together with some play. An interference fit always makes the shaft slightly larger than the hole, guaranteeing the parts have to be forced together and won't come apart on their own. A transition fit sits in between — depending on where each part actually lands within its tolerance band, an individual pair might assemble with a hint of clearance or a hint of interference, which is why transition fits are chosen when accurate location matters more than a guaranteed clearance or guaranteed grip.
Exact tolerance values depend on the nominal size
This reference describes what each fit class is used for, not the exact tolerance band in microns or thousandths — those values shift with the nominal diameter (a 10mm H7 hole and a 100mm H7 hole have different absolute tolerance bands even though they're both "H7"). For the precise numeric limits at your specific size, consult the full ISO 286 tolerance tables rather than a general-purpose reference like this one.