ISO fits describe how a hole and a shaft of the same nominal size mate once you apply their tolerances. This calculator turns an ISO 286 callout such as H7/g6 into real upper and lower limits in millimetres, then tells you whether the pair will be a clearance, transition, or interference fit and by how much.
How it works
Every size band has a standard tolerance unit i in microns, derived from the
geometric-mean diameter D of that band:
i = 0.45 × cuberoot(D) + 0.001 × D
IT = factor(grade) × iThe factor steps through the grades — IT6 is 10i, IT7 is 16i, IT8 is 25i, and so on. The letter chosen for the hole or shaft sets the fundamental deviation, the distance from the nominal line to the near edge of the tolerance zone. Combining the deviation with the IT width gives the two limits, and comparing the hole and shaft limits yields the clearance or interference range:
max clearance = largest hole − smallest shaft
min clearance = smallest hole − largest shaftA negative result means the shaft is larger than the hole, which is interference.
Common fits and their engineering uses
| ISO callout | Fit type | Typical use |
|---|---|---|
| H7/g6 | Clearance | Free-running sliding, bearing journals |
| H7/h6 | Transition (close location) | Close locating, easy assembly by hand |
| H7/k6 | Transition | Precision locating, light knock-in |
| H7/n6 | Interference (light press) | Drive fits, gear hubs |
| H7/p6 | Interference (press fit) | Standard press fits, bushings |
| H7/s6 | Interference (force fit) | Heavy-duty press fits, heated assembly |
The H hole letter is the most common choice in the basic-hole system: it places the hole’s lower deviation at zero (the hole size starts from nominal), which simplifies tool and gauge management because one reamer or boring head can be used for many different fits by varying only the shaft.
Worked example: 25 mm H7/g6
For a 25 mm diameter H7/g6:
- The hole runs from 25.000 mm (lower) to 25.021 mm (upper)
- The shaft runs from 24.980 mm (lower) to 24.993 mm (upper)
Max clearance = 25.021 − 24.980 = 0.041 mm
Min clearance = 25.000 − 24.993 = 0.007 mmThis is a guaranteed clearance in the range 0.007–0.041 mm — a classic free-running sliding fit. The shaft will always slide into the hole, never bind, and the gap enables an oil film for bearing applications.
Switching to an interference fit
Change the shaft to p6 on the same 25 mm diameter:
- The shaft now runs from 25.022 mm (lower) to 25.035 mm (upper)
Max interference = 25.035 − 25.000 = 0.035 mm
Min interference = 25.022 − 25.021 = 0.001 mmThe shaft is always larger than the hole — the parts must be pressed together. This is used for bushings, gears, and other components that must be permanently seated and resist rotational or axial loads.
Notes
The deviation values are evaluated at each band’s geometric mean, which is exactly how the official ISO 286 tables are built, so the limits track the published data. For safety-critical or contractual applications, always verify against the official ISO 286-1 table. Confirm the IT grade with your drawing’s general tolerance note if no grade is explicitly called out.