What is the difference between a clearance, transition, and interference fit?

A clearance fit always leaves a gap so the parts slide together freely. An interference fit always has overlap so the parts must be pressed or shrunk together. A transition fit can be either a slight clearance or slight interference depending on where the two parts fall within their tolerance bands.

What do the IT grade numbers mean?

IT stands for International Tolerance grade. A lower number such as IT5 is a very tight tolerance, while IT18 is very loose. Each grade scales the standard tolerance unit, which itself grows with diameter, so the same grade gives a wider band on a larger part.

Why are hole letters uppercase and shaft letters lowercase?

ISO 286 uses uppercase letters for internal features such as holes and lowercase letters for external features such as shafts. The letter sets where the tolerance zone sits relative to the nominal size, and the case keeps hole and shaft callouts from being confused.

What is the most common engineering fit?

H7/g6 is a very common sliding clearance fit and H7/h6 is a close location fit. H7/p6 and H7/s6 are typical interference fits for press-fitting parts. The H letter keeps the hole at nominal as the basic-hole reference.

How accurate are the values shown?

The tool uses the ISO 286 fundamental-deviation and standard-tolerance-unit formulas evaluated at the geometric mean of each size band, which matches the published tables closely. For safety-critical or contractual work, always confirm against the official ISO 286 table.

ISO Tolerance & Fit Calculator

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) × i

The 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 shaft

A negative result means the shaft is larger than the hole, which is interference.

Example and notes

For a 25 mm H7/g6 fit, the hole runs 25.000 to 25.021 mm and the shaft runs 24.980 to 24.993 mm, giving a guaranteed clearance of roughly 0.007 to 0.041 mm — a classic free-running sliding fit. Switch the shaft to p6 and the range goes negative, an interference fit that must be pressed together. 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; still verify the table for any safety-critical or contractual tolerance.