Educational explanation, not professional design advice. Last reviewed 24 October 2026. Disclaimer
In plain English
When a shaft goes into a hole, it can be loose enough to turn, snug enough to locate accurately, or so tight that it has to be pressed in. Which one you get depends on the limits of the hole and the shaft. This relationship is called the fit.
Technical definition
A fit is the relationship between an internal and an external feature of the same nominal size that are assembled together (ISO 286-1). A clearance fit always leaves a clearance (minimum clearance ≥ 0). An interference fit always has interference (maximum clearance ≤ 0). A transition fit may give either, depending on the actual sizes.
In the hole-basis system the hole has a lower deviation of zero (tolerance letter H) and different fits are made by changing the shaft. In the shaft-basis system the shaft has an upper deviation of zero (letter h) and the hole changes.
How it works and where it is used
An ISO fit is written hole/shaft, for example H7/g6. Shaft letters a–h generally give clearance, j–n transition and p–z interference when combined with an H hole, although the exact behaviour depends on the size and grades — always check with the actual deviations.
The hole-basis system is used most often because holes are made with standard reamers and checked with standard plug gauges, while shafts are easy to turn or grind to any size. The shaft-basis system is used when one shaft must carry several parts with different fits, such as standard drawn bar.
Formula
Maximum clearance = ES − ei Minimum clearance = EI − es
Fit tolerance = hole tolerance + shaft tolerance
ES, EI = upper and lower deviations of the hole; es, ei = those of the shaft. A negative clearance is an interference.
Worked example
For a nominal size of 25 mm (ISO 286-2): hole H7 = +21/0 µm; shafts g6 = −7/−20 µm, k6 = +15/+2 µm, p6 = +35/+22 µm.
- H7/g6: max clearance 21 − (−20) = 41 µm; min clearance 0 − (−7) = 7 µm → clearance fit (a precise sliding fit).
- H7/k6: 21 − 2 = 19 µm; 0 − 15 = −15 µm → transition fit.
- H7/p6: 21 − 22 = −1 µm; 0 − 35 = −35 µm → interference fit of 1 to 35 µm.
In the workshop and in practice
Typical uses: running and sliding fits for shafts in plain bearings and for guide pins; transition fits for gears, pulleys and couplings that must be located accurately and are secured by a key; interference fits for bushes, dowels and the rings of rolling bearings that must not creep. Bearing manufacturers publish the recommended shaft and housing tolerances for their bearings.
Common mistakes and misconceptions
- Assuming a letter always means the same fit. Deviations change with size range. Use the tables for the actual diameter.
- Forgetting form and roughness. An out-of-round hole or rough surface changes the real fit, especially for interference fits.
- Ignoring temperature. Different materials (steel shaft in an aluminium housing) expand differently, so the fit at running temperature can differ from the fit at 20 °C.
- Pressing without calculation. Interference fits create high stresses in the hub. Assembly force and hub stress need a separate calculation.
Safety note
Pressing or shrink-fitting parts stores large forces. Use a suitable press and fixtures, wear eye protection, and handle heated or cooled parts with appropriate gloves.
Related calculators
- Fits Calculator – Clearance, Transition & Interference — Fit type and maximum/minimum clearance or interference of a hole and shaft.
- Limits of Size & Tolerance Calculator — Maximum and minimum limits, tolerance and mean size from nominal size and deviations.
- Rolling Bearing Life Calculator (L10) — Basic rating life L10 in revolutions and hours for ball or roller bearings, with reliability factor.
Related topics
- Limits of Size and Tolerance — The largest and smallest acceptable sizes of a feature, and the tolerance between them.
- GD&T Symbols and Terms — Reference chart of the geometric tolerance symbols, datums and feature control frames.
- Measurement Uncertainty — The quantified doubt about a measurement result, combined from all its sources.
References
- ISO 286-1: GPS — ISO code system for tolerances on linear sizes — Part 1 (fits, hole-basis and shaft-basis systems)
- ISO 286-2: Tables of standard tolerance classes and limit deviations for holes and shafts (source of the example values)
- Shigley’s Mechanical Engineering Design (Budynas and Nisbett), chapter on limits and fits