A tolerance class like H7 or g6 packs two ideas into a few characters. The letter sets the fundamental deviation — where the tolerance band sits relative to the nominal size (the zero line). The number is the IT grade — how wide the band is. Put a hole class and a shaft class together, at a given nominal size, and you have a fit: the range of clearance or interference you'll actually get.
At Ø50, H7 gives a hole of 50.000–50.025 mm (deviation 0 to +25 µm). g6 gives a shaft of 49.975–49.991 mm (−25 to −9 µm). The loosest pairing leaves 50 µm of clearance, the tightest 9 µm — a classic close-running fit for a rotating shaft.
Swap the shaft to p6 and the zones overlap: instead of clearance you get 1–42 µm of interference — a press fit that has to be forced together.
Clearance: the hole is always bigger — parts slide. Interference: the shaft is always bigger — parts must be pressed or shrink-fit. Transition: depending on where each part lands in its band, you might get a slight gap or a slight squeeze — used for accurate location.
Hole-basis keeps the hole at H (lower deviation zero) and varies the shaft letter — the usual choice, since holes are harder to machine to size. Shaft-basis keeps the shaft at h and varies the hole. Both reach the same fits; hole-basis needs fewer reamers and gauges.
Both the IT grade width and the fundamental deviation grow with nominal size — a 10 µm band on a 5 mm part would be impossible on a 400 mm one. ISO 286 scales them using the standard tolerance unit, which rises with the cube root of size.
Start from the function: free-running (H8/d9, H9/d9), close-running (H7/g6), location/transition (H7/k6, H7/n6), press/interference (H7/p6, H7/s6), drive/shrink (H7/u6). Then confirm against your load, material and assembly method — this tool gives the numbers, not the engineering judgement.
Every material grows or shrinks with temperature, but at different rates — an aluminium housing expands roughly twice as fast as a steel shaft inside it. Push either part far enough from the 20°C reference temperature the tolerance was specified at, and a clearance fit can tighten into interference, or vice versa. The thermal section below makes that concrete for your actual hole and shaft materials.
This page has changed in 44 archived releases, but each of those was a site-wide pass, so none is attributable to this tool on its own and none is listed here. That is not a claim that the tool never changed — a release that reworked many pages at once may well have altered this one too. The changelog has them.