A shaft and hub get the H7/g6 fit at Ø25 mm. How much clearance is actually between them?
One code, infinitely many dimensions
An ISO 286 fit has two parts: a hole class (capital letter + grade, e.g. H7) and a shaft class (lowercase letter + grade, e.g. g6). Both describe a deviation from the nominal size, but that deviation is expressed in µm and depends on the diameter band the size falls in — larger diameters get a wider tolerance field, even for the same letter and the same grade. So H7/g6 tells you the shape of the fit (here: always clearance, never clamping), but you only read off the actual clearance in µm once you know which band (e.g. >18–≤30 mm) the nominal diameter falls in.
Worked example: H7/g6 at Ø25 mm
At Ø25 mm the size falls in the >18–≤30 mm band. There, H7 gives a hole deviation of ES = +21 µm, EI = 0 µm; g6 gives a shaft deviation of es = −7 µm, ei = −20 µm. The hole is therefore 25.000 to 25.021 mm, the shaft 24.980 to 24.993 mm. The clearance is between 7 µm (minimum) and 41 µm (maximum) — always clearance, matching its use as a precise sliding location fit.
Convert the deviation to a real dimension
+21/0 µm is correct, but it's one arithmetic step removed from what happens on the drawing and on the machine. Add the deviation to the nominal diameter and you get the two absolute limit dimensions actually machined: 25.000–25.021 mm for the hole, 24.980–24.993 mm for the shaft. Those absolute dimensions are what an operator checks, not the deviation pair itself.
Assumptions and applicability limits
The 0–50 mm table is hand-verified per class; beyond 50 mm the tool uses the ISO 286-1 formulas, but only for classes H, h, JS, js, G, g, F, f, D and d — those were validated against the verified table. Classes like j, k, m, n, p and s have no usable closed-form formula and stay limited to 50 mm. At the smallest band (>0–3 mm), several classes are still entirely missing (the tool shows '—') because no confirmed source table was available, and the first band of H7/p6 is flagged as disputed until a primary ISO 286-2 table confirms or refutes it. Check the tool's own notice for the combination you chose.