What does H7/g6 mean?
An ISO 286 fit callout like H7/g6 packs four pieces of information — two letters and two numbers — into seven characters; at Ø25 mm it resolves to a hole of 0 / +0.021 mm and a shaft of −0.020 / −0.007 mm.
The four symbols
| Symbol | Means | Value at Ø25 mm |
|---|---|---|
| H | Hole, fundamental deviation EI = 0 — zone sits entirely above the nominal size. | EI = 0 mm |
| 7 | Hole grade IT7 — width of the hole's tolerance zone. | IT7 = 21 µm |
| g | Shaft, fundamental deviation es tabulated by g6 — zone sits just below the nominal size. | es = −0.007 mm |
| 6 | Shaft grade IT6 — width of the shaft's tolerance zone, from the fundamental deviation table. | IT6 = 13 µm |
Result (H7/g6 at Ø25 mm)
Fit diagram (µm · zero line = nominal)
Capital vs lower case
ISO 286 always writes the hole class first, in capitals, and the shaft class second, in lower case — H7/g6, never g6/H7 or h7/G6 unless the fit really is written that way on purpose. Capital letters (A…ZC) are fundamental deviations applied to a hole; the same letters in lower case (a…zc) are the identical deviations applied to a shaft. H and h both mean “zero deviation” on their respective part, which is what makes them the reference class for the hole-basis and shaft-basis systems — see hole basis vs shaft basis.
Reading it on a drawing
On a mechanical drawing the callout sits next to the dimension it tolerances, e.g. “Ø25 H7” on the hole's diameter and “Ø25 g6” on the shaft's — each part is dimensioned and toleranced independently; the fit (H7/g6) is what you get once both parts are made to print and assembled. Some drawings add the limits in brackets after the class, e.g. “Ø25 H7 (+0.021/0)”, so the machinist doesn't need the standard to hand — those are exactly the Upper/Lower values in the table above.
Try a different size or class pair in the calculator, or use the fit finder if you know the clearance you want but not which class gives it.