ISO Fits and Tolerances Calculator —
H7, g6, and the Standard Hole Basis Fits

How to Read a Fit Callout Like H7/g6

An ISO fit callout is a hole tolerance and a shaft tolerance written together. The capital letter is the hole, the lowercase letter is the shaft, and each letter sets where the tolerance zone sits relative to the nominal size (the fundamental deviation). The number is the IT grade, which sets how wide the tolerance zone is. H is the hole zone that starts exactly at nominal and goes up, which is why hole basis systems build everything around H holes: you ream or bore one standard hole and choose the shaft letter to get the fit you want.[1]

So Ø25 H7/g6 means: hole 25.000 to 25.021 mm (H7 = +21/0 µm at this size), shaft 24.980 to 24.993 mm (g6 = −7/−20 µm). Every possible combination clears, by 7 to 41 µm, which is what makes it a sliding fit: the shaft always turns and slides, with the minimum practical play.[1]

The Preferred Hole Basis Fits

ISO 286 allows hundreds of combinations, but standard practice (ANSI B4.2 preferred fits) narrows them to a short list. The calculator covers these seven; looser (H11/c11) and heavier press fits (H7/s6, H7/u6) exist in the standard but split some size bands, so pull those from the tables directly.[2]

IT Tolerance Grades (ISO 286)

The IT grade is the width of the tolerance zone in micrometres, growing with both size and grade number. IT6 and IT7 are the precision machining grades most fit callouts use; IT9 through IT11 cover general machining and stock tolerances. For scale, IT7 at Ø25 is 21 µm, which is 0.0008 inches.[1]

All values in micrometres (µm). Table generated from the ISO 286-1 standard tolerance values; see sources below.

H7/g6 Sliding Fit Limits by Size

The most searched fit, worked through every size band. Hole and shaft deviations are in micrometres from nominal; the last two columns are the guaranteed minimum and maximum clearance.[1]