Engineering Reference

ISO 286 Fit Calculator

Nominal size plus any hole and shaft class. Exact limits, deviations, and whether the pair is a clearance, transition, or interference fit.

By Atishay Jain, founder of Mavlon · Updated July 2026

Ø25 H7/g6
Hole H7
25.000 to 25.021
+0.000 / +0.021 mm
Shaft g6
24.980 to 24.993
-0.020 / -0.007 mm
Clearance fit
Clearance: 0.007 to 0.041 mm
Size band: over 18 to 30 mm · Source: ISO 286-1 fundamental deviations + IT grades

How a fit class becomes real numbers

A callout like Ø25 H7/g6 compresses four table lookups into six characters. The letter sets the fundamental deviation, how far the tolerance zone sits from nominal: H holes start exactly at nominal, g shafts sit slightly under. The number sets the IT grade, how wide the zone is for that size band. Capital letters describe the hole, lowercase the shaft.

The machinist and the inspector are both expected to expand this by hand, and on a Form 3 that expansion is a classic error source: the designation gets copied into the report while the actual limits never get looked up. If your drawings carry fit classes into first article inspection, that lookup step is exactly what modern FAI software can now do automatically.

Scope note: this calculator covers hole classes D, E, F, G, H, JS and shaft classes d, e, f, g, h, js, k, m, n, p across IT5 to IT11, sizes up to 500 mm, which spans the fits used on the vast majority of machined parts. Values come from the ISO 286-1 fundamental deviation and IT grade tables.

Your drawing says H7. Your Form 3 needs 25.000 to 25.021.

Mavlon expands every fit class on the drawing to actual limits automatically while building your AS9102 Form 3.

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Frequently asked questions

What does H7/g6 mean?
A sliding fit on the hole-basis system. H7 describes the hole (lower limit at nominal, IT7 wide) and g6 the shaft (slightly under nominal, IT6 wide). At 25 mm this guarantees a clearance between 0.007 and 0.041 mm.
How do you calculate limits from a fit class?
Two lookups per part in ISO 286-1 for the size band: the fundamental deviation (letter) and the IT tolerance (grade number). For an H hole: lower deviation 0, upper +IT. For a g shaft: upper deviation is the g value (negative), lower is that minus IT. Add the deviations to nominal.
Clearance vs transition vs interference?
Clearance: the largest shaft is always smaller than the smallest hole, parts always slide (H7/g6). Interference: the smallest shaft is always larger than the largest hole, assembly needs force or heat (H7/p6). Transition: either outcome is possible depending on where each part lands (H7/k6).
Why do drawings show only the class instead of limits?
The class is compact and unambiguous, and the reader is expected to expand it from the tables. That expansion is a common FAI error: the designation gets ballooned while the limits never make it into the report.

Keep reading

ISO H7 Tolerance Chart (0 to 500 mm) ISO 2768 Tolerance Calculator Thread Callout Decoder Glossary: G6 Fit Class Glossary: Envelope Principle H7 Tolerance Chart for Bore Dimensions G6 Fit Class Tolerance Limits Ballooning Software: The Buyer Guide