How do you calculate hole and shaft limits from a fit class?

Look up two numbers in ISO 286-1 for the size band: the fundamental deviation (set by the letter) and the IT tolerance (set by the grade number). For a hole H, the lower deviation is 0 and the upper is +IT. For a shaft g, the upper deviation is the g value (negative) and the lower is that value minus IT. Add the deviations to the nominal size for the limits.

What is the difference between clearance, transition, and interference fits?

Clearance: the largest shaft is always smaller than the smallest hole, so parts always slide (e.g. H7/g6). Interference: the smallest shaft is always bigger than the largest hole, so assembly needs force or heat (e.g. H7/p6). Transition: depending on where each part lands in its tolerance, the result can be either (e.g. H7/k6).

Why do drawings show only H7 instead of actual limits?

The fit class is compact and unambiguous: the machinist or inspector is expected to expand it using ISO 286 tables. That expansion step is a common source of FAI errors, and it is one of the interpretation steps drawing-reading software can now do automatically.

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

Nominal size (mm)

Common fit (optional)

Hole class

Hole grade (IT)

Shaft class

Shaft grade (IT)

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.

The common fits, as a table

H7 is the hole in every row, which is why it is on so many drawings: hold one hole tolerance, change the shaft, get a different fit. Values are ISO 286-2 deviations in millimetres, lower limit first.

H7 hole with g6, k6 and p6 shaftsISO 286-2 · mm · hole basis

Nominal Ø (mm)	H7 hole	g6 shaft	k6 shaft	p6 shaft
over 3 to 6	+0.000 / +0.012	-0.004 / -0.012	+0.001 / +0.009	+0.012 / +0.020
over 6 to 10	+0.000 / +0.015	-0.005 / -0.014	+0.001 / +0.010	+0.015 / +0.024
over 10 to 18	+0.000 / +0.018	-0.006 / -0.017	+0.001 / +0.012	+0.018 / +0.029
over 18 to 30	+0.000 / +0.021	-0.007 / -0.020	+0.002 / +0.015	+0.022 / +0.035
over 30 to 50	+0.000 / +0.025	-0.009 / -0.025	+0.002 / +0.018	+0.026 / +0.042
over 50 to 80	+0.000 / +0.030	-0.010 / -0.029	+0.002 / +0.021	+0.032 / +0.051
over 80 to 120	+0.000 / +0.035	-0.012 / -0.034	+0.003 / +0.025	+0.037 / +0.059
over 120 to 180	+0.000 / +0.040	-0.014 / -0.039	+0.003 / +0.028	+0.043 / +0.068

Which fit to ask forHole basis · plain English

Fit	Type	What it is for
H7/g6	Clearance	Sliding fit. Parts move freely, located accurately. Shafts in plain bearings, spigots that must come apart.
H7/h6	Clearance (close)	Slides with no perceptible play when dry. Locating spigots, removable covers.
H7/k6	Transition	Light interference or light clearance depending where in the band you land. Gears and pulleys that need a key.
H7/n6	Transition (tight)	Needs a press. Located parts that should not move but may need to come apart once.
H7/p6	Interference	Press fit, permanent. Bushings and bearing races that transmit load through the joint.
H7/s6	Interference (heavy)	Usually assembled with heat. Not intended to come apart.

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.

ISO 286 Fit Calculator

How a fit class becomes real numbers

The common fits, as a table

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

Frequently asked questions

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