Lower-case f = shaft. Do not read 20f7 as 20 F7 (a hole). At Ø20 mm the f7 limits are 19.959–19.980 mm (-20 / -41 µm).

What are the 20 mm f7 limits?

19.959–19.980 mm, deviations -20 / -41 µm, IT7 = 21 µm in the over 18 to 24 mm band.

Engineering

Check ISO fit

20 mm f7 Tolerance (20f7)

A 20 mm f7 shaft measures 19.959–19.980 mm. Lower-case f is an external feature — not a hole. IT7 in this size band is 21 µm.

f7 · Ø20 mm example · over 18 to 24 mm

Feature

Used when a class could be a hole or a pocket. Shafts are lowercase.

Nominal size (mm)over 18 to 24 mm · IT 18–30 mmFind any class

Type H7, h6, F8, k6, B10, or “20 H7”. Every ISO letter × grade we can compute is listed.

As on a drawing

Shaft (external) · IT7 · easy running

Ø20f7

−0.020−0.041

19.9819.959limits mm

Maximum shaft size

19.98 mm

Minimum shaft size

19.959 mm

Tolerance

21 µm

es (upper)

−20 µm

ei (lower)

−41 µm

IT band

18–30 mm

Shaft catalog row

over 18 to 24 mm · Units: µm

b9	c9	d8	d9	e7	e8	e9	f6	f7	f8	g5	g6	h4*	h5	h6	h7	h8	h9	js5	js6	js7	k5	k6	m5	m6	n5*	n6	p6	r6	s6	t6	u6	x6

Catalog row is the regularly used set. The finder above also lists every other computable class (H11, h4*, n5*, T7, x6, …). * = old JIS extra.

What the code means

Lower-case f = shaft. Do not read 20f7 as 20 F7 (a hole).

Formula

f7 at Ø20 mm

es / ei -20 / -41 µm

IT7 = 21 µm

Limits 19.959 / 19.980 mm

Ø20 f7

Size band over 18 to 24 mm. The feature measures 19.959–19.980 mm.

How we use this in the toolroom

When a drawing says Ø20 f7, that is a turned or ground pin at this diameter. Confirm letter case before you machine: F7 would be a hole.

The numbers come from ISO 286 tables, the same charts Accurate Industries uses in Pune for dies, moulds and precision tooling. AlwaysReach is the browser version of that lookup — see our story.

Class notes for f7 (any diameter) live on the f7 shaft page.

Common hole/shaft pairings

Full type-ahead for every ISO class is on the ISO tolerance calculator. Convert the millimetre result with the unit converter.

Frequently asked questions

What does an H7 hole mean?+

The capital letter is the fundamental deviation for an internal feature; H means the lower deviation is zero (the hole never goes below nominal). The number is the IT grade — IT7 is a precision finishing grade used for bearings, bushings and locational fits. A 20 mm H7 hole is 20.000–20.021 mm.

What is the tolerance of H7? What is an H7 bore?+

H7 is a hole class, not a single micrometre number: the tolerance of H7 is IT7 with a lower limit at nominal. An H7 bore is a reamed location hole — dowel, bush or bearing. This page is an H7 tolerance chart: enter a diameter (20 mm, 30 mm, 65H7, 100H7, 120H7) to read max and min.

Where is the k6 tolerance chart or H8 tolerance chart?+

k6 is a shaft class (transition with H7). Open the k6 tolerance chart for deviations and a 20 mm example. H8 is a hole/pocket class; the H8 tolerance chart also covers an H8 fit such as H8/f7. Neither is the same as H7.

Is h7 the same as H7?+

No. H7 is a hole (internal). h7 is a shaft (external). For 20 mm, H7 is 20.000–20.021 mm and h7 is 19.979–20.000 mm. They sit on opposite sides of nominal and must not be mixed up.

Is a pocket H8 the same as a hole H8?+

The ISO 286 numbers are the same: an internal H8 class is +IT8 / 0 whether it is a cylindrical hole or a pocket width. The calculator lists holes and pockets separately because the usual grades differ — reamed holes are typically H7; milled pockets are typically H8, H9 or H11 — and so the result is labelled as a hole or a pocket, not as a shaft.

How do I read a fit like H7/g6?+

H7 is the hole, g6 is the shaft. Because g is a small negative shaft deviation, H7/g6 is a close running (clearance) fit: the shaft is slightly under nominal and the hole is slightly over, so parts assemble by hand with little play.

Are the values from ISO 286?+

Standard tolerances (IT grades) and fundamental deviations follow ISO 286-1/2 tables for the usual nominal ranges up to 500 mm, including the Δ correction for internal K, M, N and P–S classes. Always confirm critical fits against the current standard and your drawing notes.

Can I enter custom deviations?+

Yes. Switch to custom mode and type upper and lower deviations (in mm or µm). The calculator returns maximum size, minimum size, tolerance width and a diagram of the zone relative to nominal.