H (lower limit = nominal) with IT8. Typical of bored holes and of milled pocket widths. At Ø20 mm the H8 limits are 20.000–20.033 mm (+33 / 0 µm).

What are the 20 mm H8 limits?

20.000–20.033 mm, deviations +33 / 0 µm, IT8 = 33 µm in the over 18 to 24 mm band.

Engineering

Check ISO fit

H8 Hole Tolerance

This H8 tolerance chart is a step looser than H7. Bored holes, less critical location, and many milled slots use H8. An H8 fit (for example H8/f7) is an easy running pairing — not a dowel. The ISO numbers for a hole and a pocket of the same width are identical; the feature type on the drawing is what differs.

H8 · 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

Hole (internal cylinder) · IT8 · bored

Ø20H8

+0.0330

20.03320limits mm

Maximum hole size

20.033 mm

Minimum hole size

20 mm

Tolerance

33 µm

ES (upper)

+33 µm

EI (lower)

0 µm

IT band

18–30 mm

Hole catalog row

over 18 to 24 mm · Units: µm

B10	C9	C10	D8	D9	D10	E7	E8	E9	F6	F7	F8	G6	G7	H6	H7	H8	H9	H10	JS6	JS7	K6	K7	M6	M7	N6	N7	P6	P7	R7	S7	T7	U7	X7

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

H (lower limit = nominal) with IT8. Typical of bored holes and of milled pocket widths.

Formula

H8 at Ø20 mm

ES / EI +33 / 0 µm

IT8 = 33 µm

Limits 20.000 / 20.033 mm

Ø20 H8

Size band over 18 to 24 mm. The feature measures 20.000–20.033 mm.

How we use this in the toolroom

A bored clearance or a milled pocket in a die plate is usually H8 or H9, not H7. Calling a pocket H7 on a mill is asking for a grind that the job does not need. If the print says H8/f7, that is the running-fit page, not this hole class alone.

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.

Other hole classes

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.