ISO 273:1979

Clearance holes for metric bolts

The complete ISO 273 table, M1 through M150, in fine, medium and coarse series. Read off the standard itself rather than copied from another chart — which turned out to matter.

Find a clearance hole

Bolt size

Series

9mm

M8 through a medium clearance hole — 9mm nominal, tolerance field H13 if you need one on the drawing. That leaves 1mm total play, so the bolt can sit up to 0.5mm off centre.

The default for general assembly, and the one to reach for unless something says otherwise. Absorbs ordinary tolerance stack-up without letting the part wander.

Supra’s fastener check verifies this size against ISO 273 automatically.

The full table

Nominal clearance hole diameter d_h in millimetres, for thread diameter d. Rows marked are the sizes Supra’s fastener check verifies automatically.

ISO 273:1979 clearance hole diameters, M1 to M150
Bolt	fineH12	mediumH13	coarseH14	playmedium
M1	1.1	1.2	1.3	0.2
M1.2	1.3	1.4	1.5	0.2
M1.4	1.5	1.6	1.8	0.2
M1.6	1.7	1.8	2	0.2
M1.8	2	2.1	2.2	0.3
M2	2.2	2.4	2.6	0.4
M2.5	2.7	2.9	3.1	0.4
M3	3.2	3.4	3.6	0.4
M3.5	3.7	3.9	4.2	0.4
M4	4.3	4.5	4.8	0.5
M4.5	4.8	5	5.3	0.5
M5	5.3	5.5	5.8	0.5
M6	6.4	6.6	7	0.6
M7	7.4	7.6	8	0.6
M8	8.4	9	10	1
M10	10.5	11	12	1
M12	13	13.5	14.5	1.5
M14	15	15.5	16.5	1.5
M16	17	17.5	18.5	1.5
M18	19	20	21	2
M20	21	22	24	2
M22	23	24	26	2
M24	25	26	28	2
M27	28	30	32	3
M30	31	33	35	3
M33	34	36	38	3
M36	37	39	42	3
M39	40	42	45	3
M42	43	45	48	3
M45	46	48	52	3
M48	50	52	56	4
M52	54	56	62	4
M56	58	62	66	6
M60	62	66	70	6
M64	66	70	74	6
M68	70	74	78	6
M72	74	78	82	6
M76	78	82	86	6
M80	82	86	91	6
M85	87	91	96	6
M90	93	96	101	6
M95	98	101	107	6
M100	104	107	112	7
M105	109	112	117	7
M110	114	117	122	7
M115	119	122	127	7
M120	124	127	132	7
M125	129	132	137	7
M130	134	137	144	7
M140	144	147	155	7
M150	155	158	165	8

The last column is not in the standard. It is d_h − d for the medium series: the total play, which is the number that actually tells you how far the joint can shift, and the one no chart prints.

Which series do you want?

This is the part the numbers cannot tell you, and the reason a bare table sends people back to search again.

fineH12

Use when the joint locates the part: little room to shift, so the hole position has to be right. Needs accurate drilling and matching hole patterns.

mediumH13

The default for general assembly, and the one to reach for unless something says otherwise. Absorbs ordinary tolerance stack-up without letting the part wander.

coarseH14

Use when parts must go together despite accumulated error: long bolt patterns, weldments, sheet metal, or anything positioned by something other than the fastener.

If nothing in your drawing says otherwise, use medium. It is the general-assembly default and the one most bolt patterns assume.

What the standard actually says

This International Standard specifies clearance hole diameters for general purpose applications. These values result from bearing area calculations in connection with ISO bolt and nut product standards.
NOTE — Clearance holes for special applications should be selected on the basis of design requirements.
ISO 273:1979, clause 1 — Scope and field of application

Two things in that are worth slowing down for. The sizes are not arbitrary round-ups: they come out of bearing-area calculations against the matching ISO bolt and nut standards, which is why the jump from M8 to M10 is bigger than you would guess. And general purpose is a real limit — a hole carrying a fitted bolt, a dowel, or a shear load is a design decision, not a table lookup.

The tolerance grades are informative

Nearly every clearance hole chart online prints H12, H13 and H14 beside the three series as though they were part of the specification. The standard is explicit that they are not:

The following tolerance fields are given for information only, for use where it is desirable to specify tolerances:
fine series : H12
medium series : H13
coarse series : H14

So the hole diameter is the requirement; the grade is offered for people who need to put a tolerance on a drawing. You are not out of spec for tolerancing it differently.

The chamfer note

In cases where it is necessary to avoid interference between the edge of the hole and the underhead fillet of the bolt, a chamfer is recommended.

This one catches people out on machined parts. A bolt has a radius where the shank meets the underside of the head, and a hole drilled exactly to size with a sharp edge can foul it — the head then sits proud and the joint never fully clamps. It reads as an over-torqued bolt or a warped flange when it is a missing chamfer.

ISO has withdrawn this edition

First edition, 1979-06-15. ISO’s catalogue now lists it as withdrawn, though it remains the table in universal use and is the one adopted by EN 20273 / DIN EN 20273:1992, whose title names ISO 273:1979 directly. It is published here because it is what people mean when they say ISO 273 — but if you are writing it into a drawing for a regulated part, cite the regional adoption you are actually held to, and check it against ISO’s catalogue entry rather than this page.

ISO 273 is also not the only answer. ASME B18.2.8 covers clearance holes on the US side and agrees at most common sizes while diverging at others; if your drawing calls out a standard, use that one.

Where these numbers came from

Off a scan of ISO 273:1979 itself — the dimensions table on printed page 2 — rendered and read rather than copied from another website.

That was not the original plan. The first draft of this table was checked against the engineering sites that rank for these terms, and they did not agree: two published different values for M14, M22, M27, M30, M33, M36, M39, M42, M45 and M48, and a third published this exact table under the heading ASME B18.2.8. Any one of them would have produced a page that looked identical to this one and was wrong above M20, with nothing on it to reveal that.

So the standard was read directly and every digit checked against it. We mention this less as a boast than as a warning about the genre: a clearance hole chart with no stated source is a chart somebody copied, and you cannot tell from looking at it which one they copied.

What Supra does with this

Supra is an AI CAD tool that runs engineering checks against every part it generates and shows you the result of each one. Fastener fit is one of them: given a declared fastener, it checks the modelled hole against ISO 273 and reports pass, fail, or deferred with the measured diameter.

It knows 8 sizes — M2, M2.5, M3, M4, M5, M6, M8, M10 — which is M2 through M10 without the intermediate sizes. Hand it an M12, or an M7, and it returns deferred, naming the sizes it does know, rather than quietly passing a hole it never checked. That distinction is most of the point: a check that was skipped and a check that passed look the same when all you are shown is a green tick, and the difference between them is exactly where a part goes wrong.

Check an STL free — no account, no key. Or see what Supra is.