Spanner Size Chart: Metric Bolts M1.6 to M64 (ISO and DIN)

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Reviewed by MLC Editorial Team | 2 October 2026

A variety of chrome vanadium tools including pliers and wrenches on burlap. — Photo by mefodiy on Pexels

Spanner and hex-key sizes for metric hexagon bolts, nuts and socket cap screws, M1.6 to M64. The spanner size is the across-flats (A/F) width of the bolt head, set by ISO 4014 and ISO 4017 for hex bolts and ISO 4032 for nuts. An M10 hex bolt takes a 16 mm spanner under the current ISO standard, an M12 takes 18 mm, an M16 takes 24 mm and an M36 takes 55 mm. A hex nut takes the same spanner as the bolt it fits.

Bolt size	Spanner A/F ISO (mm)	Older DIN A/F (mm)	Hex key (mm)
M1.6	3.2	-	1.5
M2	4	-	1.5
M2.5	5	-	2
M3	5.5	-	2.5
M3.5	6	-	-
M4	7	-	3
M5	8	-	4
M6	10	-	5
M8	13	-	6
M10	16	17	8
M12	18	19	10
M14	21	22	12
M16	24	-	14
M18	27	-	14
M20	30	-	17
M22	34	32	17
M24	36	-	19
M27	41	-	19
M30	46	-	22
M33	50	-	-
M36	55	-	-
M39	60	-	-
M42	65	-	-
M45	70	-	-
M48	75	-	-
M52	80	-	-
M56	85	-	-
M60	90	-	-
M64	95	-	-

What size bolt fits my spanner?

Read this table when you know the spanner and need the thread. Measure across the flats of the head or nut, find that figure in the left column and read off the bolt size. Sizes marked older DIN are the pre-ISO head widths still found on older machinery and on a lot of current stock.

Spanner size (mm)	Fits bolt and nut size
3.2	M1.6
4	M2
5	M2.5
5.5	M3
6	M3.5
7	M4
8	M5
10	M6
13	M8
16	M10
17	M10 (older DIN bolts)
18	M12
19	M12 (older DIN bolts)
21	M14
22	M14 (older DIN bolts)
24	M16
27	M18
30	M20
32	M22 (older DIN bolts)
34	M22
36	M24
41	M27
46	M30
50	M33
55	M36
60	M39
65	M42
70	M45
75	M48
80	M52
85	M56
90	M60
95	M64

A 13 mm spanner fits an M8 bolt, a 17 mm spanner fits an M10 bolt made to the older DIN standard, a 19 mm spanner fits a DIN M12 and a 24 mm spanner fits an M16.

Large bolts: M33 to M64

Above M30 the spanner sizes climb quickly: M36 takes 55 mm, M42 takes 65 mm, M48 takes 75 mm, M56 takes 85 mm and M64 takes 95 mm. M33, M39, M45, M52 and M60 are second-choice sizes in the ISO series, so they are less often stocked but turn up on older plant and structural work.

Imperial spanner sizes: AF, Whitworth, BSF and BA

Imperial fasteners use three unrelated spanner systems, and a spanner from one will not fit another. AF spanners fit Unified (UNC and UNF) bolts and are marked with the across-flats width. Whitworth and BSF spanners are marked with the bolt diameter, not the head width. BA spanners are marked with the BA number.

AF spanner sizes for UNC and UNF bolts

Across-flats widths for standard hex bolts and hex nuts to ASME B18.2.1 and B18.2.2. The nut takes the same spanner as the bolt head on every size except 7/16 inch, where the nut is 11/16 inch and the bolt head is 5/8 inch and 9/16 inch, where the nut is 7/8 inch and the bolt head is 13/16 inch.

Bolt diameter (UNC/UNF)	Bolt head A/F	Bolt head A/F (mm)	Hex nut A/F	Hex nut A/F (mm)
1/4"	7/16"	11.11	7/16"	11.11
5/16"	1/2"	12.70	1/2"	12.70
3/8"	9/16"	14.29	9/16"	14.29
7/16"	5/8"	15.88	11/16"	17.46
1/2"	3/4"	19.05	3/4"	19.05
9/16"	13/16"	20.64	7/8"	22.23
5/8"	15/16"	23.81	15/16"	23.81
3/4"	1-1/8"	28.58	1-1/8"	28.58
7/8"	1-5/16"	33.34	1-5/16"	33.34
1"	1-1/2"	38.10	1-1/2"	38.10
1-1/8"	1-11/16"	42.86	1-11/16"	42.86
1-1/4"	1-7/8"	47.63	1-7/8"	47.63
1-3/8"	2-1/16"	52.39	2-1/16"	52.39
1-1/2"	2-1/4"	57.15	2-1/4"	57.15

Whitworth and BSF spanner sizes

A Whitworth spanner is marked with the diameter of the Whitworth bolt it was originally made for, so a spanner marked 3/8 W measures 0.710 inch (18.03 mm) across the jaws. BSF bolts use a head one size smaller than Whitworth bolts of the same diameter, so the same spanner fits the BSF bolt one size up: the 3/8 W spanner fits a 7/16 inch BSF bolt. Spanners are often marked with both sizes for that reason.

Spanner marking	A/F (in)	A/F (mm)	Fits BSW bolt (original hexagon)	Fits BSF bolt
3/16 W	0.445	11.30	3/16"	1/4"
1/4 W	0.525	13.34	1/4"	5/16"
5/16 W	0.600	15.24	5/16"	3/8"
3/8 W	0.710	18.03	3/8"	7/16"
7/16 W	0.820	20.83	7/16"	1/2"
1/2 W	0.920	23.37	1/2"	9/16"
9/16 W	1.010	25.65	9/16"	5/8"
5/8 W	1.100	27.94	5/8"	11/16"
11/16 W	1.200	30.48	11/16"	3/4"
3/4 W	1.300	33.02	3/4"	7/8"
7/8 W	1.480	37.59	7/8"	1"
1 W	1.670	42.42	1"	1-1/8"
1-1/8 W	1.860	47.24	1-1/8"	1-1/4"

BA spanner sizes

British Association (BA) threads run from 0BA, the largest, to 10BA and smaller. They are found on older British electrical and instrument work.

BA size	A/F (in)	A/F (mm)
0BA	0.413	10.5
1BA	0.365	9.275
2BA	0.324	8.225
3BA	0.282	7.175
4BA	0.248	6.3
5BA	0.220	5.6
6BA	0.193	4.9
7BA	0.172	4.375
8BA	0.152	3.85
9BA	0.131	3.325
10BA	0.117	2.975

How to find the right spanner size

Match the bolt thread size in the first column to the across-flats spanner size. The thread size (M10) is the diameter of the thread, not the spanner size - an M10 bolt doesn't take a 10 mm spanner. If you don't know the thread size, measure the across-flats width of the head with calipers and read the chart backwards.

Two width standards exist. Current ISO 4014/4017 sets M10 at 16 mm and M12 at 18 mm. Older DIN 931/933 bolts, still common in machinery built before the 1990s, used 17 mm for M10 and 19 mm for M12. The DIN column lists every size that differs: M10 (17 mm DIN against 16 mm ISO), M12 (19 mm DIN against 18 mm ISO), M14 (22 mm DIN against 21 mm ISO), M22 (32 mm DIN against 34 mm ISO). Carry both spanners for those sizes if you work on mixed-age equipment.

Socket cap screws (ISO 4762) use a hex key, not a spanner, and the key size is much smaller than the head: an M10 cap screw takes an 8 mm hex key. See the metric bolt torque chart for the tightening torque once you have the right tool engaged.

Related standards

ISO 4014 / ISO 4017 - Hexagon head bolts and set screws (across-flats widths)
ISO 4032 - Hexagon regular nuts (same across-flats widths)
ISO 4762 - Hexagon socket head cap screws (hex-key sizes)
DIN 931 / DIN 933 - Older hex bolt standard (17 mm for M10, 19 mm for M12)

References

ISO 4014:2011 - Hexagon head bolts
ISO 4762:2004 - Hexagon socket head cap screws

Per-size nut cards: M6 · M8 · M10 · M12 · M16 · M20 (A/F, thickness and DIN 934 differences per size).

Related reference pages: washer dimensions chart.

Every MLC reference tool in one place: engineering calculators and charts.

Frequently Asked Questions

What size spanner for an M10 bolt?

A standard M10 hexagon bolt takes a 16 mm spanner under current ISO 4014/4017. Older DIN 931 bolts use a 17 mm spanner for M10. An M10 socket cap screw (ISO 4762) takes an 8 mm hex key, not a spanner. The M10 refers to the 10 mm thread diameter, not the spanner size.

Why doesn't an M10 bolt take a 10mm spanner?

The M10 designation is the thread diameter (10 mm), not the head width. The spanner fits across the flats of the hexagon head, which is wider than the thread - 16 mm for an M10 under ISO 4014. Thread size and spanner size are two different measurements.

What is the difference between ISO and DIN spanner sizes?

Current ISO 4014/4017 reduced the head width on two sizes versus the older DIN 931/933 standard: M10 went from 17 mm to 16 mm, and M12 from 19 mm to 18 mm. All other common sizes match. Machinery built before the 1990s often uses DIN heads, so keep both 16/17 mm and 18/19 mm spanners for mixed equipment.

What size bolt takes a 17 mm spanner?

A 17 mm spanner fits an M10 bolt or nut made to the older DIN 931, 933 or 934 standards. A current ISO M10 takes a 16 mm spanner instead.

What size bolt takes a 13 mm spanner?

A 13 mm spanner fits an M8 hexagon bolt or nut. The width is the same under ISO and the older DIN standards.

What size spanner for an M36 bolt?

An M36 hexagon bolt or nut takes a 55 mm spanner. M42 takes 65 mm and M48 takes 75 mm.

Do nuts and bolts of the same thread take the same spanner?

Yes. An ISO 4032 hexagon nut has the same across-flats width as the ISO 4014 or ISO 4017 bolt of the same thread size, from M1.6 to M64.

What size is a 3/8 Whitworth spanner in mm?

A spanner marked 3/8 W measures 0.710 inch across the jaws, which is 18.03 mm. It fits a 3/8 inch Whitworth bolt with the original hexagon and a 7/16 inch BSF bolt.

What spanner fits a 1/2 inch UNC bolt?

A 1/2 inch UNC or UNF hex bolt takes a 3/4" AF spanner.

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