What size wrench for a 1/2 inch bolt?

3/4 in for a hex bolt or cap screw and its hex nut, per ASME B18.2.1 and B18.2.2 as published. A 1/2 in heavy hex structural bolt or heavy hex nut takes 7/8 in, per RCSC Table C-2.1.

What size wrench for a 3/8 bolt?

9/16 in for a hex bolt and its hex nut. A 3/8 in heavy hex nut takes 11/16 in.

What size wrench for a 5/16 bolt?

1/2 in for the hex bolt head and the hex nut. A 5/16 in heavy hex nut is 9/16 in.

What size wrench for a 1/4 bolt?

7/16 in for a 1/4 in hex nut, and normally for the bolt head. Engineering ToolBox also lists 3/8 in for some 1/4 in heads. A 1/4 in heavy hex nut is 1/2 in.

What size wrench for a 5/8 bolt?

15/16 in for a hex bolt and hex nut, 1 1/16 in for a heavy hex structural bolt or heavy hex nut.

What size socket for an M10 bolt?

16 mm if it was made to ISO 4014 or 4017, 17 mm if made to DIN 931 or 933. M12 is 18 or 19 mm and M14 is 21 or 22 mm the same way.

What size wrench for an M8 bolt?

13 mm. ISO and DIN agree at M8.

Is 19 mm the same as 3/4 inch?

Close: 3/4 in is 19.05 mm. Wikipedia lists the pair as one of the few close enough to swap for most purposes. 13 mm and 1/2 in (12.70 mm) are not.

What size wrench for a 7/16 bolt?

5/8 in on the hex bolt head and 11/16 in on the hex nut, so one bolt needs two wrenches. A 7/16 in heavy hex nut is 3/4 in.

Home › Framing › Wrench Size Chart

Wrench Size Chart

ASME B18.2.1 and B18.2.2 as published, RCSC Table C-2.1, ISO 4014 and DIN 931

A 1/2 in bolt takes a 3/4 in wrench, unless it is a structural bolt, which takes 7/8. A 7/16 in bolt takes a 5/8 in wrench on the head and an 11/16 in wrench on its own nut. And an M10 bolt is 16 mm or 17 mm depending on whether it was made to ISO or the older DIN standard. The chart below gives the wrench or socket for each bolt and nut, from the dimension tables the fastener standards publish, with every size in both inches and millimeters.

Look up a wrench size

Fastener

Bolt size

Inch sizes go with the four inch fastener types; M sizes with ISO or DIN.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

Worked example

Three bolts that do not take the wrench people expect.

1/2 in hex bolt. 3/4 in on the head and 3/4 in on the hex nut. A 1/2 in heavy hex structural bolt takes 7/8 in on both.
7/16 in hex bolt. 5/8 in on the head, 11/16 in on the hex nut: two wrenches for one bolt.
M10. 16 mm made to ISO 4014 or 4017, 17 mm made to DIN 931 or 933.

In millimeters a 3/4 in wrench is 19.05 mm; in inches a 16 mm wrench is 0.630 in. Those are conversions, not interchangeable tools.

The formula

Converting a wrench size between the two systems is one multiplication:

millimeters = inches × 25.4

3/4 in: 0.75 × 25.4 = 19.05 mm
16 mm: 16 ÷ 25.4 = 0.630 in

The inch tables also follow a pattern, which none of the sources prints as a rule: a hex bolt head is 1.5 times the bolt diameter at 3/8 in, 1/2 in and every size from 5/8 in up, and a heavy hex structural bolt head is 1.5 times the diameter plus 1/8 in at every size RCSC lists, 1/2 to 1 1/2 in. The hex bolt pattern breaks at 1/4, 5/16, 7/16 and 9/16 in, which is why a chart beats arithmetic.

Where the numbers come from

The ASME standards themselves are sold, not published free, so this page reads their tables as fastener makers and engineering references reprint them, and cross-checks one against another. Hex bolt heads are ASME B18.2.1 as printed by Engineering ToolBox and, from 3/8 in up, identically by Atlanta Rod and Manufacturing. Hex nuts and heavy hex nuts are ASME B18.2.2 as printed by Atlanta Rod.

Heavy hex structural bolts are RCSC Table C-2.1 (Research Council on Structural Connections, June 5, 2025), which reprints ASME B18.2.6 and states that the bolt head and the nut share one width across flats so one wrench fits both.

Metric heads are ISO 4014 as printed by Industrial Threaded Products, whose table footnotes the DIN 931 sizes at M10, M12 and M14. Eurolink Fastener Supply Solutions gives the same three and adds M22, where DIN is the smaller head.

SAE wrench size chart: inch bolts and nuts

Wrench or socket size (width across flats), inches. Hex bolt: ASME B18.2.1; hex and heavy hex nut: ASME B18.2.2; heavy hex bolt head = heavy hex nut (RCSC Table C-2.1)
Bolt diameter	Hex bolt or cap screw head	Hex nut	Heavy hex nut and structural bolt	Hex head in mm
1/4 in	7/16 in	7/16 in	1/2 in	11.11 mm
5/16 in	1/2 in	1/2 in	9/16 in	12.70 mm
3/8 in	9/16 in	9/16 in	11/16 in	14.29 mm
7/16 in	5/8 in	11/16 in	3/4 in	15.88 mm
1/2 in	3/4 in	3/4 in	7/8 in	19.05 mm
9/16 in	13/16 in	7/8 in	15/16 in	20.64 mm
5/8 in	15/16 in	15/16 in	1 1/16 in	23.81 mm
3/4 in	1 1/8 in	1 1/8 in	1 1/4 in	28.57 mm
7/8 in	1 5/16 in	1 5/16 in	1 7/16 in	33.34 mm
1 in	1 1/2 in	1 1/2 in	1 5/8 in	38.10 mm
1 1/8 in	1 11/16 in	1 11/16 in	1 13/16 in	42.86 mm
1 1/4 in	1 7/8 in	not printed	2 in	47.62 mm
1 3/8 in	2 1/16 in	2 1/16 in	2 3/16 in	52.39 mm
1 1/2 in	2 1/4 in	2 1/4 in	2 3/8 in	57.15 mm

The heavy hex column covers structural bolts (ASTM F3125, which includes Grades A325 and A490) from 1/2 in up, and heavy hex nuts at every size. Heavy hex structural bolt heads below 1/2 in are not in RCSC's table. The 1 1/4 in hex nut is blank because the table read for this page skips that size; the bolt head is listed.

Engineering ToolBox lists 7/16 in or 3/8 in for a 1/4 in hex bolt. 7/16 in is the figure the hex nut table gives for the same size, and the one in the chart.

Metric wrench size chart: ISO and DIN

Wrench or socket size (width across flats), millimeters. ISO 4014 / 4017 and DIN 931 / 933 hex bolts and cap screws
Thread	ISO 4014 or 4017	DIN 931 or 933	ISO size in inches
M3	5.5 mm	5.5 mm	0.217 in
M4	7 mm	7 mm	0.276 in
M5	8 mm	8 mm	0.315 in
M6	10 mm	10 mm	0.394 in
M8	13 mm	13 mm	0.512 in
M10	16 mm	17 mm (differs)	0.630 in
M12	18 mm	19 mm (differs)	0.709 in
M14	21 mm	22 mm (differs)	0.827 in
M16	24 mm	24 mm	0.945 in
M20	30 mm	30 mm	1.181 in
M22	34 mm	32 mm (differs)	1.339 in
M24	36 mm	36 mm	1.417 in
M30	46 mm	46 mm	1.811 in
M36	55 mm	55 mm	2.165 in

Four sizes take a different wrench depending on the standard: M10 (16 or 17 mm), M12 (18 or 19), M14 (21 or 22) and M22 (34 or 32). The DIN standards were withdrawn in favor of ISO, but Eurolink notes they are still widely requested and made, so both heads are in circulation. Wikipedia, citing ISO 272, dates the change to 1982.

A third M10 exists. Engineering ToolBox lists 15 mm for M10 under the US metric standard ASME B18.2.3; this page could not check that against the standard, so it is not in the chart.

Bolts whose head and nut take different wrenches

Same bolt diameter, different wrench, from the tables above
Bolt	Head	Nut	Why
7/16 in hex	5/8 in	11/16 in	B18.2.1 head against B18.2.2 nut
9/16 in hex	13/16 in	7/8 in	B18.2.1 head against B18.2.2 nut
1/2 in hex bolt with a heavy hex nut	3/4 in	7/8 in	heavy hex nut is a larger hex
1/2 in structural bolt	7/8 in	7/8 in	RCSC: head and nut match on purpose

Before buying a wrench for a bolt, check what kind of nut is on it: the nut, not the bolt diameter, sets the size you need at that end.

SAE and metric sizes in both units

Plain unit conversion, millimeters = inches × 25.4. A size that converts close to the other system's size is still a different tool.

Every inch wrench size on this page, in millimeters
SAE size	Decimal	Millimeters
7/16 in	0.4375 in	11.11 mm
1/2 in	0.5 in	12.70 mm
9/16 in	0.5625 in	14.29 mm
5/8 in	0.625 in	15.88 mm
11/16 in	0.6875 in	17.46 mm
3/4 in	0.75 in	19.05 mm
13/16 in	0.8125 in	20.64 mm
7/8 in	0.875 in	22.22 mm
15/16 in	0.9375 in	23.81 mm
1 1/16 in	1.0625 in	26.99 mm
1 1/8 in	1.125 in	28.57 mm
1 1/4 in	1.25 in	31.75 mm
1 5/16 in	1.3125 in	33.34 mm
1 7/16 in	1.4375 in	36.51 mm
1 1/2 in	1.5 in	38.10 mm
1 5/8 in	1.625 in	41.27 mm
1 11/16 in	1.6875 in	42.86 mm
1 13/16 in	1.8125 in	46.04 mm
1 7/8 in	1.875 in	47.62 mm
2 in	2 in	50.80 mm
2 1/16 in	2.0625 in	52.39 mm
2 3/16 in	2.1875 in	55.56 mm
2 1/4 in	2.25 in	57.15 mm
2 3/8 in	2.375 in	60.32 mm

Every metric wrench size on this page, in inches
Metric size	Inches
5.5 mm	0.217 in
7 mm	0.276 in
8 mm	0.315 in
10 mm	0.394 in
13 mm	0.512 in
16 mm	0.630 in
17 mm	0.669 in
18 mm	0.709 in
19 mm	0.748 in
21 mm	0.827 in
22 mm	0.866 in
24 mm	0.945 in
30 mm	1.181 in
32 mm	1.260 in
34 mm	1.339 in
36 mm	1.417 in
46 mm	1.811 in
55 mm	2.165 in

Which ones can stand in for each other? Wikipedia names 19 mm for 3/4 in, 8 mm for 5/16 in and 4 mm for 5/32 in as close enough for most purposes, and gives a 13 mm wrench on a 1/2 in head as the kind of swap that rounds a nut. The tables above show why: 3/4 in is 19.05 mm and 5/16 in is 7.94 mm. This page does not extend that list by its own judgment.

Frequently asked questions

What size wrench for a 1/2 inch bolt?: 3/4 in for a hex bolt or cap screw and its hex nut, per ASME B18.2.1 and B18.2.2 as published. A 1/2 in heavy hex structural bolt or heavy hex nut takes 7/8 in, per RCSC Table C-2.1.
What size wrench for a 3/8 bolt?: 9/16 in for a hex bolt and its hex nut. A 3/8 in heavy hex nut takes 11/16 in.
What size wrench for a 5/16 bolt?: 1/2 in for the hex bolt head and the hex nut. A 5/16 in heavy hex nut is 9/16 in.
What size wrench for a 1/4 bolt?: 7/16 in for a 1/4 in hex nut, and normally for the bolt head. Engineering ToolBox also lists 3/8 in for some 1/4 in heads. A 1/4 in heavy hex nut is 1/2 in.
What size wrench for a 5/8 bolt?: 15/16 in for a hex bolt and hex nut, 1 1/16 in for a heavy hex structural bolt or heavy hex nut.
What size socket for an M10 bolt?: 16 mm if it was made to ISO 4014 or 4017, 17 mm if made to DIN 931 or 933. M12 is 18 or 19 mm and M14 is 21 or 22 mm the same way.
What size wrench for an M8 bolt?: 13 mm. ISO and DIN agree at M8.
Is 19 mm the same as 3/4 inch?: Close: 3/4 in is 19.05 mm. Wikipedia lists the pair as one of the few close enough to swap for most purposes. 13 mm and 1/2 in (12.70 mm) are not.
What size wrench for a 7/16 bolt?: 5/8 in on the hex bolt head and 11/16 in on the hex nut, so one bolt needs two wrenches. A 7/16 in heavy hex nut is 3/4 in.

Check these numbers yourself

US Hex Bolts - InchesEngineering ToolBox, ANSI/ASME B18.2.1 dimensions of hex bolts, width across flats 5/16 to 4 in. Read 2026-10-03.
U.S. Bolts - Head and Wrench SizesEngineering ToolBox, head and wrench size by bolt diameter for hex and heavy hex bolts, citing ASME B18.2.1. Source of the 1/4 in and 9/16 in hex bolt heads. Read 2026-10-03.
Finished Hex Bolt DimensionsAtlanta Rod and Manufacturing, Lavonia, Georgia, a US bolt maker. Width across flats 3/8 to 2 1/2 in, undated. Read 2026-10-03.
Finished Hex Nut DimensionsAtlanta Rod and Manufacturing. Hex nut width across flats 1/4 to 2 3/4 in (no 1 1/4 in row), undated. Read 2026-10-03.
Heavy Hex Nut DimensionsAtlanta Rod and Manufacturing. Heavy hex nut width across flats 1/4 to 4 in, undated. Read 2026-10-03.
Specification for Structural Joints Using High-Strength BoltsResearch Council on Structural Connections, June 5, 2025, superseding June 11, 2020. Commentary to Section 2.7 and Table C-2.1, heavy hex bolt and nut nominal dimensions (ASME B18.2.6). Read 2026-10-03.
Hex Bolts Grade A and B, Metric, ISO 4014Industrial Threaded Products, Inc., Brea, California, a fastener distributor. ISO 4014 product grade A and B tables, width across flats M1.6 to M64, with the DIN 931 footnote for M10 to M14. Undated (uploaded 2015). Read 2026-10-03.
DIN 933 vs. ISO 4017 and DIN 931 vs. ISO 4014Eurolink Fastener Supply Solutions blog. Width across flats at the four sizes where DIN and ISO differ: M10, M12, M14, M22. Read 2026-10-03.
Width across flatsWikipedia, citing ISO 262 and ISO 272 (1982 change at M10, M12, M14, M22) and naming 19 mm, 8 mm and 4 mm as close to 3/4, 5/16 and 5/32 in. The article carries a more-citations-needed notice, so it is used here only for those statements. Read 2026-10-03.

How every figure here is verified: Sources & Method. Who builds this: Lucas Pradella, and the method is on About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-10-03. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Nominal wrench sizes, not tolerances or torque. Heads are made slightly under the nominal size so a wrench fits; a worn or damaged head may not take the listed wrench. Torque values come from the bolt maker or the design, not from this chart.