Metric Hex Bolt Head Size

Do you ever struggle to remember the relationship between a metric hex bolt thread size and the related hex head size (between the flats, or ‘wrench’ size).

Or, do you have a design with limited clearance?  Use this table to see if you have room, but don’t forget to check that you have enough room for a socket or a wrench to get in there.  Otherwise, you might be better off using a Socket Head Cap Screw (SHCS) instead of a Hex Head fastener.

The first column (ANSI/ISO) is pertinent to ‘standard’ metric  hex bolts:

Bolt Diameter
(mm)
Head/Wrench Size
(mm)
ANSI/ISO DIN JIS DIN/ISO
Heavy Hex
4 7 7 7
5 8 8 8
6 10 10 10
7 11
8 13 13 12
10 16 17 14
12 18 19 17 22/21*
14 21 22 19
16 24 24 22 27
18 27
20 30 30 34/32*
ANSI – American National Standards Institute
ISO – International Organization for Standardisation
DIN – Deutsches Institut fur Normung
JIS – Japanese Industrial Standard
* Indicates ISO standard.

Learn more about threaded joints.

 

Hex Bolt Identifying Marks, Material Grade and Mechanical Properties

Are you confused about the modest hex bolt, or are you wondering what all those marks mean?  This table will help you identify any hex bolt right on the nuts, and it will also tell you about the material grade and mechanical properties.  What more could you ask for from a super boring table?

Hex Bolt Identification
Grade Mark
Specification Fastener
Description
Material Nominal Size
Range (in.)
Mechanical Properties
Proof Load
(psi)
Yield Strength
Min (psi)
Tensile Strength
Min (psi)

No
Grade
Mark
SAE J429
Grade 1
Bolts,
Screws,
Studs
Low or Medium Carbon Steel 1/4 thru 1-1/2 33,000 36,000 60,000
ASTM A307
Grades A&B
Low Carbon Steel 1/4 thru 4
SAE J429
Grade 2
Low or Medium Carbon Steel 1/4 thru 3/4 Over 3/4 to 1-1/2 55,000
33,000
57,000
36,000
74,000
60,000

No
Grade
Mark
SAE J429
Grade 4
Studs Medium Carbon Cold Drawn Steel 1/4 thru 1-1/2 100,000 115,000

B5
ASTM A193
Grade B5
AISI 501 1/4 Thru 4 80,000 100,000

B6
ASTM A193
Grade B6
AISI 410 85,000 110,000

B7
ASTM A193
Grade B7
AISI 4140, 4142, OR 4105 1/4 thru 2-1/2
Over 2-1/2 thru 4
Over 4 thru 7


105,000
95,000
75,000
125,000
115,000
100,000

B16
ASTM A193
Grade B16
CrMoVa Alloy Steel 105,000
95,000
85,000
125,000
115,000
100,000

B8
ASTM A193
Grade B8
AISI 304 1/4 and larger 30,000 75,000

B8C
ASTM A193
Grade B8C
AISI 347

B8M
ASTM A193
Grade B8M
AISI 316

B8T
ASTM A193
Grade B8T
Bolts,
Screws,
Studs for High-Temperature Service
AISI 321 1/4 and larger 30,000 75,000

B8
ASTM A193
Grade B8
AISI 304
Strain Hardened
1/4 thr 3/4
Over 3/4 thru 1
Over 1 thru 1-1/4
Over 1-1/4 thru 1-1/2



100,000
80,000
65,000
50,000
125,000
115,000
105,000
100,000

B8C
ASTM A193
Grade B8C
AISI 347
Strain Hardened

B8M
ASTM A193
Grade B8M
AISI 316
Strain Hardened
95,000
80,000
65,000
50,000
110,000
100,000
95,000
90,000

B8T
ASTM A193
Grade B8T
AISI 321
Strain Hardened
100,000
80,000
65,000
50,000
125,000
115,000
105,000
100,000

L7
ASTM A320
Grade L7
Bolts,
Screws,
Studs for Low-Temperature Service
AISI 4140,
4142 or 4145
1/4 thru 2-1/2 105,000 125,000

L7A
ASTM A320
Grade L7A
AISI 4037

L7B
ASTM A320
Grade L7B
AISI 4137

L7C
ASTM A320
Grade LC7
AISI 8740

L43
ASTM A320
Grade L43
AISI 4340 1/4 thru 4 105,000 125,000

B8
ASTM A320
Grade B8
Bolts,
Screws,
Studs for Low-Temperature Service
AISI 304 1/4 and larger 30,000 75,000

B8C
ASTM A320
Grade B8C
AISI 347

B8T
ASTM A320
Grade B8T
AISI 321

B8F
ASTM A320
Grade B8F
AISI 303
or 303Se

B8M
ASTM A320
Grade B8M
AISI 316

B8
ASTM A320
Grade B8
AISI 304 1/4 thru 3/4
Over 3/4 thru 1
Over 1 thru 1-1/4
Over 1-1/4 thru 1-1/2



100,000
80,000
65,00
50,00
100,000
80,000
65,00
50,00

B8C
ASTM A320
Grade B8C
AISI 347

B8F
ASTM A320
Grade B8F
AISI 303
or 303Se

B8M
ASTM A320
Grade B8M
AISI 316

B8T
ASTM A320
Grade B8T
AISI 321
SAE J429
Grade 5
Bolts,
Screws,
Studs
Medium Carbon Steel, Quenched and Tempered 1/4 thru 1
Over 1 to 1-1/2
85,000
74,000
92,000
81,000
120,000
105,000
ASTM A449 1/4 thru 1
Over 1 to 1-1/2
Over 1-1/2 thru 3
85,000
74,000
55,000
92,000
81,000
58,000
120,000
105,000
90,000
SAE J429
Grade 5.1
Sems Low or Medium Carbon Steel, Quenched and Tempered No. 6
thru 3/8
85,000 120,000
SAE J429
Grade 5.2
Bolts,
Screws,
Studs
Low Carbon Martensitic Steel, Quenched and Tempered 1/4 thru 1 85,000 92,000 120,000

A325
ASTM A325
Type 1
High Strength
Structural Bolts
Medium Carbon Steel, Quenched and Tempered 1/2 thru 1
1-1/8 thru 1-1/2
85,000
74,000
92,000
81,000
120,000
105,000

A325
ASTM A325
Type 2
Low Carbon Martensitic Steel, Quenched and Tempered 1/2 thru 1 85,000 92,000 120,000

A325
ASTM A325
Type 3
Atmospheric Corrosion Resisting Steel, Quenched and Tempered 1/2 thru 1
1-1/8 thru 1-1/2
85,000
74,000
92,000
81,000
120,000
105,000

BB
ASTM A354
Grade BB
Bolts,
Studs
Alloy Steel, Quenched and Tempered 1/4 thru 2-1/2
2-3/4 thru 4
80,000
75,000
83,000
78,000
105,000
100,000

BC
ASTM A354
Grade BC
105,000
95,000
109,000
99,000
125,000
115,000
SAE J429
Grade 7
Bolts,
Screws,
Medium Carbon Alloy Steel, Quenched and Tempered 4 1/4 thru 1-1/2 105,000 115,000 133,000
SAE J429
Grade 8
Bolts,
Screws,
Studs
Medium Carbon Alloy Steel, Quenched and Tempered 1/4 thru 1-1/2 120,000 130,000 150,000
ASTM A354
Grade BD
Alloy Steel, Quenched and Tempered 4

No Grade
Mark
SAE J429
Grade 8.1
Studs Medium Carbon Alloy or SAE 1041 Modified Elevated Temperature Drawn Steel 1/4 thru 1-1/2 120,000 130,000 150,000

A490
ASTM A490 High Strength Structural Bolts Alloy Steel, Quenched and Tempered 1/2 thru 1-1/2 120,000 130,000 150,000 min
170,000 max

No Grade
Mark
ISO R898
Class 4.6
Bolts,
Screws,
Studs
Medium Carbon Steel, Quenched and Tempered All Sizes
thru 1-1/2
33,000 36,000 60,000

No Grade
Mark
ISO R898
Class 5.8
55,000 57,000 74,000
8.8

or

88
ISO R898
Class 8.8
Alloy Steel, Quenched and Tempered 85,000 92,000 120,000
10.9

or

109
ISO R898
Class 10.9
120,000 130,000 150,000

Your hex bolt may require a hex nut.

Hex Nut Dimensions

There are lots of reasons you need to know hex nut dimensions.  You need to hex nut height to determine the best length of a fastener in a bolted assembly.  You need to know the size of the hex to make sure you have sufficient clearance for the nut itself, the washer and a wrench.


Size Across Flats* Height
Hex Nut Machine Screw Nut Hex Nut Jam Nut Nylock Nut Machine Screw Nut
0 5/32 3/64
1 5/32 3/64
2 3/16 9/64 1/16
3 3/16 9/64 1/16
4 1/4 9/64 3/32
6 5/16 11/64 7/64
8 11/32 15/64 1/8
10 3/8 15/64 1/8
12 7/16 5/16 5/32
1/4 7/16 7/16 7/32 5/32 5/16 3/16
5/16 1/2 9/16 17/64 3/16 11/32 7/32
3/8 9/16 5/8 21/64 7/32 29/64 1/4
7/16 11/16 3/8 1/4 29/64
1/2 3/4 7/16 5/16 19/32
9/16 7/8 31/64 5/16 41/64
5/8 15/16 35/64 3/8 3/4
3/4 1-1/8 41/64 27/64 7/8
7/8 1-5/16 3/4 31/64 63/64
1 1-1/2 55/64 35/64 1-3/64
* This is also the size of wrench to use.

Hex Cap Bolt Dimensions

Hex cap bolt dimensions are trickier than you think.  By default, they are not threaded all the way, so you need to be careful in your design to make sure you are not threading beyond the capability of the bolt itself.

Just as with the standard nut and washer, it it important to consider the geometric constraints of the hex cap bolt to ensure proper function and easier assembly / disassembly.


Hex Bolt Dimensions s
Basic Product Diameter
Full-Size Body Diameter
Width Across Flats
Width Across Corners
Head Height
Thread Length
Thread Length
Max
Min
Max
Min
Up to 6in
Over 6 in
1/4
.26
.237
7/16
.505
0.484
11/64
0.75
1.00
5/16
.324
.298
1/2
.577
0.552
7/32
0.88
1.13
3/8
.388
.360
9/16
.650
0.62
1/4
1.00
1.25
7/16
.452
.421
5/8
.722
0.687
19/64
1.13
1.38
1/2
.515
.482
3/4
.866
0.826
11/32
1.25
1.50
5/8
.642
.605
15/16
1.083
1.033
27/64
1.50
1.75
3/4
.768
.729
1-1/8
1.299
1.24
1/2
1.75
2.00
7/8
.895
.852
1-5/16
1.516
1.447
37/64
2.00
2.25
1
1.022
.976
1-1/2
1.732
1.653
43/64
2.25
2.50
1-1/8
1.149
1.098
1-11/16
1.949
1.859
3/4
2.50
2.75
1-1/4
1.277
1.223
1-7/8
2.165
2.066
27/32
2.75
3.00
1-3/8
1.404
1.345
2-1/16
2.382
2.273
29/32
3.00
3.25
1-1/2
1.531
1.47
2-1/4
2.598
2.48
1
3.25
3.50
1-3/4
1.785
1.716
2-5/8
3.031
2.893
1-5/32
3.75
4.00
2
2.039
1.964
3
3.464
3.306
1-11/32
4.25
4.50
2-1/4
2.305
2.214
3-3/8
3.897
3.719
1-1/2
4.75
5.00
2-1/2
2.559
2.461
3-3/4
4.33
4.133
1-21/32
5.25
5.5
2-3/4
2.827
2.711
4-1/8
4.763
4.546
1-13/16
5.75
6.00
3
3.081
2.961
4-1/2
5.196
4.959
2
6.25
6.50
3-1/4
3.335
3.21
4-7/8
5.629
5.372
2-3/16
6.75
7.00
3-1/2
3.589
3.461
5-1/4
6.062
5.786
2-5/16
7.25
7.50
3-3/4
3.858
3.726
5-5/8
6.495
6.198
2-1/2
7.75
8.00
4
4.111
3.975
6
6.928
6.612
2-11/16
8.25
8.50