Bolt Head Dimensions and Markings
The strength of a bolt is determined by its diameter and the strength of the material from which it is made. Minimum material strengths for various grades of bolts are given below. First, we deal with the diameter.
It may seem that the determination of a bolt's diameter is a simple matter of measurement, and this is indeed the case when the shank is exposed. In many cases, however, the shank is hidden, and the engineer who is charged with determining the strength of the bolted joint must infer the bolt's diameter from the size of its head. Fortunately, there are standards that govern the relationship between bolt size and head size. The following tables are based on information in
- ANSI B18.2.1 (Square, Hex, and Heavy Hex)
- ASTM A325 and A490 (High Strength Structural)
- AISC Manual of Steel Construction (all)
Square Heads
| D | F | C | H |
| 1/4 | 3/8 | 1/2 | 3/16 |
| 3/8 | 9/16 | 13/16 | 1/4 |
| 1/2 | 3/4 | 1-1/16 | 5/16 |
| 5/8 | 15/16 | 1-5/16 | 7/16 |
| 3/4 | 1-1/8 | 1-9/16 | 1/2 |
| 7/8 | 1-5/16 | 1-7/8 | 5/8 |
| 1 | 1-1/2 | 2-1/8 | 11/16 |
| 1-1/8 | 1-11/16 | 2-3/8 | 3/4 |
| 1-1/4 | 1-7/8 | 2-5/8 | 7/8 |
| 1-3/8 | 2-1/16 | 2-15/16 | 15/16 |
| 1-1/2 | 2-1/4 | 3-3/16 | 1 |
| All dimensions in inches | |||
Hex Heads
| D | F | C | H |
| 1/4 | 7/16 | 1/2 | 3/16 |
| 3/8 | 9/16 | 5/8 | 1/4 |
| 1/2 | 3/4 | 7/8 | 3/8 |
| 5/8 | 15/16 | 1-1/16 | 7/16 |
| 3/4 | 1-1/8 | 1-5/16 | 1/2 |
| 7/8 | 1-5/16 | 1-1/2 | 9/16 |
| 1 | 1-1/2 | 1-3/4 | 11/16 |
| 1-1/8 | 1-11/16 | 1-15/16 | 3/4 |
| 1-1/4 | 1-7/8 | 2-3/16 | 7/8 |
| 1-3/8 | 2-1/16 | 2-3/8 | 15/16 |
| 1-1/2 | 2-1/4 | 2-5/8 | 1 |
| 1-3/4 | 2-5/8 | 3 | 1-3/16 |
| 2 | 3 | 3-7/16 | 1-3/8 |
| 2-1/4 | 3-3/8 | 3-7/8 | 1-1/2 |
| 2-1/2 | 3-3/4 | 4-5/16 | 1-11/16 |
| 2-3/4 | 4-1/8 | 4-3/4 | 1-13/16 |
| 3 | 4-1/2 | 5-3/16 | 2 |
| 3-1/4 | 4-7/8 | 5-5/8 | 2-3/16 |
| 3-1/2 | 5-1/4 | 6-1/16 | 2-5/16 |
| 3-3/4 | 5-5/8 | 6-1/2 | 2-1/2 |
| 4 | 6 | 6-15/16 | 2-11/16 |
| All dimensions in inches | |||
Heavy Hex Heads
| D | F | C | H |
| 1/2 | 7/8 | 1 | 3/8 |
| 5/8 | 1-1/16 | 1-1/4 | 7/16 |
| 3/4 | 1-1/4 | 1-7/16 | 1/2 |
| 7/8 | 1-7/16 | 1-11/16 | 9/16 |
| 1 | 1-5/8 | 1-7/8 | 11/16 |
| 1-1/8 | 1-13/16 | 2-1/16 | 3/4 |
| 1-1/4 | 2 | 2-5/16 | 7/8 |
| 1-3/8 | 2-3/16 | 2-1/2 | 15/16 |
| 1-1/2 | 2-3/8 | 2-3/4 | 1 |
| 1-3/4 | 2-3/4 | 3-3/16 | 1-3/16 |
| 2 | 3-1/8 | 3-5/8 | 1-3/8 |
| 2-1/4 | 3-1/2 | 4-1/16 | 1-1/2 |
| 2-1/2 | 3-7/8 | 4-1/2 | 1-11/16 |
| 2-3/4 | 4-1/4 | 4-15/16 | 1-13/16 |
| 3 | 4-5/8 | 5-5/16 | 2 |
| All dimensions in inches | |||
High Strength Structural Bolts
| D | F | H | |
| 1/2 | 7/8 | 5/16 | |
| 5/8 | 1-1/16 | 25/64 | |
| 3/4 | 1-1/4 | 15/32 | |
| 7/8 | 1-7/16 | 35/64 | |
| 1 | 1-5/8 | 39/64 | |
| 1-1/8 | 1-13/16 | 11/16 | |
| 1-1/4 | 2 | 25/32 | |
| 1-3/8 | 2-3/16 | 27/32 | |
| 1-1/2 | 2-3/8 | 15/16 | |
| All dimensions in inches | |||
Hex Head Bolt Markings
The strength and type of steel used in a bolt is supposed to be indicated by a raised mark on the head of the bolt. The type of mark depends on the standard to which the bolt was manufactured. Most often, bolts used in machinery are made to SAE standard J429, and bolts used in structures are made to various ASTM standards. The tables below give the head markings and some of the most commonly-needed information concerning the bolts. For further information, see the appropriate standard.
SAE Bolt Designations
|
SAE Grade No. |
Size range |
Tensile strength, ksi |
Material | Head marking |
|
1 2 |
1/4 thru 1-1/2 1/4 thru 3/4 7/8 thru 1-1/2 |
60 74 60 |
Low or medium carbon steel |
|
| 5 |
1/4 thru 1 1-1/8 thru 1-1/2 |
120 105 |
Medium carbon steel, quenched & tempered |
|
| 5.2 | 1/4 thru 1 | 120 |
Low carbon martensite steel, quenched & tempered |
|
| 7 | 1/4 thru 1-1/2 | 133 |
Medium carbon alloy steel, quenched & tempered |
|
| 8 | 1/4 thru 1-1/2 | 150 |
Medium carbon alloy steel, quenched & tempered |
|
| 8.2 | 1/4 thru 1 | 150 |
Low carbon martensite steel, quenched & tempered |
|
ASTM Bolt Designations
|
ASTM standard |
Size range |
Tensile strength, ksi |
Material | Head marking |
| A307 | 1/4 thru 4 | 60 | Low carbon steel |
|
|
A325 Type 1 |
1/2 thru 1 1-1/8 thru 1-1/2 |
120 105 |
Medium carbon steel, quenched & tempered |
|
|
A325 Type 2 |
1/2 thru 1 1-1/8 thru 1-1/2 |
120 105 |
Low carbon martensite steel, quenched & tempered |
|
|
A325 Type 3 |
1/2 thru 1 1-1/8 thru 1-1/2 |
120 105 |
Weathering steel, quenched & tempered |
|
| A449 |
1/4 thru 1 1-1/8 thru 1-1/2 1-3/4 thru 3 |
120 105 90 |
Medium carbon steel, quenched & tempered |
|
|
A490 Type 1 |
1/4 thru 1-1/2 | 150 |
Alloy steel, quenched & tempered |
|
|
A490 Type 3 |
1/4 thru 1-1/2 | 150 |
Weathering steel, quenched & tempered |
|
Head markings and strengths for metric hex-head cap screws
| Head marking | Grade, material & condition | Nominal size range [mm] | Proof strength [MPa] | Yield strength (min) [MPa] | Tensile strength (min) [MPa] | Core hardness [Rockwell] |
|---|---|---|---|---|---|---|
|
Class 3.6 | 1.6–36 | 180 | 190 | 330 | B52–95 |
|
Class 4.6 Low or medium carbon steel |
5–100 | 225 | 240 | 400 | B67–95 |
|
Class 4.8 Low or medium carbon steel; fully or partially annealed |
1.6–16 | 310 | 340 | 420 | B71–95 |
|
Class 5.8 Low or medium carbon steel; cold worked |
5–24 | 380 | 420 | 520 | B82–95 |
|
Class 8.8 Medium carbon steel; quench and tempered |
Under 16 (inc.) | 580 | 640 | 800 | |
| 17–72 | 600 | 660 | 830 | C23–34 | ||
|
Class 8.8 low carbon Low carbon boron steel; quench and tempered |
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|
Class 8.8.3 Atmospheric corrosion resistant steel; quench and tempered |
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|
ASTM A325M - Type 1 Medium carbon steel; quench and tempered |
12–36 | ||||
|
ASTM A325M - Type 3 Atmospheric corrosion resistant steel; quench and tempered |
|||||
|
Class 9.8 Medium carbon steel; quench and tempered |
1.6–16 | 650 | 720 | 900 | C27–36 |
|
Class 9.8 low carbon Low carbon boron steel; quench and tempered |
|||||
|
Class 10.9 Alloy steel; quench and tempered |
5–100 | 830 | 940 | 1040 | C33–39 |
|
Class 10.9 low carbon Low carbon boron steel; quench and tempered |
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|
Class 10.9.3 Atmospheric corrosion resistant steel; quench and tempered |
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|
ASTM A490M - Type 1 Alloy steel; quench and tempered |
12–36 | ||||
|
ASTM A490M - Type 3 Atmospheric corrosion resistant steel; quench and tempered |
|||||
|
Class 12.9 Alloy steel; quench and tempered |
1.6–100 | 970 | 1100 | 1220 | C38–44 |
|
A2 Stainless steel with 17–19% chromium and 8–13% nickel |
Up to 20 |
210 minimum 450 typical |
500 minimum 700 typical |
||
|
ISO 3506-1 A2-50 304 stainless steel-class 50 (annealed) |
210 | 500 | ||||
|
ISO 3506-1 A2-70 304 stainless steel-class 70 (cold worked) |
450 | 700 | ||||
|
ISO 3506-1 A2-80 304 stainless steel-class 80 |
600 | 800 |
Often one will find "extra" marks on a bolt head--marks in addition to those shown above. Usually these marks indicate the bolt's manufacturer.
ASTM A325 Type 2 bolts have been discontinued, but are included above because they can be found in existing structures. Their properties can be important in failure investigations.
While the bolts shown above are among the most common in the U.S., the list is far from exhaustive. In addition to the other bolts covered by the SAE and ASTM standards, there are a host of international standards, of which ISO is perhaps the most well known.
