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Bolt Head Dimensions
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 in a related page. Here, 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 | |||