Standard Hex Cap Screw Dimensions
Hex cap screws (also called hex cap bolts or hex head bolts) are the workhorse fastener of mechanical assembly. This table covers dimensions per ASME B18.2.1 — the data you need for designing bolt holes, counterbores, clearances, and grip lengths.
Key Dimensions Explained
- Width across flats (F) — your wrench/socket size
- Width across corners (C) — the diagonal of the hex head; determines the minimum counterbore diameter
- Head height (H) — needed for counterbore depth calculations
- Thread length (LT) — for bolts ≤6″ long: LT = 2D + 1/4″. For bolts >6″: LT = 2D + 1/2″
| Bolt Size | Width Across Flats (in) | Width Across Corners (in) | Head Height (in) | Thread Length (≤6″ bolt) |
|---|---|---|---|---|
| 1/4″ | 7/16 | 0.505 | 5/32 | 0.750 |
| 5/16″ | 1/2 | 0.577 | 13/64 | 0.875 |
| 3/8″ | 9/16 | 0.650 | 15/64 | 1.000 |
| 7/16″ | 5/8 | 0.722 | 9/32 | 1.125 |
| 1/2″ | 3/4 | 0.866 | 5/16 | 1.250 |
| 9/16″ | 13/16 | 0.938 | 23/64 | 1.375 |
| 5/8″ | 15/16 | 1.083 | 25/64 | 1.500 |
| 3/4″ | 1-1/8 | 1.299 | 15/32 | 1.750 |
| 7/8″ | 1-5/16 | 1.516 | 35/64 | 2.000 |
| 1″ | 1-1/2 | 1.732 | 39/64 | 2.250 |
| 1-1/8″ | 1-11/16 | 1.949 | 11/16 | 2.500 |
| 1-1/4″ | 1-7/8 | 2.165 | 25/32 | 2.750 |
| 1-3/8″ | 2-1/16 | 2.382 | 27/32 | 3.000 |
| 1-1/2″ | 2-1/4 | 2.598 | 15/16 | 3.250 |
Counterbore Dimensions
If you’re recessing the bolt head (socket head cap screws too), here are recommended counterbore sizes:
| Bolt Size | Counterbore Ø (in) | Counterbore Depth (in) |
|---|---|---|
| 1/4″ | 9/16 | 11/64 |
| 5/16″ | 5/8 | 7/32 |
| 3/8″ | 23/32 | 1/4 |
| 1/2″ | 15/16 | 21/64 |
| 5/8″ | 1-3/16 | 27/64 |
| 3/4″ | 1-3/8 | 1/2 |
| 1″ | 1-13/16 | 43/64 |
Thread Length Formula
Bolts ≤ 6″ long: Thread length = 2 × diameter + 0.25″
Bolts > 6″ long: Thread length = 2 × diameter + 0.50″
Related: Hex Bolt Grade ID & Properties | Hex Nut Dimensions | Flat Washer Dimensions
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