How it connects
Driver socket slides axially over the hex head; torque is transmitted through the six flats/corners. No seal; retention is by fit, sometimes a magnet in the nut setter.
Male and female halves
This is the male half (external hex head). The female half is the hex socket/nut driver recess.
Usually connects to
- Matching hex nut driver or magnetic nut-setter bit
- 6-point or 12-point hex socket
- Open-end or box wrench
- Adjustable wrench
Common sizes
- Sized across flats: inch drives span roughly 1/8 in to over 4 in; metric about 3.2 mm to over 100 mm.
- Common small sizes include 1/4, 5/16, 3/8 and 7/16 in, and 7, 8 and 10 mm; 1/4 in AF is typical for #10/#12 hex washer-head screws.
- Standard hardware ranges commonly run 1/4–1-1/2 in or 4–36 mm, extending to 2+ in/50+ mm and larger for structural bolts.
Where it is used
construction fasteners; construction and structural framingsheet metal and roofing screwsself-drilling screws; machine screws and boltsautomotiveheavy machineryplumbing and pipinggeneral mechanical assemblywoodworking
Variations
- Plain or heavy hex head
- Integral washer/flange head, flat or indented
- Slotted hex washer head
- Combined external hex/internal recess, such as T25
How to recognise it
Six-sided prismatic head on top of a screw/bolt, often with an integral round washer flange below; measure across opposite flats to get the driver size. May also carry an internal recess (Torx, Phillips, slot) for combination drive.
Often confused with
| Confused with | How to tell them apart |
|---|---|
| 1/4 in hex shank (bit holder) drive | A 1/4 in hex shank is a long male hex bar on a bit that inserts into a female chuck; a hex head is a short head on a fastener engaged from outside by a socket. |
| Internal hex (Allen) socket drive | Internal hex is a female recess in the head driven by a hex key; external hex is the outside head shape. External hex is a protruding 6-sided head driven from the outside; internal hex is a female 6-sided recess in the head driven by an Allen key or male hex bit. |
| Internal hex (Allen / socket head) | External hex features six protruding outer flats driven from the outside, whereas internal hex features a recessed hexagonal pocket driven by a hex key. |
| Square head drive | Square head has four exterior flats at 90-degree angles rather than six flats at 120-degree angles. |
| External Torx (E-Torx) | External Torx has a 6-lobed star shape protruding from the head rather than 6 straight flat faces. |
Also called
External hex/hexagon wrench driveHex bolt headHex washer head drive (HWH)
Good to know
This is a fastener-to-driver interface. A combination-drive example uses T25 internally and 1/4 in external hex. For 6.35 mm across flats, the ideal corner-to-corner dimension is 7.332 mm (AF × 1.1547). Additional standard references: SAE J429.
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