1/4"-20 UNC Thread
Direct Answer
1/4"-20 UNC is a fractional Unified Coarse machine screw thread with a major diameter of 0.2500 inches (6.350 mm) and 20 threads per inch (TPI). Requiring a #7 (0.2010 inch) tap drill, it is the universal global standard under ISO 1222 for camera tripod mounting sockets, studio light stands, and general machinery bolting.
Source: ANSI/ASME B1.1 / ISO 1222 (Photography — Tripod Connections)
Specs
| Nominal Major Diameter | 0.2500 in (6.350 mm) |
| Threads Per Inch (TPI) | 20 TPI (Coarse pitch: 1.270 mm) |
| Fine Pitch Equivalent | 1/4"-28 UNF (0.907 mm pitch) |
| Tap Drill Size | #7 drill bit (0.2010 in / 5.10 mm) or 13/64 in |
| Minor Diameter (Internal) | 0.1959 in (4.976 mm) |
| Tensile Stress Area | 0.0318 in² (20.5 mm²) |
| Common Drive Size | 7/16" Hex (11.11 mm) / 3/16" Allen (4.76 mm) |
The Global Camera Tripod Standard (ISO 1222)
Since the early 20th century, 1/4"-20 UNC has been the mandated mounting thread for all consumer and professional cameras (DSLRs, mirrorless bodies, action cameras, smartphone tripod clamps, and projector mounts). The standard thread engagement depth in camera baseplates is between 5.5 mm and 7.5 mm.
General Mechanical Fastening
1/4"-20 is the most widely stocked bolt in North American hardware, used universally across automotive brackets, wood furniture assembly (with T-nuts), and machine framing.