Fasteners reference
1/4-20 bolt reference: diameter, TPI, wrench, and Allen key
A bench reference for the common 1/4-20 UNC thread and the sizes most often confused with it.
Reviewed against GaugeFox data and the sources below · Updated August 4, 2026
Read 1/4-20 from left to right
The first number is the nominal outside thread diameter in inches. The second is threads per inch. UNC means Unified National Coarse.
The fine-thread partner is 1/4-28 UNF. It has the same nominal diameter, so diameter measurement cannot separate the two. Match several crests to the thread gauge or count threads over a known distance.
| Thread | Diameter | Threads per inch | Pitch equivalent |
|---|---|---|---|
| 1/4-20 UNC | 0.250 in / 6.35 mm | 20 | 1.27 mm |
| 1/4-28 UNF | 0.250 in / 6.35 mm | 28 | 0.91 mm |
Common head tools
These are common standardized combinations, not a promise about every screw. Measure the recess or try the tool without force when the head is damaged, plated heavily, or made to another standard.
| Head profile | Common tool |
|---|---|
| Regular external hex | 7/16 in wrench or socket |
| Socket-head cap | 3/16 in Allen key |
| Button-head socket | 5/32 in Allen key |
| Flat / countersunk socket | 5/32 in Allen key |
The M6 comparison
M6 × 1.0 is the nearby metric size. M6 is 0.35 mm smaller in diameter and its 1.00 mm pitch is finer than the 1.27 mm spacing of 1/4-20. A 10 mm wrench on an M6 hex head versus 7/16 inch on a regular 1/4 inch hex head is a useful clue, but the thread measurements are the confirmation.
Reference trail
Sources and review method
These source pages identify the standards or industry references used for this guide. Full standards may require purchase. The guide is an identification aid, not a substitute for a calibrated inspection or the complete applicable standard.
Read the editorial standardUse it at the bench
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GaugeFox adds calibrated on-screen gauges, searchable nearby sizes, and a local Parts Bin to the reference data in this guide.