1/4” Bolt & Screw Dimensions: Diameter & Head Size
1/4″ bolt dimensions and screw sizes are foundational in automotive repair, structural assemblies, and general mechanical fastening. This size is standardized with a 0.250″ nominal major diameter and is available in 1/4″-20 coarse (UNC), 1/4″-28 fine (UNF), or 1/4″-32 extra-fine (UNEF) thread pitches, representing 20, 28, and 32 threads per inch (TPI) respectively. This reference chart follows ANSI/ASME B18.6.3 and B18.3 standards for the head size and ANSI/ASME B1.1 standards for the thread dimensions to ensure compatibility across imperial fastening applications.
Disclaimer: Dimensions and tolerances may vary slightly depending on head style, manufacturer, or material — always verify with the official product datasheet before use.
1/4-20 UNC Screw & Bolt – ANSI/ASME B1.1
Dimension
Size (in)
Min
Max
Major Diameter
0.2367
0.2489
Pitch Diameter
0.2108
0.2164
Minor Diameter
Not specified
0.1894
Threads Per Inch (TPI)
20
Major Diameter
0.2408
0.2489
Pitch Diameter
0.2127
0.2164
Minor Diameter
Not specified
0.1894
Threads Per Inch (TPI)
20
Major Diameter
0.2419
0.25
Pitch Diameter
0.2147
0.2175
Minor Diameter
Not specified
0.1905
Threads Per Inch (TPI)
20
1/4-28 UNF Screw & Bolt – ANSI/ASME B1.1
Dimension
Size (in)
Min
Max
Major Diameter
0.2392
0.249
Pitch Diameter
0.2208
0.2258
Minor Diameter
Not specified
0.2065
Threads Per Inch (TPI)
28
Major Diameter
0.2425
0.249
Pitch Diameter
0.2225
0.2258
Minor Diameter
Not specified
0.2065
Threads Per Inch (TPI)
28
Major Diameter
0.2435
0.25
Pitch Diameter
0.2243
0.2268
Minor Diameter
Not specified
0.2075
Threads Per Inch (TPI)
28
1/4-32 UNEF Screw & Bolt – ANSI/ASME B1.1
Dimension
Size (in)
Min
Max
Major Diameter
0.243
0.249
Pitch Diameter
0.2255
0.2287
Minor Diameter
Not specified
0.2118
Threads Per Inch (TPI)
32
Major Diameter
0.244
0.25
Pitch Diameter
0.2273
0.2297
Minor Diameter
Not specified
0.2128
Threads Per Inch (TPI)
32
Major Diameter –
The largest diameter of the external thread, measured crest to crest.
This defines the nominal size of the screw or bolt.
Pitch Diameter –
The diameter of an imaginary cylinder where the thread thickness equals
the space between threads. This dimension primarily controls thread fit
and tolerance class of 2A and 3A.
Minor Diameter –
The smallest diameter of the external thread, measured root to root.
This affects thread strength and clearance.
Threads Per Inch (TPI) –
The number of complete threads within one inch of thread length.
Higher TPI values indicate a finer thread pitch.
What is the tap drill size for 1/4″-20, 1/4″-28, and 1/4″-32 screws or bolts?
The appropriate tap drill depends on the material and required thread engagement. For aluminum, brass, or plastic (75% thread), a #7 (0.201″) drill bit is specified for 1/4″-20, a #3 (0.213″) for 1/4″-28, and a 7/32″ (0.219″) for 1/4″-32. For steel, stainless, or iron (50% thread), a 7/32″ (0.219″) drill bit is recommended for 1/4″-20, and a #1 (0.228″) drill bit is specified for both 1/4″-28 and 1/4″-32.
What clearance hole size is needed for a 1/4″ bolt or screw?
For a Close Fit, an F (0.257″) drill bit is utilized. For a Normal/Free Fit (standard clearance), an H (0.266″) drill bit is specified to allow the screw or bolt body to pass through the material without binding.
What is the difference between a bolt and a screw?
The difference is based on the intent of the fastener’s use; If the primary intent is for the fastener to pass through an unthreaded holes and be assembled with a nut, it is a Bolt. In this application, you usually hold the head and turn the nut to apply tension. But if the primary intent is for the fastener to be installed into a threaded (tapped) hole, it is a Screw. In this application, the connection is tightened by turning the head of the fastener itself.
