1” Bolt & Screw Dimensions: Diameter & Head Size

Q: What clearance hole size is needed for a 1″ bolt or screw?

For a Close Fit, a 1-1/64″ (1.016″) drill bit is utilized. For a Normal/Free Fit (standard clearance), a 1-1/32″ (1.031″) drill bit is specified to allow the screw body to pass through the material without binding.

1″ screw dimensions and bolt sizes are widely used for heavy industrial machinery, structural base plates, and large-scale mechanical engineering projects. This size have a 1.000″ nominal major diameter and is available in 1″-8 coarse (UNC), 1″-12 fine (UNF), or 1″-20 extra-fine (UNEF) thread pitches, representing 8, 12, and 20 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.

Thread diagram fot bolt and screw depicting the major diameter, minor diameter, pitch diameter, and threads per inch

1-8 UNC Screw & Bolt – ANSI/ASME B1.1
Dimension	Size (in)
Dimension	Min	Max
Major Diameter	0.9755	0.998
Pitch Diameter	0.9067	0.9168
Minor Diameter	Not specified	0.8492
Threads Per Inch (TPI)	8
Major Diameter	0.983	0.998
Pitch Diameter	0.9101	0.9168
Minor Diameter	Not specified	0.8492
Threads Per Inch (TPI)	8
Major Diameter	0.985	1
Pitch Diameter	0.9137	0.9188
Minor Diameter	Not specified	0.8512
Threads Per Inch (TPI)	8

1-12 UNF Screw & Bolt – ANSI/ASME B1.1
Dimension	Size (in)
Dimension	Min	Max
Major Diameter	0.981	0.9982
Pitch Diameter	0.9353	0.9441
Minor Diameter	Not specified	0.899
Threads Per Inch (TPI)	12
Major Diameter	0.9868	0.9982
Pitch Diameter	0.9382	0.9441
Minor Diameter	Not specified	0.899
Threads Per Inch (TPI)	12
Major Diameter	0.9886	1
Pitch Diameter	0.9415	0.9459
Minor Diameter	Not specified	0.9008
Threads Per Inch (TPI)	12

1-14 UNS Screw & Bolt – ANSI/ASME B1.1
Dimension	Size (in)
Dimension	Min	Max
Major Diameter	0.9881	0.9984
Pitch Diameter	0.9467	0.952
Minor Diameter	Not specified	0.9133
Threads Per Inch (TPI)	14
Major Diameter	0.9897	1
Pitch Diameter	0.9496	0.9536
Minor Diameter	Not specified	0.9149
Threads Per Inch (TPI)	14

1-16 UN Screw & Bolt – ANSI/ASME B1.1
Dimension	Size (in)
Dimension	Min	Max
Major Diameter	0.9891	0.9985
Pitch Diameter	0.9529	0.9579
Minor Diameter	Not specified	0.9241
Threads Per Inch (TPI)	16
Major Diameter	0.9906	1
Pitch Diameter	0.9557	0.9594
Minor Diameter	Not specified	0.9256
Threads Per Inch (TPI)	16

1-20 UNEF Screw & Bolt – ANSI/ASME B1.1
Dimension	Size (in)
Dimension	Min	Max
Major Diameter	0.9905	0.9986
Pitch Diameter	0.9616	0.9661
Minor Diameter	Not specified	0.9391
Threads Per Inch (TPI)	20
Major Diameter	0.9919	1
Pitch Diameter	0.9641	0.9675
Minor Diameter	Not specified	0.9405
Threads Per Inch (TPI)	20

1-28 UN Screw & Bolt – ANSI/ASME B1.1
Dimension	Size (in)
Dimension	Min	Max
Major Diameter	0.9923	0.9988
Pitch Diameter	0.9716	0.9756
Minor Diameter	Not specified	0.9563
Threads Per Inch (TPI)	28
Major Diameter	0.9935	1
Pitch Diameter	0.9738	0.9768
Minor Diameter	Not specified	0.9575
Threads Per Inch (TPI)	28

1-32 UN Screw & Bolt – ANSI/ASME B1.1
Dimension	Size (in)
Dimension	Min	Max
Major Diameter	0.9929	0.9989
Pitch Diameter	0.9748	0.9786
Minor Diameter	Not specified	0.9617
Threads Per Inch (TPI)	32
Major Diameter	0.994	1
Pitch Diameter	0.9769	0.9797
Minor Diameter	Not specified	0.9628
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.

Hex Head – ANSI/ASME B18.2.1
Dimension	Size (in)
Dimension	Min	Max
Width Across Flats	1.45	1.5
Width Across Corners	1.653	1.732
Head Height	0.591	0.7

Hex Socket Cap Head – ANSI/ASME B18.3
Dimension	Size (in)
Dimension	Min	Max
Head Diameter	1.476	1.5
Head Height	0.988	1
Hex Socket Size	3/4 (0.75 in)

Hex Socket Flat Head – ANSI/ASME B18.3
Dimension	Size (in)
Dimension	Min	Max
Head Diameter	1.841	1.938
Head Height	0.54	Not specified
Hex Socket Size	5/8 (0.625 in)

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Frequently Asked Questions

What is the tap drill size for 1″-8, 1″-12, and 1″-20 screws or bolts?

Tap drill selection is determined by the material and required thread engagement. For aluminum, brass, or plastic (75% thread), a 7/8″ (0.875″) drill bit is specified for 1″-8, a 15/16″ (0.938″) for 1″-12, and a 61/64″ (0.953″) for 1″-20. For steel, stainless, or iron (50% thread), a 59/64″ (0.922″) drill bit is recommended for 1″-8, a 61/64″ (0.953″) for 1″-12, and a 31/32″ (0.969″) for 1″-20.

What clearance hole size is needed for a 1″ bolt or screw?

For a Close Fit, a 1-1/64″ (1.016″) drill bit is utilized. For a Normal/Free Fit (standard clearance), a 1-1/32″ (1.031″) drill bit is specified to allow the screw body to pass through the material without binding.