Imperial Bolt Size Chart

This chart gives head and body dimensions for imperial (BSW/BSF-threaded) bolts across the common head styles — hexagon, socket, countersunk, and button head — so you can check across-flats size, head height, and body diameter without measuring a sample bolt by hand. For the thread form itself, see our Imperial Threads chart; for the North American inch-based Unified system, which uses a different thread angle and isn't interchangeable with BSW/BSF, see Unified Bolt Dimensions.

The charts below gives the dimensions for Hexagon Head, Socket Cap, Socket Cap - Low Head, Socket Countersink and Socket Button screws with British Standard Whitworth (BSW) and British Standard Fine (BSF) Threads.

Dimensions shown are in Inches, if you wish to convert to mm, please use our universal converter.

We also have information on Metric and Unified Bolts.

HEXAGON HEAD DIMENSIONS - BSW + BSF - BS916:1953

Nominal Size	Threads Per Inch (TPI)		Across Flats (A/F)	Across Corners (A/C)	Head Thickness (H)
	BSW	BSF
1/4"	20	26	0.525	0.606	0.188
5/16"	18	22	0.600	0.693	0.250
3/8"	16	20	0.710	0.820	0.312
7/16"	14	18	0.820	0.947	0.375
1/2"	12	16	0.920	1.062	0.438
5/8"	11	14	1.100	1.270	0.562
3/4"	10	12	1.300	1.501	0.688

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SOCKET CAP SCREW DIMENSIONS - BSW + BSF - BS2470:1973

Nominal Size	Body Dia (D)	Head Dia (Dk)	Head Height (K)	Hex Key Size (S)	Key Engagement (T)
1/4"	0.250	0.375	0.250	3/16	0.125
5/16"	0.312	0.469	0.312	7/32	0.156
3/8"	0.375	0.562	0.375	5/16	0.188
7/16"	0.438	0.656	0.438	5/16	0.219
1/2"	0.500	0.750	0.500	3/8	0.250
5/8"	0.625	0.938	0.625	1/2	0.312
3/4"	0.750	1.125	0.750	9/16	0.375

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SOCKET CAP LOW HEAD SCREW DIMENSIONS - BSW + BSF - BS 2470:1973

Nominal Size	Body Dia (D)	Head Dia (Dk)	Head Height (K)	Hex Key Size (S)	Key Engagement (T)
1/4"	0.250	0.375	0.156	5/32	0.094
5/16"	0.312	0.469	0.194	3/16	0.117
3/8"	0.375	0.562	0.234	7/32	0.141
1/2"	0.500	0.750	0.312	5/16	0.188

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SOCKET COUNTERSINK SCREW DIMENSIONS - BSW + BSF - BS 2470:1973

Nominal Size	Body Dia (D)	Head Dia (Dk)	Head Height (K)	Hex Key Size (S)
1/4"	0.250	0.500	0.144	5/32
5/16"	0.312	0.625	0.181	3/16
3/8"	0.375	0.750	0.217	7/32
7/16"	0.438	0.812	0.212	1/4
1/2"	0.500	0.938	0.254	5/16
5/8"	0.625	1.188	0.324	3/8
3/4"	0.750	1.438	0.395	1/2

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SOCKET BUTTON SCREW DIMENSIONS - BSW + BSF - BS 2470:1973

Nominal Size	Body Dia (D)	Head Dia (Dk)	Head Height (K)	Hex Key Size (S)
1/4"	0.250	0.437	0.132	5/32
5/16"	0.312	0.547	0.166	3/16
3/8"	0.375	0.656	0.199	7/32
1/2"	0.500	0.875	0.265	5/16

RELATED PAGES

• Imperial Threads (BA/BSF/BSW/BSPP/BSPT)
• Unified Bolt Dimensions
• Metric Bolt Dimensions
• British Association Fastener Dimensions
• Spanner Size Comparison Chart

FREQUENTLY ASKED QUESTIONS

Are imperial (BSW/BSF) and Unified (UNC/UNF) bolts interchangeable?

No — despite both being inch-based, BSW/BSF threads use a 55° thread angle while Unified threads use a 60° angle, so they won't mate safely even at matching nominal diameters. Always check the thread standard, not just the diameter, before substituting one for the other.

Is BSW/BSF still used on new equipment?

It's largely been superseded by metric threads in general UK engineering, but BSW/BSF fasteners are still needed for maintaining and repairing older machinery, vehicles, and structures built to imperial standards.

How do I identify an unmarked imperial bolt?

Measure the major diameter and count threads per inch, then compare against the dimensions on this page and our Imperial Threads chart to identify the closest matching standard size.