Unified Bolt Dimensions

Unified (UNC/UNF/UNEF) bolts follow the inch-based thread standard used across North America and still common on legacy British and aerospace equipment. This chart gives head and body dimensions for Unified bolts across the common head styles — hexagon, socket, countersunk, and button head — so you can check across-flats size, head height, and body diameter for a given nominal size. For the thread form itself (diameter, pitch, and tolerance class), see our Unified Threads (UNC/UNF/UNEF) chart.

The charts below gives the dimensions for Hexagon head, Socket cap, Socket countersink, and Socket button screws with Unified (UNC/UNF) Threads.

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

We also have information on Imperial and Metric Bolts.

HEXAGON HEAD DIMENSIONS - UNIFIED - BS 1768:1963

Nominal Size	Threads Per Inch (TPI)		Body Diameter (D)		Across Flats (A/F)		Across Corners (A/C) Max	Head Thickness (H)
	UNC	UNF	Max	Min	Max	Min		Max	Min
1/4"	20	28	0.2500	0.2465	0.4375	0.4305	0.505	0.163	0.153
5/16"	18	24	0.3125	0.3090	0.5000	0.4930	0.577	0.211	0.201
3/8"	16	24	0.3750	0.3715	0.5625	0.5545	0.650	0.243	0.233
7/16"	14	20	0.4375	0.4335	0.6250	0.6170	0.722	0.291	0.281
1/2"	13	20	0.5000	0.4960	0.7500	0.7420	0.866	0.323	0.313
9/16"	12	18	0.5625	0.5585	0.8125	0.8045	0.938	0.371	0.361
5/8"	11	18	0.6250	0.6190	0.9375	0.9295	1.083	0.403	0.393
3/4"	10	16	0.7440	0.7500	1.1250	1.1150	1.300	0.483	0.463
7/8"	9	14	0.8750	0.8670	1.3125	1.3005	1.515	0.563	0.543
1"	8	12	1.0000	0.9920	1.5000	1.4880	1.732	0.627	0.597

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SOCKET CAP SCREW DIMENSIONS - UNIFIED - BS 2470:1973

Nominal Size	Body Dia (D)	Head Dia (Dk)	Head Height (K)	Hex Key Size (S)	Key Engagement (T)
#4	0.112	0.183	0.112	3/32	0.051
#6	0.138	0.226	0.138	7/64	0.064
#8	0.164	0.270	0.164	9/64	0.077
#10	0.190	0.312	0.190	5/32	0.090
1/4"	0.250	0.375	0.250	3/16	0.120
5/16"	0.312	0.469	0.312	1/4	0.151
3/8"	0.375	0.562	0.375	5/16	0.182
1/2"	0.500	0.750	0.500	3/8	0.245

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SOCKET COUNTERSINK SCREW DIMENSIONS - UNIFIED - BS 2470:1973

Nominal Size	Body Dia (D)	Head Dia (Dk)	Head Height (K)	Hex Key Size (S)
#4	0.112	0.224	0.067	1/16
#6	0.138	0.276	0.083	5/64
#8	0.164	0.311	0.090	3/32
#10	0.190	0.359	0.101	1/8
1/4"	0.250	0.480	0.134	5/32
5/16"	0.312	0.600	0.169	3/16
3/8"	0.375	0.720	0.203	7/32
1/2"	0.500	0.931	0.248	5/16

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SOCKET BUTTON SCREW DIMENSIONS - UNIFIED - BS EN ISO 7380

Nominal Size	Body Dia (D)	Head Dia (Dk)	Head Height (K)	Hex Key Size (S)
#4	0.112	0.213	0.059	1/16
#6	0.138	0.262	0.073	5/64
#8	0.164	0.312	0.087	3/32
#10	0.190	0.361	0.101	1/8
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

• Unified Threads (UNC/UNF/UNEF)
• Unified Nut Dimensions
• Imperial Threads
• Imperial Bolt Dimensions
• Thread Designation Charts

FREQUENTLY ASKED QUESTIONS

What's the difference between a Unified bolt and an imperial (BSW/BSF) bolt?

Both use inch-based dimensions, but Unified threads use a 60° thread angle (matching ISO metric) while British Whitworth-based threads (BSW/BSF) use a 55° angle — the two are not interchangeable even at the same nominal diameter. See Imperial Threads for BSW/BSF dimensions.

How do I read a Unified bolt size, e.g. 1/4-20 UNC?

1/4 is the nominal diameter in inches, 20 is the number of threads per inch, and UNC identifies the series (Unified National Coarse). A UNF designation would indicate a finer thread at the same nominal diameter.

Are Unified bolts still commonly used?

Yes, particularly in North America, and on aerospace, automotive, and legacy equipment manufactured to imperial/Unified standards worldwide. Metric has largely superseded Unified threads in general engineering outside North America, but replacement and maintenance parts for existing Unified-threaded equipment are still widely needed.