Allen Bolt Size Chart (Metric & Inch) – Dimensions, Key Sizes & Torque Guide

Allen bolts, also known as hex socket head screws or Allen screws, are common fasteners found in machinery, automotive, electronics, and many more fields. Recognized by their cylindrical head with an internal hexagonal socket, they are tightened or loosened using an Allen key (hex wrench). This guide, based on the expertise of experienced engineers from CNCLATHING, will provide you with a detailed explanation of Allen Bolts, covering what they are, where and when to use them, standards, key sizes, torque values, engineering consideration when using them, and how to handle common issues.

Allen Bolt Size Chart (Quick Answer)

Allen bolts (hex socket head screws) follow standard sizes defined by ISO 4762, DIN 912, and ASME B18.3. The most commonly used sizes include:

• M3 → 2.5 mm hex key
• M6 → 5 mm hex key
• M8 → 6 mm hex key
• M10 → 8 mm hex key

These sizes ensure compatibility across global engineering and manufacturing applications.

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What is an Allen Bolt?

An Allen bolt, also known as a hex socket head screw, is a fastener with an internal hexagonal drive. It is designed to deliver:

• Higher torque transmission
• Compact installation in tight spaces
• Reduced risk of stripping compared to external hex bolts

Unlike traditional bolts, Allen bolts are tightened using a hex key instead of a wrench.

When Should You Use Allen Bolts?

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Allen Bolt (Screw) Dimensions

Metric Standard Allen Bolt Size Chart in MM (IS0 4762 / DIN 912)

Nom. Size	Thread Pitch	Body Diameter		Head Diameter		Key Engagement
		Max	Min	Max	Min	Min
M1.6	0.35	1.60	1.46	3.00	2.87	0.80
M2	0.40	2.00	1.86	3.80	3.00	1.00
M2.5	0.45	2.50	2.36	4.50	4.33	1.25
M3	0.50	3.00	2.86	5.50	5.32	1.50
M4	0.70	4.00	3.82	7.00	6.80	2.00
M5	0.80	5.00	4.82	8.20	8.02	2.50
M6	1.00	6.00	5.82	10.00	9.78	3.00
M8	1.25	8.00	7.78	13.00	12.73	4.00
M10	1.50	10.00	9.78	16.00	15.73	5.00
M12	1.75	12.00	11.73	18.00	17.73	6.00
M14	2.00	14.00	13.73	21.00	20.67	7.00
M16	2.00	16.00	15.73	24.00	23.58	8.00
M18	2.50	18.00	17.73	27.00	26.67	9.00
M20	2.50	20.00	19.73	30.00	29.53	10.00
M24	3.00	24.00	23.73	36.00	35.48	12.00
M30	3.50	30.00	29.67	45.00	44.42	15.00
M36	4.00	36.00	35.61	54.00	53.37	18.00
M42	4.50	42.00	41.61	63.00	62.31	21.00
M48	5.00	48.00	47.61	72.00	71.27	24.00

Imperial Standard Allen Bolt Size Chart in Inches (ASME B18.3)

Nom. Size	Body Diameter (D)		Head Diameter (A)		Key Engagement (T)
	Max	Min	Max	Min	Min
0	0.0600	0.0568	0.096	0.091	0.025
1	0.0730	0.0695	0.118	0.112	0.031
2	0.0860	0.0822	0.140	0.134	0.038
3	0.0990	0.0949	0.161	0.154	0.044
4 (#4)	0.1120	0.1075	0.183	0.176	0.051
5	0.1250	0.1205	0.198	0.192	0.051
6 (#6)	0.1380	0.1329	0.226	0.218	0.064
8	0.1640	0.1585	0.270	0.262	0.077
10 (#10)	0.1900	0.1840	0.312	0.303	0.090
1/4	0.2500	0.2435	0.375	0.365	0.120
5/16	0.3125	0.3053	0.469	0.457	0.151
3/8	0.3750	0.3678	0.562	0.550	0.182
7/16	0.4375	0.4294	0.656	0.642	0.213
1/2	0.5000	0.4919	0.750	0.735	0.245
5/8	0.6250	0.6163	0.938	0.921	0.307
3/4	0.7500	0.7380	1.125	1.102	0.370
7/8	0.8750	0.8647	1.312	1.293	0.432
1	1.0000	0.9886	1.500	1.479	0.495
1-1/4	1.2500	1.2336	1.875	1.852	0.620
1-1/2	1.5000	1.4818	2.250	2.224	0.745

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How to Read an Allen Bolt Size

Example: M8 × 1.25

• M8 = 8 mm thread diameter
• 1.25 = thread pitch
• Uses 6 mm hex key

This standardized naming ensures global compatibility in engineering design.

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Allen Bolt Key Size

An Allen bolt key, commonly called an Allen key or hex key, is a six-sided tool designed to fit into the hexagonal socket of an Allen bolt. These keys are typically made of hardened steel and are available in both straight and L-shaped forms. The size of the Allen key must match the socket size of the bolt for proper engagement.

Allen Bolt Key Size Chart (MM/Inches)

Allen key sizes correspond to the width across flats of the hex socket in the bolt head. Common metric and imperial sizes include:

Allen Key Sizes for Metric Allen Bolts (ISO 4762 / DIN 912)

Metric Bolt Size	Hex Key Size (mm)	Hex Key Size (inches)
M1.6	1.5	0.059
M2	1.5	0.059
M2.5	2.0	0.079
M3	2.5	0.098
M4	3.0	0.118
M5	4.0	0.157
M6	5.0	0.197
M8	6.0	0.236
M10	8.0	0.315
M12	10.0	0.394
M14	12.0	0.472
M16	14.0	0.551
M18	14.0	0.551
M20	17.0	0.669
M22	17.0	0.669
M24	19.0	0.748
M27	19.0	0.748
M30	22.0	0.866
M33	24.0	0.945
M36	27.0	1.063
M39	27.0	1.063
M42	32.0	1.260
M45	32.0	1.260
M48	36.0	1.417
M52	36.0	1.417
M56	41.0	1.614
M64	46.0	1.811

Allen Key Sizes for Inch (Imperial) Allen Bolts (ASME B18.3)

Inch Bolt Size	Hex Key Size (inches)	Hex Key Size (mm)
0	0.050	1.27
1	1/16 (0.0625)	1.59
2	5/64 (0.0781)	2.00
3	5/64 (0.0781)	2.00
4	3/32 (0.0938)	2.38
5	3/32 (0.0938)	2.38
6	7/64 (0.1094)	2.78
8	9/64 (0.1406)	3.57
10	5/32 (0.1562)	3.97
1/4	3/16 (0.1875)	4.76
5/16	1/4 (0.2500)	6.35
3/8	5/16 (0.3125)	7.94
7/16	3/8 (0.3750)	9.53
1/2	3/8 (0.3750)	9.53
5/8	1/2 (0.5000)	12.70
3/4	5/8 (0.6250)	15.88
7/8	3/4 (0.7500)	19.05
1	3/4 (0.7500)	19.05
1-1/4	7/8 (0.8750)	22.23
1-1/2	1 (1.0000)	25.40

The hex key size corresponds to the internal socket width across flats. Using the correct size prevents:

• Tool slipping
• Socket damage
• Improper torque application

How to Calculate and Determine the Key Size for an Allen Bolt？

The correct key size matches the internal width across flats of the Allen socket. For metric bolts, this is usually standardized by size, e.g., an M6 bolt generally uses a 5 mm Allen key. For imperial bolts, check the standard chart or measure the socket across flats and match to the nearest hex key.

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Allen Bolt Torque Guide

Tightening torque depends on multiple engineering factors, including

• Clamping force requirements
• Bolt material and strength grade
• Thread friction and lubrication

How to Calculate Allen Bolt Torque

Torque can be estimated using the formula:

T = K × F × d

Where:

• K is the torque coefficient (typically 0.12–0.25),
• F is the preload force (0.6–0.7 × yield strength × stress area),
• d is the nominal diameter.

However, real-world torque varies depending on application conditions.

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Engineering Considerations When Using Allen Bolts

When designing or machining parts for Allen bolts:

• Ensure proper thread tolerance (ISO fit)
• Avoid insufficient socket depth
• Consider tool access in deep cavities
• Prevent burrs in internal hex features

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Standards for Allen Bolts

Allen bolts are manufactured according to:

• ISO 4762
• DIN 912
• ASME B18.3

These standards ensure interchangeability across industries.

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More FAQs About Allen Bolts

What size Allen key for M6 bolt?

→ 5 mm

Are Allen bolts stronger than hex bolts?

→ Typically yes, due to better torque transfer

Can torque values be universal?

→ No, they vary based on material and application