When it comes to mechanical assemblies, furniture fittings, machinery, or even DIY projects, Allen bolts (also called hex socket head cap screws) are one of the most reliable fasteners. Unlike standard bolts tightened with a wrench, Allen bolts are driven with a hex key or Allen wrench, allowing for a clean, flush fit and high torque capacity.
If you’ve ever been confused by the different Allen bolt sizes, diameters, and thread types, you’re not alone. Many engineers, mechanics, and DIY enthusiasts often struggle to pick the correct size for their application. That’s where a comprehensive Allen bolt size chart comes in handy.
This guide will walk you through everything you need to know: Allen bolt dimensions, standards, applications, size conversion, and how to use an Allen bolt size chart effectively.
What Are Allen Bolts?
Allen bolts are fasteners with a cylindrical head and a hexagonal recess that requires an Allen wrench (hex key) for tightening. They are commonly used where:
Space is limited, and a wrench cannot fit.
A flush or recessed installation is needed.
Extra torque is required compared to standard screws.
A neat, professional look is important.
They are popular in:
Automotive assemblies
Furniture (like IKEA products)
Bicycles and motorcycles
Industrial machinery
Electronics
Why an Allen Bolt Size Chart Matters
Choosing the correct bolt size isn’t just about convenience — it’s about safety, durability, and performance.
Using the wrong size can strip the hex socket.
Incorrect length can either cause weak fastening or damage the material.
Mismatched threads may lead to loosening or breakage.
That’s why Allen bolt size charts are essential references. They help match the diameter, length, thread pitch, and wrench size to your specific need.
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Allen Bolt Size Standards
Allen bolts are manufactured according to international standards, ensuring interchangeability across industries. The most common standards include:
ISO (International Organization for Standardization) – ISO 4762 covers hex socket head cap screws.
DIN (German Standard) – DIN 912 is a widely used metric standard.
ANSI/ASME (American Standard) – Covers inch-based Allen bolts.
This means Allen bolts can be found in both metric (mm) and imperial (inch) units.
Allen Bolt Size Chart (Metric)
Here’s a quick reference for standard metric Allen bolts (DIN 912 / ISO 4762):
| Nominal Diameter (d) | Thread Pitch (mm) | Head Diameter (mm) | Head Height (mm) | Hex Key Size (mm) |
|---|---|---|---|---|
| M3 | 0.5 | 5.5 – 6 | 3 | 2.5 |
| M4 | 0.7 | 7 – 8 | 4 | 3 |
| M5 | 0.8 | 8.5 – 9 | 5 | 4 |
| M6 | 1.0 | 10 – 11 | 6 | 5 |
| M8 | 1.25 | 13 – 14 | 8 | 6 |
| M10 | 1.5 | 16 – 17 | 10 | 8 |
| M12 | 1.75 | 18 – 20 | 12 | 10 |
| M16 | 2.0 | 24 – 26 | 16 | 14 |
| M20 | 2.5 | 30 – 32 | 20 | 17 |
💡 Example: An M8 Allen bolt means:
8 mm nominal diameter
1.25 mm thread pitch
Requires a 6 mm hex key
Allen Bolt Size Chart (Imperial / Inch)
For projects in the U.S. and some industries, inch-based Allen bolts are common.
| Nominal Size (inch) | Threads per Inch (TPI) | Head Diameter (inch) | Head Height (inch) | Hex Key Size (inch) |
|---|---|---|---|---|
| #4-40 | 40 | 0.22 | 0.09 | 1/16 |
| #6-32 | 32 | 0.28 | 0.11 | 5/64 |
| #8-32 | 32 | 0.34 | 0.13 | 9/64 |
| 1/4″-20 | 20 | 0.43 | 0.19 | 3/16 |
| 5/16″-18 | 18 | 0.52 | 0.24 | 1/4 |
| 3/8″-16 | 16 | 0.62 | 0.29 | 5/16 |
| 1/2″-13 | 13 | 0.83 | 0.41 | 3/8 |
💡 Example: A 1/4″-20 Allen bolt means:
1/4 inch diameter
20 threads per inch
Requires a 3/16 inch Allen wrench
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Allen Bolt Length Selection
When selecting length:
Too short → weak grip, may fail under load.
Too long → may damage internal parts or protrude unnecessarily.
Rule of Thumb:
For steel parts, thread engagement should be at least 1 x bolt diameter.
For aluminum or softer metals, aim for 1.5 x diameter.
How to Read an Allen Bolt Size
An Allen bolt size usually looks like this:
Metric: M8 × 30 – 8.8
M8 = 8 mm diameter
30 = 30 mm length
8.8 = strength grade
Imperial: 1/4″-20 × 1.5″
1/4″ = diameter
20 = threads per inch
1.5″ = length
Common Applications of Allen Bolts
Furniture: Flat-pack furniture often uses Allen bolts for easy assembly.
Bicycles & Motorcycles: Handlebar clamps, brake rotors, and engine covers.
Automotive: Engine assemblies, chassis parts, and aftermarket accessories.
Industrial Machines: Securing precision components.
Electronics: Securing PC cases, mounts, and brackets.
Tips for Using Allen Bolts
Always use the correct hex key size (too small = rounding, too big = won’t fit).
Apply anti-seize or thread locker depending on application.
Use high-strength bolts (grade 12.9) for heavy loads.
Replace damaged bolts immediately — stripped sockets are unsafe.
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Allen Bolt Size Conversion (Quick Reference)
Sometimes you need to convert metric ↔ imperial:
| Metric Bolt | Closest Imperial Equivalent |
|---|---|
| M3 | #4 (0.112″) |
| M4 | #8 (0.164″) |
| M5 | 3/16″ (0.190″) |
| M6 | 1/4″ (0.250″) |
| M8 | 5/16″ (0.312″) |
| M10 | 3/8″ (0.375″) |
Final Thoughts
Allen bolts are small but mighty components that play a huge role in engineering, furniture, automotive, and countless other industries. Having a clear Allen bolt size chart makes it much easier to choose the right fastener for the job, preventing costly mistakes and ensuring strong, durable assemblies.
Whether you’re working with metric M-series bolts or imperial inch-based bolts, always double-check the diameter, length, and hex key size before installation. A properly selected Allen bolt means safety, reliability, and a professional finish.