Allen Wrenches (Hex Keys): History, Uses & Size Charts
Quick answer: Allen wrenches—also called hex keys—are six-sided L-shaped tools used to drive socket (hex-recess) screws. They provide snug, slip-resistant engagement, work in tight spaces, and come in both US (inch) and Metric sizes. Browse Monster Bolts Hex Keys to outfit your kit.
Overview & Benefits
From the mid-1800s to modern manufacturing
The modern L-shaped hex key was popularized in the early 1900s by Allen Manufacturing (Connecticut). Today it’s ubiquitous—from furniture and bicycles to machinery and robotics—because it’s compact, precise, and easy to use.
Why people love hex keys
- Six-point contact for snug, slip-resistant driving
- Resists cam-out vs. Phillips/slotted drivers
- Low profile reaches tight or recessed spaces
- Small, light, and cost-effective; ideal for tiny screws
- Durable—blunt tips can be re-cut to extend life
- Available in US & Metric sizes
- Use with hex driver bits in cordless tools
Shop Hex Keys
Choose inch for US hardware, metric for ISO/DIN hardware.
Compatible Fasteners
Quick Reference: Typical Screw → Hex Key
Typical pairings for socket head cap screws. Other head styles can use different drives—always verify.
| Metric Screw (Socket Cap) | Typical Hex Key | US/Imperial Screw (Socket Cap) | Typical Hex Key |
|---|---|---|---|
| M2 | 1.5 mm | #4-40 | 3/32" |
| M2.5 | 2.0 mm | #6-32 | 7/64" |
| M3 | 2.5 mm | #8-32 | 9/64" |
| M4 | 3.0 mm | #10-24 | 5/32" |
| M5 | 4.0 mm | 1/4-20 | 3/16" |
| M6 | 5.0 mm | 5/16-18 | 1/4" |
| M8 | 6.0 mm | 3/8-16 | 5/16" |
| M10 | 8.0 mm | 1/2-13 | 3/8" |
| M12 | 10.0 mm | 5/8-11 | 1/2" |
US Short-Arm Hex / Allen Key Sizes ASME B18.3-2012
| Hex Key Size | Min Width Across Flats | Short Section Max | Short Section Min | Long Section Max | Long Section Min |
|---|---|---|---|---|---|
| 0.028" | 0.0275" | 0.312" | 0.125" | 1.312" | 1.125" |
| 0.035" | 0.0345" | 0.438" | 0.250" | 1.312" | 1.125" |
| 0.050" | 0.0490" | 0.625" | 0.438" | 1.750" | 1.562" |
| 1/16" | 0.0615" | 0.656" | 0.469" | 1.844" | 1.656" |
| 5/64" | 0.0771" | 0.703" | 0.516" | 1.969" | 1.781" |
| 3/32" | 0.0927" | 0.750" | 0.562" | 2.094" | 1.906" |
| 7/64" | 0.1077" | 0.797" | 0.609" | 2.219" | 2.031" |
| 1/8" | 0.1235" | 0.844" | 0.656" | 2.344" | 2.156" |
| 9/64" | 0.1391" | 0.891" | 0.703" | 2.469" | 2.281" |
| 5/32" | 0.1547" | 0.938" | 0.750" | 2.594" | 2.406" |
| 3/16" | 0.1860" | 1.031" | 0.844" | 2.844" | 2.656" |
| 7/32" | 0.2172" | 1.125" | 0.938" | 3.094" | 2.906" |
| 1/4" | 0.2480" | 1.219" | 1.031" | 3.344" | 3.156" |
| 5/16" | 0.3110" | 1.344" | 1.156" | 3.844" | 3.656" |
| 3/8" | 0.3730" | 1.469" | 1.281" | 4.344" | 4.156" |
| 7/16" | 0.4355" | 1.594" | 1.406" | 4.844" | 4.656" |
| 1/2" | 0.4975" | 1.719" | 1.531" | 5.344" | 5.156" |
| 9/16" | 0.5600" | 1.844" | 1.656" | 5.844" | 5.656" |
| 5/8" | 0.6225" | 1.969" | 1.781" | 6.344" | 6.156" |
| 3/4" | 0.7470" | 2.219" | 2.031" | 7.344" | 7.156" |
| 7/8" | 0.8720" | 2.469" | 2.281" | 8.344" | 8.156" |
| 1" | 0.9970" | 2.719" | 2.531" | 9.344" | 9.156" |
Metric Short-Arm Hex / Allen Key Sizes DIN 911
| Hex Key Size | Min Width Across Flats | Short Section Max | Short Section Min | Long Section Max | Long Section Min |
|---|---|---|---|---|---|
| M0.9 | 0.876 | 11 | 6 | 34 | 28 |
| M1.3 | 1.244 | 16 | 11 | 44 | 39 |
| M1.5 | 1.470 | 14 | 13 | 45 | 43 |
| M2 | 1.970 | 16 | 15 | 50 | 48 |
| M2.5 | 2.470 | 18 | 17 | 56 | 53 |
| M3 | 2.960 | 20 | 18 | 63 | 60 |
| M4 | 3.960 | 25 | 23 | 70 | 66 |
| M5 | 4.960 | 28 | 26 | 80 | 76 |
| M6 | 5.950 | 32 | 30 | 90 | 86 |
| M8 | 7.950 | 36 | 34 | 100 | 95 |
| M10 | 9.950 | 40 | 38 | 112 | 106 |
| M12 | 11.950 | 45 | 43 | 125 | 119 |
| M14 | 13.930 | 56 | 53 | 140 | 133 |
| M17 | 16.930 | 63 | 60 | 160 | 152 |
| M19 | 18.930 | 70 | 67 | 280 | 271 |
Frequently Asked Questions
Are “Allen wrench” and “hex key” the same?
Yes—two names for the same six-sided L-shaped tool for socket screws.
Can I use an Allen wrench in a drill?
Yes. Use hex driver bits in a drill or impact driver for faster installs.
What sizes are most common?
Inch: 1/16", 5/64", 3/32", 1/8", 5/32", 3/16". Metric: 2, 2.5, 3, 4, 5, 6 mm.
Can I use metric Allen keys on inch screws (or vice versa)?
No—even slight mismatches can strip screw heads. Always match exact sizing.
Why are some Allen keys rounded at one end?
That’s the ball-end design, allowing angled entry in tight spaces. It’s for access, not high-torque use.
What size for garbage disposals or shower handles?
Many disposals use 1/4". Delta shower handles commonly 3/32" or 1/8" (check your model’s specs).
What hex key fits an M4 socket screw?
Typically 3.0 mm for socket head cap screws. Button/low-head styles may differ—verify the spec.
Important Note: Screw Size ≠ Hex Key Size
Hex key size refers to the across-flats dimension of the tool, which is not the same as the screw’s nominal diameter. For example, an M4 button socket screw typically uses a 2.0 mm hex key, not an M4 key.