Allen Wrench Sizes: Metric & SAE Hex Key Conversion Chart
Metric Hex Key → SAE
Direct Answer
Allen wrenches, also called hex keys, come in metric sizes from 1.5mm to 19mm and SAE sizes from 1/16 inch to 3/4 inch. The two systems are not interchangeable. Forcing the wrong system into a recess strips the corners, since even a close size mismatch concentrates force on two opposing points instead of spreading it across all six sides of the hex. Most modern furniture, bicycles, and automotive components built after the 1980s use metric hex hardware.
This page covers the full metric and SAE hex key charts with converted equivalents, the difference between an L-key and a hex socket bit, common applications by product category, and how to recover a stripped hex recess without destroying the fastener.
Metric Allen Wrench Sizes
| Metric (mm) | Decimal Inches | Closest SAE |
|---|---|---|
| 1.5mm | 0.0591" | 1/8" (-0.0659") |
| 2mm | 0.0787" | 1/8" (-0.0463") |
| 2.5mm | 0.0984" | 1/8" (-0.0266") |
| 3mm | 0.1181" | 1/8" (-0.0069") |
| 4mm | 0.1575" | 5/32" (+0.0012") |
| 5mm | 0.1969" | 3/16" (+0.0094") |
| 6mm | 0.2362" | 1/4" (-0.0138") |
| 7mm | 0.2756" | 9/32" (-0.0057") |
| 8mm | 0.3150" | 5/16" (+0.0025") |
| 10mm | 0.3937" | 3/8" (+0.0187") |
| 12mm | 0.4724" | 15/32" (+0.0037") |
| 14mm | 0.5512" | 9/16" (-0.0113") |
| 17mm | 0.6693" | 21/32" (+0.0130") |
| 19mm | 0.7480" | 3/4" (-0.0020") |
SAE Allen Wrench Sizes
| SAE | Decimal Inches | Metric (mm) |
|---|---|---|
| 1/16" | 0.0625" | 1.59mm |
| 5/64" | 0.0781" | 1.98mm |
| 3/32" | 0.0938" | 2.38mm |
| 7/64" | 0.1094" | 2.78mm |
| 1/8" | 0.1250" | 3.17mm |
| 9/64" | 0.1406" | 3.57mm |
| 5/32" | 0.1563" | 3.97mm |
| 3/16" | 0.1875" | 4.76mm |
| 7/32" | 0.2188" | 5.56mm |
| 1/4" | 0.2500" | 6.35mm |
| 5/16" | 0.3125" | 7.94mm |
| 3/8" | 0.3750" | 9.52mm |
| 7/16" | 0.4375" | 11.11mm |
| 1/2" | 0.5000" | 12.70mm |
Allen Wrench vs Hex Socket
An Allen wrench, also called an L-key, is a length of hex steel bent into an L shape. It's cheap, portable, and fine for low-torque assembly, but the short lever arm limits how much torque a person can realistically apply. A hex socket bit fits onto a ratchet or impact wrench instead, using the longer handle and, for impact tools, hammering torque to break loose a stuck or over-tightened fastener that an L-key can't budge. Keep an L-key set for quick assembly work and a hex bit socket set for anything seized or high-torque.
Ball-End vs Standard Hex Keys
A standard hex key only engages a fastener straight on, with the shaft square to the recess. A ball-end hex key rounds off the working tip, letting it engage a fastener at up to roughly 25 degrees off center, useful when a bolt sits at an angle or behind an obstruction that blocks a straight approach. The ball tip contacts less surface area inside the recess than a straight hex tip, so it delivers less maximum torque and carries a higher risk of rounding the recess on a stubborn fastener. Use the ball end for speed and access, and switch to the straight end for the final tightening or an initial stuck-fastener break.
Applications
- Furniture assembly: metric (IKEA and most flat-pack brands)
- Bicycle components: metric (2, 3, 4, 5, 6, 8mm cover most use)
- Automotive: both metric and SAE, depending on vehicle origin and age
- Industrial machinery: metric in most of the world, SAE on older US-built equipment
Identifying a Stripped Hex Bolt
A stripped recess feels rounded: the key slides without engaging any real resistance. Try seating the next size up firmly, which can bite into the deformed walls of a lightly rounded recess. A Torx bit one size up sometimes grips where a hex key won't, since its star shape can wedge into rounded hex corners. For badly damaged heads, a screw extractor or drilling the head off are the remaining options, in roughly that order of preference. For the wrench and socket equivalents of a stripped bolt's head, not just its hex recess, check the wrench size chart.
Allen Wrench Sizes FAQ
Can I use a metric Allen wrench on an SAE hex bolt?
No, not reliably. Metric and SAE hex sizes rarely match closely enough to avoid rounding the recess. A 5mm key is close to 3/16 inch but not identical, and forcing a mismatched key strips the corners of the recess, especially under real torque.
What size Allen wrench do I need for IKEA furniture?
Most flat-pack furniture uses 3mm, 4mm, 5mm, and 6mm metric hex keys, which is why IKEA and similar brands include a single multi-size metric key with most kits.
What is the most common bicycle Allen key size?
5mm is the single most common size on a modern bicycle, used on stem bolts, seat clamps, and many brake and derailleur mounts. A set covering 2, 2.5, 3, 4, 5, 6, and 8mm covers nearly everything on a typical bike.
Why do some Allen keys have a ball end?
A ball-end hex key can engage the fastener at an angle, up to about 25 degrees off center, which helps in tight spaces where a straight approach isn't possible. The tradeoff is lower torque capacity at an angle, since the ball tip contacts less of the recess than a straight hex tip.
Can I use a Torx bit in a hex socket recess?
No. Torx uses a six-point star profile that is a different shape from a hex recess, not just a different size, so a Torx bit will not seat properly in a hex socket or vice versa.
What should I do if my hex key won't grip a stripped bolt?
Try firmly seating the next size up to bite into the rounded walls of the recess. If that fails, a screw extractor or an EZ-out designed for hex or socket-head fasteners can grip from inside the stripped recess. As a last resort, drilling the head off separates the fastener from whatever it's threaded into.