Allen Wrench Sizes | The Complete Hex Key Chart

Nothing stalls a furniture build or bike repair like reaching for the wrong hex key. Whether you need an Allen wrench for a M6 bolt or a 5/32 in setscrew, the size chart below matches common fasteners to their correct hex keys — so you stop guessing and start turning. For a tested roundup of quality tools, check out our guide to the best Allen wrench sets.

Why Size Matching Matters

An Allen wrench must seat fully into the screw’s hex socket for torque to transfer cleanly. A loose-fitting key rounds the socket corners, and a tight one can cam out under load. Either mistake strips the fastener and turns a five-minute job into a drill-out project.

The terms “Allen wrench,” “hex key,” and “hex wrench” all describe the same tool — an L-shaped or T-shaped bar whose ends fit hexagonal sockets. Major reference pages treat the names as interchangeable, so don’t let labeling confusion send you to the wrong drawer. What matters is the drive size, not what you call the tool.

Standard SAE And Metric Size Charts

Hex key sizes split into two families: inch-based SAE sizes governed by ASME B18.3, and metric sizes defined under DIN 911 / ISO 2936. The two systems are not interchangeable, even when numbers look close — a 6 mm key does not properly fit a 1/4 in socket, and forcing it can damage both tools.

Typical sets include 0.028 in, 0.035 in, 0.050 in, 1/16 in, 5/64 in, 3/32 in, 7/64 in, 1/8 in, 9/64 in, 5/32 in, 3/16 in, 7/32 in, 1/4 in, 5/16 in, and 3/8 in. Wikipedia’s hex key reference documents both size families in detail.

Matching Screw Sizes To Hex Keys

The screw’s nominal diameter tells you which key to grab for metric fasteners. These pairings come from standard metric screw charts and hold across furniture, bicycle, and automotive hardware.

Pro Tips: Avoiding Stripped Sockets

Common mistakes cause most stripped hex sockets. Confusing near-equivalent metric and SAE sizes — like 5 mm vs 3/16 in, or 6 mm vs 1/4 in — tops the list. Inserting the key only partway also increases rounding and cam-out risk, so push it fully into the socket before applying torque.

If the wrench feels loose in the socket, stop and verify the size rather than forcing it. A worn or undersized key is another stripping hazard; replace keys that show rounded edges. For stubborn fasteners, use the correct long or short arm rather than improvising with a near-size key — published hex key reference charts show different arm lengths by size, and the right arm gives the best leverage without risking damage. Some conversion charts list approximate nearest equivalents between metric and SAE, but the fit can still be imperfect, so verify compatibility before applying force on critical fasteners.

Precision sizes below 1/16 in and 1.5 mm serve small electronics and eyeglass repair, while larger 14 mm, 17 mm, and 19 mm keys handle industrial and heavy automotive work. Small sizes need a light touch — over-torquing a 0.7 mm or 0.9 mm key snaps it instantly and can leave a broken tip lodged in the screw.