Hex keys, commonly called Allen wrenches or Allen keys, are made to two parallel dimensional standards: DIN 911/ISO 2936 for metric sizes and ASME B18.3 for inch (SAE) sizes. Common metric sizes run from about 1.5 mm up to 41 mm, and common SAE sizes run from 1/16" up to 3/8" and beyond. The two size series don't line up exactly — a metric key is never precisely the same as an inch key of similar size — so the conversions below are the closest available substitutes, not exact equivalents, and should be checked for fit before use on a fastener that matters.
What Is a Hex Key (Allen Wrench)?
A hex key is an L-shaped or T-handled bar of hexagonal cross-section that fits into a matching hexagonal socket machined into a fastener head, most often a socket-head cap screw or set screw. "Allen" is a genericized trademark: the Allen Manufacturing Company of Hartford, Connecticut, founded in 1910 by engineer William G. Allen, built its business on a cold-forming process for socket-head set screws and became closely associated with the hex-drive fastener system, to the point that "Allen wrench" is now used generically for any hex key regardless of maker. Metric hex keys are made to DIN 911/ISO 2936; inch (SAE) hex keys are made to ASME B18.3 — the two standards define slightly different across-flats tolerances, which is why a metric and an inch key of nominally similar size are not true substitutes for each other.
Metric Hex Key Size Chart
The table below is our own factory production spec for chrome-vanadium (#6150) L-shaped hex keys, covering the short-arm-driving/long-arm-driving dimensions (L1 = long arm, L2 = short arm) across the full metric range we manufacture.
| Item No. | Size (mm) | L1 (mm) | L2 (mm) |
|---|---|---|---|
| TAH100-0127 | 1.27 | 40 | 13 |
| TAH100-015 | 1.5 | 46 | 15 |
| TAH100-020 | 2 | 51 | 17 |
| TAH100-025 | 2.5 | 57 | 20 |
| TAH100-030 | 3 | 64 | 22 |
| TAH100-035 | 3.5 | 68 | 25 |
| TAH100-040 | 4 | 72 | 28 |
| TAH100-045 | 4.5 | 78 | 30 |
| TAH100-050 | 5 | 83 | 32 |
| TAH100-060 | 6 | 94 | 37 |
| TAH100-080 | 8 | 105 | 43 |
| TAH100-090 | 9 | 111 | 46 |
| TAH100-100 | 10 | 119 | 49 |
| TAH100-120 | 12 | 134 | 56 |
| TAH100-140 | 14 | 151 | 69 |
| TAH100-170 | 17 | 174 | 79 |
| TAH100-190 | 19 | 196 | 88 |
| TAH100-220 | 22 | 216 | 100 |
| TAH100-240 | 24 | 242 | 112 |
| TAH100-270 | 27 | 250 | 100 |
| TAH100-300 | 30 | 280 | 112 |
| TAH100-320 | 32 | 315 | 125 |
| TAH100-360 | 36 | 355 | 140 |
| TAH100-410 | 41 | 401 | 153 |
Note the smallest size, 1.27 mm: this is the metric-marked equivalent of the precision inch size 0.050", included specifically because sets that need to service both metric and inch precision hardware standardize on it.
SAE (Inch) Hex Key Sizes
Common inch hex key sizes, from smallest to largest typically stocked in a general-purpose set, are: 0.028", 0.035", 0.050", 1/16", 5/64", 3/32", 7/64", 1/8", 9/64", 5/32", 3/16", 7/32", 1/4", 5/16", and 3/8". The first three (0.028"–0.050") are precision sizes found mainly in electronics and instrument sets rather than general mechanical toolboxes.
Metric to SAE Conversion (Closest Substitutes)
Because the metric and inch series are built on different base units, no inch size is mathematically identical to a metric size — the pairs below are the closest commercially available substitutes, commonly used when the exact matching size isn't on hand. Treat them as a stop-gap, not a replacement for owning both sets, and always check for a snug fit (no play, no forcing) before applying real torque.
| Inch size | Closest metric substitute |
|---|---|
| 5/64" | 2 mm |
| 3/32" | 2.5 mm |
| 7/64" | 3 mm |
| 1/8" | 3.5 mm |
| 5/32" | 4 mm |
| 3/16" | 5 mm |
| 7/32" | 5.5 mm or 6 mm |
| 1/4" | 7 mm |
| 5/16" | 8 mm |
| 3/8" | 10 mm |
Some metric sizes, such as 9 mm, have no reasonably close inch counterpart at all — for those, the only real option is to use the matching metric key rather than force an inch substitute, since an undersized key rounds the fastener's socket and an oversized one won't seat.
Ball-End vs. Standard Hex Keys
A ball-end hex key has a rounded tip that lets it engage a fastener at an angle — commonly cited as up to roughly 25–30° off-axis — which is useful for start-threading a screw in a recessed or angled bore where a straight key can't line up. The trade-off is torque capacity: because the ball only contacts the hex socket's flats at points rather than across their full face, published estimates put the loss at around 25% less torque capacity than a standard straight hex end, and pushing a ball end too hard risks rounding the fastener's socket. The common practice is to put a ball end on the long arm for quick spinning and angled starts, and rely on the short, straight end for final tightening or breaking loose a stuck fastener.
What Fasteners Use a Hex Key
The most common fastener driven by a hex key is the socket head cap screw (DIN 912 / ISO 4762), which has a cylindrical head with a flat underside and an internal hex recess; it's the workhorse of machine building because the flat head seats flush in a counterbore and distributes clamping load evenly, and the hex drive resists cam-out far better than a cross-head recess under high torque. Hex keys are also the standard drive for set screws (grub screws) — headless fasteners used to lock pulleys, collars, knobs, and couplings onto a shaft, where the hex socket lets the screw sit flush or below the surface with nothing to snag. Because both fastener families rely entirely on the hex key for installation and removal, a worn or undersized key is one of the most common causes of a rounded-out socket head cap screw or set screw.
Shop Metric and SAE Hex Keys
We manufacture L-shaped hexagon Allen keys and ball-end hexagon Allen keys across the metric range above, plus matching inch (SAE) sizes and full sets. For custom sizes, sets, or private-label packaging, contact our team.