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Complete reference for hex key sizes in SAE and metric, which key fits socket head cap screws and set screws, SAE-to-metric conversions, and how to measure and choose the right Allen wrench.
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A hex key (Allen wrench) drives the hexagonal socket found in cap screws, set screws, and countless pieces of furniture, bicycles, and machinery. This guide covers every common SAE and metric size, shows which key fits each socket head cap screw and set screw, explains why SAE and metric keys are not interchangeable, and helps you pick between L-key, T-handle, folding, and ball-end styles.
"Hex key" and "Allen wrench" are the same tool — a hexagonal L- or T-shaped key measured across the flats. They come in SAE/inch sizes (0.028" up to 3/8") and metric (0.7mm up to 12mm). Common metric set sizes are 1.5, 2, 2.5, 3, 4, 5, 6, 8, and 10mm; common SAE are 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 key size is determined by the socket-head screw, not the screw's thread diameter. See the chart below.
Hex keys are measured across the flats — the distance between two opposite flat faces of the hexagon — which is the same dimension as the socket in the fastener. The table below lists the common SAE (inch) sizes and metric (mm) sizes, with decimal-inch equivalents so you can compare the two systems directly.
| SAE (inch) Size | Metric Size | Metric in Decimal Inches |
|---|---|---|
| 1/16" (0.0625") | 1.5mm | 0.059" |
| 5/64" (0.078") | 2mm | 0.079" |
| 3/32" (0.094") | 2.5mm | 0.098" |
| 7/64" (0.109") | 3mm | 0.118" |
| 1/8" (0.125") | 4mm | 0.157" |
| 9/64" (0.141") | 5mm | 0.197" |
| 5/32" (0.156") | 6mm | 0.236" |
| 3/16" (0.188") | 8mm | 0.315" |
| 7/32" (0.219") | 10mm | 0.394" |
| 1/4" (0.250") | — | — |
| 5/16" (0.313") | — | — |
| 3/8" (0.375") | — | — |
The SAE and metric columns are listed in ascending order, not as exact equivalents — each row simply places the two systems side by side. Use the decimal-inch column to see how close a given metric key is to an SAE size. The full smaller sizes (0.028", 0.035", 0.050", 0.7mm, 0.9mm, 1.27mm) appear on precision and electronics sets but are uncommon in general workshops.
For standard metric socket head cap screws (DIN 912 / ISO 4762), the hex key size is fixed by the screw's socket, not its thread diameter. Use this chart to match the bolt size to the correct hex key.
| Screw Size | Hex Key Size |
|---|---|
| M3 | 2.5mm |
| M4 | 3mm |
| M5 | 4mm |
| M6 | 5mm |
| M8 | 6mm |
| M10 | 8mm |
| M12 | 10mm |
| M14 | 12mm |
| M16 | 14mm |
The chart above is for standard socket head cap screws. Button-head (ISO 7380) and low-head (DIN 7984) screws use a smaller hex socket — for example, an M6 button-head screw typically takes a 4mm key rather than 5mm. Always confirm against the specific screw if it is not a standard cap screw.
Imperial (SAE) socket head cap screws follow the ASME/ANSI standard. As with metric, the hex key size is determined by the socket, not the thread diameter.
| Screw Size | Hex Key Size |
|---|---|
| #4 | 3/32" |
| #6 | 7/64" |
| #8 | 9/64" |
| #10 | 5/32" |
| 1/4" | 3/16" |
| 5/16" | 1/4" |
| 3/8" | 5/16" |
| 1/2" | 3/8" |
Socket set screws (grub screws) use a smaller hex socket than cap screws of the same thread size, because the socket has to fit within the screw's outside diameter. These are the common pairings.
| Set Screw Size | Hex Key Size |
|---|---|
| M3 | 1.5mm |
| M4 | 2mm |
| M5 | 2.5mm |
| M6 | 3mm |
| M8 | 4mm |
| M10 | 5mm |
| M12 | 6mm |
| #8 (SAE) | 5/64" |
| #10 (SAE) | 3/32" |
| 1/4" (SAE) | 1/8" |
| 5/16" (SAE) | 5/32" |
| 3/8" (SAE) | 3/16" |
A set screw has no head, so the entire body is just the threaded diameter. The hex socket must be recessed inside that diameter, leaving room for the screw walls. That is why an M6 set screw uses a 3mm key while an M6 cap screw uses a 5mm key.
Several SAE and metric hex key sizes are close enough to tempt substitution, but using the wrong one is the fastest way to strip a fastener. The clearest example: a 3/16" key measures 0.1875" while a 5mm key measures 0.197" — about 0.01" larger. A 5mm key will not fully seat in a 3/16" socket, and a 3/16" key will rattle loosely inside a 5mm socket. Under torque, that gap lets the key cam against the socket corners and round them out.
| Near-Match Pair | SAE (decimal) | Metric (decimal) | Difference |
|---|---|---|---|
| 5/64" vs 2mm | 0.078" | 0.079" | ~0.001" (very close) |
| 3/16" vs 5mm | 0.188" | 0.197" | ~0.010" (strips easily) |
| 1/4" vs 6mm | 0.250" | 0.236" | ~0.014" (do not mix) |
| 5/16" vs 8mm | 0.313" | 0.315" | ~0.002" (close, still risky) |
Always match the system to the fastener. If you do not know whether a screw is SAE or metric, try keys in ascending size and use the largest one that fully inserts with a snug fit and no wobble. A loose key plus high torque equals a rounded socket you will have to drill out.
Hex keys come in several physical formats. The size chart is identical across all of them — they differ only in handling, reach, and torque.
Many quality L-key sets have a ball end on the long arm and a straight (broached) end on the short arm — giving you reach and angle on one side and full-torque grip on the other. For heavy-duty work, keep a T-handle set in the matching sizes you use most.
If a hex key is unmarked or worn, you can identify its size in a couple of ways.
Across-the-flats is the standard measurement for all hex tooling, including hex keys, hex bolt heads, and hex stock. If your caliper reads a point-to-point distance, divide by about 1.155 to get the across-flats size.
A good hex key set covers both SAE and metric in the common sizes and includes ball ends for reach. These are the formats worth owning. All links are affiliate links — we may earn a commission at no cost to you.
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SAE and metric socket sizes for bolts and nuts, with close-match conversions.
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Browse All →Yes. "Hex key" and "Allen wrench" (or "Allen key") are different names for the same tool — a hexagonal L- or T-shaped key that drives a hex-socket fastener. "Allen" comes from the Allen Manufacturing Company, an early maker of the tool, and became a generic name. The size is measured across the flats of the hexagon, which matches the hex socket in the screw.
A standard M6 socket head cap screw takes a 5mm hex key. Note that the hex key size is set by the screw's socket, not the M6 thread diameter, so it does not match the bolt size. Some low-head or button-head M6 screws use a 4mm hex key, and M6 set screws typically use a 3mm hex key, so always check the specific screw type.
Not safely. Some SAE and metric sizes are close — for example, a 3/16" key is 0.1875" and a 5mm key is 0.197", a difference of about 0.01". Using the wrong one leaves a loose fit that can round out (strip) the socket or the key under torque. Always use the matching system. When in doubt, try the largest key that fully inserts without forcing.
Common metric hex key sizes are 1.5, 2, 2.5, 3, 4, 5, 6, 8, and 10mm. Common SAE (inch) sizes are 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 inch. A good general-purpose set in both systems covers nearly all household furniture, bicycle, and machinery fasteners you are likely to encounter.
A ball-end hex key has a rounded tip that lets you insert the key at an angle — up to about 25 to 30 degrees off-axis — to reach screws you cannot approach straight on. Use the ball end to start and spin fasteners quickly, then switch to the straight (broached) end for final tightening, because the ball end contacts less of the socket and can round it out under high torque.