Sources and scope for this page: ASME B18.3-2012 (R2017), covering the hex key across-flats sizes and the socket screw socket dimensions; and ISO 2936:2014, covering the metric hex key series (one more source is tabulated below). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. Full sources, values and limits on this page.
Page last changed 2026-10-19 · based on ASME B18.3-2012 (R2017)
Quick Answer
Hex Key Sizes — Imperial and Metric
| Key size[1] | Series[1] | Across flats in[2] | Across flats mm[2] | Typically drives[1] |
|---|---|---|---|---|
| 0.028 # | Inch | 0.0280 | 0.711 | #0 socket screws |
| 0.035 # | Inch | 0.0350 | 0.889 | #1 socket screws |
| 0.050 # | Inch | 0.0500 | 1.270 | #2 socket screws |
| 1/16 # | Inch | 0.0625 | 1.587 | #3 socket screws; also a common set screw key |
| 5/64 # | Inch | 0.0781 | 1.984 | #5 socket screws |
| 3/32 # | Inch | 0.0938 | 2.383 | #4 socket screws |
| 7/64 # | Inch | 0.1094 | 2.779 | #6 socket screws |
| 1/8 # | Inch | 0.1250 | 3.175 | #8 socket screws; the most common small key |
| 9/64 # | Inch | 0.1406 | 3.571 | #10 socket screws |
| 5/32 # | Inch | 0.1562 | 3.967 | Small set screws and #10 heavy heads |
| 3/16 # | Inch | 0.1875 | 4.762 | 1/4 in socket cap screws |
| 7/32 # | Inch | 0.2188 | 5.558 | 5/16 in socket set screws |
| 1/4 # | Inch | 0.2500 | 6.350 | 5/16 in socket cap screws |
| 5/16 # | Inch | 0.3125 | 7.938 | 3/8 in socket cap screws |
| 3/8 # | Inch | 0.3750 | 9.525 | 1/2 in socket cap screws |
| 7/16 # | Inch | 0.4375 | 11.112 | 9/16 in socket head screws |
| 1/2 # | Inch | 0.5000 | 12.700 | 5/8 in socket cap screws |
| 9/16 # | Inch | 0.5625 | 14.287 | 3/4 in socket head screws |
| 5/8 # | Inch | 0.6250 | 15.875 | 7/8 in socket head screws |
| 3/4 # | Inch | 0.7500 | 19.050 | 1 in and larger socket head screws |
| 0.7 mm # | Metric | 0.0276 | 0.700 | Small metric set screws |
| 0.9 mm # | Metric | 0.0354 | 0.900 | Small metric set screws |
| 1.3 mm # | Metric | 0.0512 | 1.300 | Small metric set screws |
| 1.5 mm # | Metric | 0.0591 | 1.500 | Small metric set screws |
| 2 mm # | Metric | 0.0787 | 2.000 | Small metric set screws |
| 2.5 mm # | Metric | 0.0984 | 2.500 | Small metric set screws |
| 3 mm # | Metric | 0.1181 | 3.000 | M3 socket screws |
| 4 mm # | Metric | 0.1575 | 4.000 | M4 socket screws |
| 5 mm # | Metric | 0.1969 | 5.000 | M5 socket screws |
| 6 mm # | Metric | 0.2362 | 6.000 | M6 socket screws |
| 8 mm # | Metric | 0.3150 | 8.000 | M8 socket screws |
| 10 mm # | Metric | 0.3937 | 10.000 | M10 socket screws |
| 12 mm # | Metric | 0.4724 | 12.000 | M12 socket screws |
| 14 mm # | Metric | 0.5512 | 14.000 | M14 socket screws |
| 17 mm # | Metric | 0.6693 | 17.000 | M17 socket screws |
| 19 mm # | Metric | 0.7480 | 19.000 | M19 socket screws |
| 22 mm # | Metric | 0.8661 | 22.000 | M22 socket screws |
| 24 mm # | Metric | 0.9449 | 24.000 | M24 socket screws |
| 27 mm # | Metric | 1.0630 | 27.000 | M27 socket screws |
| 32 mm # | Metric | 1.2598 | 32.000 | M32 socket screws |
| 36 mm # | Metric | 1.4173 | 36.000 | M36 socket screws |
Across-flats is the distance between two opposite flats of the hexagonal key, which is the dimension that matters for fit — the corner-to-corner distance is about 15.5% larger. Both series are listed in one table because the two overlap confusingly: a 5 mm key is 0.1969 in, which sits between the 3/16 in (0.1875) and 7/32 in (0.2188) imperial keys. Never substitute across series. A metric key in an inch socket is either too loose and rounds the corners, or too tight and will not enter.
Why Imperial and Metric Keys Are Not Interchangeable
The two series were designed independently and do not coincide anywhere except at 1/4 in and 6.35 mm. Close is worse than far in this application, because a key that almost fits will be forced, and forcing a hex key rounds the socket corners instead of failing visibly.
The practical traps are the near-misses. A 5 mm key measures 0.1969 in and the closest imperial keys are 3/16 in (0.1875) and 7/32 in (0.2188) — both about 0.010 in away, which is a 5% error. A 3 mm key at 0.1181 in against a 1/8 in socket at 0.125 in is a 5.5% error. A 6 mm key at 0.2362 in against 1/4 in at 0.2500 is 5.5% the other way. In every case the key will go in and the socket will round.
The rule is to measure the key, not to measure the screw. If the across-flats dimension is 0.1875 in the key is imperial 3/16; if it is 0.1969 in it is metric 5 mm. A caliper resolves this in seconds, and guessing does not.
Key Fit, Socket Screw Strength and Torque
The hex socket in a cap screw is a deliberate compromise. It puts the driving feature inside the screw head, which keeps the head small, but it removes material from the centre of the head where the stress is highest. The socket is therefore the weakest part of the fastener, and it fails by rounding — which is why socket screws have lower torque ratings than external hex bolts of the same size and grade.
Three practical consequences. Engagement depth matters: a hex key that only enters part of the socket concentrates load on a few corners and rounds them. Use a key long enough to reach the bottom, or a ball-end key for angled access with the understanding that it transmits far less torque. Wear matters: a worn key with rounded corners damages every socket it touches, so replace keys rather than keeping them. Torque matters: a socket screw is tightened to a specification, not until it feels tight — over-torquing rounds the socket and there is no way to recover the fastener once it has.
For heavily loaded or frequently adjusted joints, an external hex head or a Torx socket is a better choice: both transmit torque over a larger area and are far more resistant to cam-out. Torx in particular was designed to eliminate the corner rounding that limits the hex socket.
Frequently Asked Questions
- What size Allen wrench fits a 1/4-20 socket screw?
- A 3/16 in hex key. The key size for a socket screw is set by the socket dimension, not by the thread: 1/4 in socket cap screws use a 3/16 in key, 5/16 in screws use 1/4 in, 3/8 in screws use 5/16 in. The metric equivalent sizes are 5 mm, 6 mm and 8 mm respectively, and they are close enough to feel right and wrong enough to round the socket.
- Is a 5 mm Allen wrench the same as 3/16?
- No. 3/16 in is 4.763 mm and 5 mm is 0.1969 in — a difference of about 5%. That is enough that a 5 mm key in a 3/16 in socket will round the corners, and a 3/16 key in a 5 mm socket will not seat properly. Measure the key with a caliper: 4.76 mm is imperial 3/16 and 5.00 mm is metric.
- What is the difference between a hex key and a ball-end key?
- A ball-end key has a spherical tip that lets it drive a socket at up to about 25° off axis, which is useful in tight spaces. The trade-off is much lower torque capacity — the ball engages only a small area of the socket and will round it under load. Use ball-end keys for running a screw in and out, and switch to a standard square-cut key for final tightening.
- How much torque can a hex key take?
- Less than the socket screw it drives, in most cases — the key is usually stronger than the socket. The limit is set by the screw's socket, which is why socket screw torque tables give lower values than the equivalent external hex bolt. Keys fail by twisting at the short end. If you are regularly rounding sockets or twisting keys, the screw is being over-tightened or the key is worn.
- Do I need both metric and imperial hex key sets?
- If you work on equipment from more than one market, yes. North American equipment is predominantly imperial, European and Asian equipment predominantly metric, and the two series overlap without matching anywhere except at 1/4 in and 6.35 mm. A set that covers both is cheaper than the damage caused by forcing the wrong key into a socket.
Related
Value Sources
References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.
| # | Source | Type | Revision / method |
|---|---|---|---|
| [1] | ASME B18.3 — Socket Cap, Shoulder, Set Screws and Hex Keys | reference | ASME B18.3-2012 (R2017) |
| [2] | Value computed from the standard's defining relationship | derived | Computed at build time from the defining formula stated on the page and verified against every row, then checked against known standard values held as anchors. Nothing in this column was transcribed. |
Data Sources
| Standard | Revision | What it covers on this page |
|---|---|---|
| ASME B18.3 — Socket Cap, Shoulder and Set Screws, Hex Keys and Spline Keys | ASME B18.3-2012 (R2017) | the hex key across-flats sizes and the socket screw socket dimensions |
| ISO 2936 — Assembly tools for screws and nuts, hexagon socket screw keys | ISO 2936:2014 | the metric hex key series |
| DIN 911 / DIN 912 — Hexagon socket set screws and cap screws | DIN 912:1983 | the socket dimensions the key sizes correspond to |
Verification scope
No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.
Derived values — the following values on this page are calculated, not taken directly from the standard:
| Value | How it is derived |
|---|---|
| Across flats in inches and millimetres | Imperial keys from the fraction itself; metric keys are exact millimetres. Inches = mm ÷ 25.4 and mm = inches × 25.4, both recomputed at build time for every row. |
Across-flats dimensions are nominal. Both ASME and ISO specify tolerances on the key, and a worn key is out of tolerance long before it looks damaged — measure rather than eyeball when fit matters. Metric and inch keys are not interchangeable at any size, and forcing a near-miss key rounds the socket rather than failing visibly.
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