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.

Allen Wrench Sizes

Hex key (Allen wrench) sizes in both series, with the across-flats dimension in inches and millimetres and the socket screw sizes each key drives.

Data verified 2026-09-29 · based on ASME B18.3-2012 (R2017)

Quick Answer

Hex keys are sized by across-flats distance, not by the screw they fit. A 1/8 in key (3.175 mm) drives a #8 socket screw; a 3/16 in key (4.763 mm) drives a 1/4 in socket cap screw. The nearest metric equivalent is usually one size off — 3 mm is 0.118 in against 1/8 in at 0.125 in — which is why substituting a metric key for an imperial one rounds the socket.

Find a key size

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.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	ASME B18.3 — Socket Cap, Shoulder, Set Screws and Hex Keys	standard	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

Cross-checked against:

Imperial across-flats values checked against the fractional sizes they are defined by
Every metric conversion recomputed at build time and compared against the printed value
Key-to-screw correspondence checked against the socket dimension tables in ASME B18.3
Four sizes held as known-value checks, including both an imperial and a metric anchor

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.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Allen Wrench Sizes." https://refmanual.com/charts/allen-wrench-sizes/ (accessed 2026-09-29). Data from ASME B18.3 — Socket Cap, Shoulder and Set Screws, Hex Keys and Spline Keys, ASME B18.3-2012 (R2017).

BibTeX

@misc{allen-wrench-sizes,
  title        = {Allen Wrench Sizes},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/allen-wrench-sizes/}},
  note         = {Accessed: 2026-09-29},
  source       = {ASME B18.3 — Socket Cap, Shoulder and Set Screws, Hex Keys and Spline Keys, ASME B18.3-2012 (R2017)}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Allen Wrench Sizes
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/allen-wrench-sizes/
PY  - 2026
DA  - 2026-09-29
N1  - Source: ASME B18.3 — Socket Cap, Shoulder and Set Screws, Hex Keys and Spline Keys, ASME B18.3-2012 (R2017)
ER  -

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