Hex (Allen) key
5 mm
0.1969" / 5 mm across flats
Hex key
- Across flats
- 0.197" · 5.00 mm
- Across corners
- 0.227" · 5.77 mm
- Nearest other system
- 3/16"−0.0094"
- Largest key that fits
- 3/16"Loose: rounding risk
Full chart 37 rows
Sort any column, filter by series, print a shop sheet. Click a row to load it on the gauge.
Allen (hex) key size chart
37 rowsAllen (hex) key size chart · ScrewBoltChart
screwboltchart.com · inch-first · tap drills 75% (soft metals) · torque dry, K=0.20 · verify against the governing standard
| 0.028" · 0.7 mm | 0.032" · 0.81 mm | 0.028" (largest that fits: N/A) | Set #0 M1.6 | ||
| 0.028" · 0.71 mm | 0.032" · 0.82 mm | 0.7 mm (largest that fits: 0.7 mm) | Set #0 M1.6 | ||
| 0.035" · 0.89 mm | 0.041" · 1.03 mm | 0.9 mm (largest that fits: 0.7 mm) | BHCS/FHCS #0 · Set #1 #2 M2 | ||
| 0.035" · 0.9 mm | 0.041" · 1.04 mm | 0.035" (largest that fits: 0.035") | BHCS/FHCS #0 · Set #1 #2 M2 | ||
| 0.050" · 1.27 mm | 0.058" · 1.47 mm | 1.3 mm (largest that fits: 0.9 mm) | SHCS #0 · BHCS/FHCS #1 #2 · Set #3 #4 | ||
| 0.051" · 1.3 mm | 0.059" · 1.5 mm | 0.050" (largest that fits: 0.050") | Set M2.5 | ||
| 0.059" · 1.5 mm | 0.068" · 1.73 mm | 1/16" (largest that fits: 0.050") | SHCS M1.6 M2 · Set M3 M3.5 | ||
| 0.063" · 1.59 mm | 0.072" · 1.83 mm | 1.5 mm (largest that fits: 1.5 mm) | SHCS #1 · BHCS/FHCS #3 #4 · Set #5 #6 | ||
| 0.078" · 1.98 mm | 0.090" · 2.29 mm | 2 mm (largest that fits: 1.5 mm) | SHCS #2 #3 · BHCS/FHCS #5 #6 · Set #8 | ||
| 0.079" · 2 mm | 0.091" · 2.31 mm | 5/64" (largest that fits: 5/64") | SHCS M2.5 · BHCS/FHCS M3 · Set M4 | ||
| 0.094" · 2.38 mm | 0.108" · 2.75 mm | 2.5 mm (largest that fits: 2 mm) | SHCS #4 #5 · BHCS/FHCS #8 · Set #10 | ||
| 0.098" · 2.5 mm | 0.114" · 2.89 mm | 3/32" (largest that fits: 3/32") | SHCS M3 M3.5 · BHCS/FHCS M4 · Set M5 | ||
| 0.109" · 2.78 mm | 0.126" · 3.21 mm | 3 mm (largest that fits: 2.5 mm) | SHCS #6 | ||
| 0.118" · 3 mm | 0.136" · 3.46 mm | 1/8" (largest that fits: 7/64") | SHCS M4 · BHCS/FHCS M5 · Set M6 | ||
| 0.125" · 3.18 mm | 0.144" · 3.67 mm | 3 mm (largest that fits: 3 mm) | BHCS/FHCS #10 · Set 1/4" | ||
| 0.141" · 3.57 mm | 0.162" · 4.12 mm | 4 mm (largest that fits: 3 mm) | SHCS #8 | ||
| 0.156" · 3.97 mm | 0.180" · 4.58 mm | 4 mm (largest that fits: 3 mm) | SHCS #10 · BHCS/FHCS 1/4" · Set 5/16" | ||
| 0.157" · 4 mm | 0.182" · 4.62 mm | 5/32" (largest that fits: 5/32") | SHCS M5 · BHCS/FHCS M6 · Set M8 | ||
| 0.187" · 4.76 mm | 0.217" · 5.5 mm | 5 mm (largest that fits: 4 mm) | SHCS 1/4" · BHCS/FHCS 5/16" · Set 3/8" | ||
| 0.197" · 5 mm | 0.227" · 5.77 mm | 3/16" (largest that fits: 3/16") | SHCS M6 · BHCS/FHCS M8 · Set M10 | ||
| 0.219" · 5.56 mm | 0.253" · 6.42 mm | 6 mm (largest that fits: 5 mm) | BHCS/FHCS 3/8" · Set 7/16" | ||
| 0.236" · 6 mm | 0.273" · 6.93 mm | 1/4" (largest that fits: 7/32") | SHCS M8 · BHCS/FHCS M10 · Set M12 M14 | ||
| 0.250" · 6.35 mm | 0.289" · 7.33 mm | 6 mm (largest that fits: 6 mm) | SHCS 5/16" · BHCS/FHCS 7/16" · Set 1/2" | ||
| 0.276" · 7 mm | 0.318" · 8.08 mm | 1/4" (largest that fits: 1/4") | — | ||
| 0.313" · 7.94 mm | 0.361" · 9.17 mm | 8 mm (largest that fits: 7 mm) | SHCS 3/8" · BHCS/FHCS 1/2" · Set 5/8" | ||
| 0.315" · 8 mm | 0.364" · 9.24 mm | 5/16" (largest that fits: 5/16") | SHCS M10 · BHCS/FHCS M12 · Set M16 M18 | ||
| 0.375" · 9.52 mm | 0.433" · 11 mm | 10 mm (largest that fits: 8 mm) | SHCS 7/16" 1/2" · BHCS/FHCS 5/8" · Set 3/4" | ||
| 0.394" · 10 mm | 0.455" · 11.55 mm | 3/8" (largest that fits: 3/8") | SHCS M12 · BHCS/FHCS M14 M16 · Set M20 M22 | ||
| 0.437" · 11.11 mm | 0.505" · 12.83 mm | 12 mm (largest that fits: 10 mm) | — | ||
| 0.472" · 12 mm | 0.546" · 13.86 mm | 1/2" (largest that fits: 7/16") | SHCS M14 · BHCS/FHCS M18 M20 · Set M24 | ||
| 0.500" · 12.7 mm | 0.577" · 14.66 mm | 12 mm (largest that fits: 12 mm) | SHCS 5/8" · BHCS/FHCS 3/4" · Set 7/8" | ||
| 0.551" · 14 mm | 0.637" · 16.17 mm | 9/16" (largest that fits: 1/2") | SHCS M16 M18 · BHCS/FHCS M22 M24 | ||
| 0.563" · 14.29 mm | 0.650" · 16.5 mm | 14 mm (largest that fits: 14 mm) | BHCS/FHCS 7/8" · Set 1" | ||
| 0.625" · 15.88 mm | 0.722" · 18.33 mm | 17 mm (largest that fits: 14 mm) | SHCS 3/4" · BHCS/FHCS 1" | ||
| 0.669" · 17 mm | 0.773" · 19.63 mm | 5/8" (largest that fits: 5/8") | SHCS M20 M22 | ||
| 0.748" · 19 mm | 0.864" · 21.94 mm | 3/4" (largest that fits: 5/8") | SHCS M24 | ||
| 0.750" · 19.05 mm | 0.866" · 22 mm | 19 mm (largest that fits: 19 mm) | SHCS 7/8" 1" |
05 / Field guide
Hex keys,
sorted.
A hex key is sized across its flats. Inch and metric keys interleave closely, and a wrong-system key that almost fits is the fastest way to strip a socket.
Across flats and across corners
The size on the key is the distance across the flats. Lying on a table, a key rests on a flat and shows its across-corners width, about 15% larger, which is why the V-gauge ticks keys at their corner width.
across corners = across flats × 1.1547
Inch and metric neighbours
Some pairs are close: 0.050" (1.27 mm) against 1.3 mm, 5/32" (3.97 mm) against 4 mm. Close is not the same, though: a 4 mm key in a 5/32" socket is too big to enter, and a 5/32" key in a 4 mm socket is 0.03 mm loose, fine for light work and risky at full torque.
Which key a screw needs
Socket cap screws use the largest key for their size, button and flat heads one size down, and set screws one or two sizes down again. For 1/4" that is 3/16", 5/32" and 1/8". For M6 it is 5, 4 and 3 mm.
Getting the most from a key
Seat the key fully before you turn it; a key that only enters halfway concentrates the load on the top of the socket and rounds it. Use the long arm for speed and the short arm in the socket for torque. If a socket is already rounded, a slightly larger Torx bit tapped into it often grips well enough to remove the screw. Replace keys when their corners look shiny or rounded.
Questions
Answered plainly.
01What is 5/32" in mm?+−
3.97 mm. A 4 mm key is 0.03 mm larger and won’t go into a true 5/32" socket; a 5/32" key is slightly loose in a 4 mm socket.
02What Allen key for an M6 screw?+−
5 mm for a socket head cap screw, 4 mm for button and flat heads, 3 mm for a set screw.
03What Allen key for a 1/4" screw?+−
3/16" for a socket head cap screw, 5/32" for button and flat heads, 1/8" for a set screw.
04How do I measure an unmarked hex key?+−
Measure across two opposite flats with calipers, or lay the key on a flat across the V-gauge above, which reads the across-corners width and converts it.
05Ball end or straight end?+−
Ball ends let you turn at an angle for fast running-in. Use the straight end for breaking loose and final tightening; ball ends carry much less torque.
06Why do keys come in 0.028" and 0.035"?+−
Those tiny keys fit #0–#2 set screws and miniature cap screws used in instruments, knobs and model engineering.
07What Allen key for a #8 cap screw?+−
A 9/64" key. #8 button and flat heads take 3/32", and #8 set screws take 5/64".
08Is a 1/4" Allen key the same as 6 mm?+−
No. 1/4" is 6.35 mm, so it won’t enter a 6 mm socket, and a 6 mm key is 0.35 mm loose in a 1/4" socket, enough to round it.
09What size Allen key is 3 mm in inches?+−
3 mm is 0.118". The nearest inch key is 1/8" (0.125"), which is too large to enter a 3 mm socket. Use the metric key.
10What Allen key for an M4 cap screw?+−
A 3 mm key. M4 button and flat heads take 2.5 mm, and M4 set screws take 2 mm.
11What is 3/16" in mm?+−
4.76 mm. A 5 mm key won’t enter a 3/16" socket; a 4.5 mm key would be loose. Use the inch key.
12What Allen key fits a 10-32 set screw?+−
A 3/32" key. #10 socket head cap screws take 5/32" and #10 button heads take 1/8".