Hex Key Size Chart: Allen Wrench Table for Every Screw (Free PDF)

The Hex key size (mm) column reads what type of Allen key to drive it with; you find the thread size in the first column. So M6 uses a 5 mm, an M8 uses a 6 mm, and an M10 use an 8 mm. You read the column labeled Hex Key Size (mm).

To drive any standard socket head cap screw of a given thread size you use the Allen key whose key size is listed in the hex key column. The key size is the width across flats of the hexagon socket in the head of the bolt.

This doesn't correspond evenly to thread size: M6 = 5 mm, M8 = 6 mm, than M10 = 8 mm. Guessing one size smaller than the thread doesn't work once past M8.

The thread pitch (mm) is coarse pitch. Under ISO 261, this is the normal thread pitch for that diameter. Use it to verify you have the size you believe you do.

Key size (in, decimal): convert the hex key width to decimal inches. If all you have to measure with is imperial or if you’re wondering if the SAE key you’ve got in-hand is actualy the metric one you need, here’s how to double-check.

Head diameter and head height is taken from ISO 4762. These are the max dimensions of the head. The head diameter gives you the counterbore needed, and the head height is always equal to the nominal thread diameter on a standard cap screw.

Look at the three plateau pairs in the key column. Both M16 and M18 each has a 14 mm key; both M20 and M22 each have 17 mm; both M24 and M27 each have 19 mm.

Look at the thread head diameter on the table. If the head of the big screw is bigger than it, and your key fits, you’re probably one size up from the correct pair.

The reverse trap is at the lower end of the table. M1.6 and M2 each have a 1.5 mm key, they share a key, but differ by their thread.

These are the ISO 4762 / DIN 912 socket cap screws with coarse thread (standard head). Socket cap screws of other heads like button head, set screws, low head, etc., or countersunk flat head of the same thread require smaller keys. See metric variants table below for those.

For fine pitch sockets, the same socket can be used as for coarse thread versions, so the pitch column doesn’t apply, though the key column still does.

There are two useful warnings. Plated and/or worn out sockets will fit loosely or tightly. The socket may be slightly oversized as allowed by ISO. Paint or zinc plating in the socket can also prevent the key from engaging properly, so it never bottoms out. So clean the socket before you conclude the key just doesn't work.

Secondly, you must always bottom a key out into place before tightening. Usually, cap screws is class 12.9. These have seating torques that will round off a partially engaged socket well before the screw gives. You should of checked the fit first.

In the first column, look up the screw size. Go down and read the Hex key size column to find the Allen key for that size of socket head cap screw.

Small machine screw gauges runs #0 to #12 numbers. Fractional diameters starts at 1/4 in and get larger.

By looking at the number gauges, you can use the Basic diameter column and your calipers to identify an unknown screw. A #8 gauge have a shank measuring about 0.164 in. A 9/64 key will fit it.

Fine and coarse thread pitches for a given diameter is listed under TPI (threads per inch). Fine and coarse pitch use the same socket, so do they for a key. A 3/16" key fit a 1/4-20 and a 1/4-28 cap screw.

The key size is the same measurement as a decimal number. This comes into play if you're using a caliper or comparing to a metric key. It will spell out the three-number keys as well. For example, #0 is 0.050 in, a common size in most inch L-key sets.

See those same keys? That’s number two and three; they’re 5/64. Four and five is also 3/32. Ten and twelve is both 5/32. Seven-sixteenths and 1/2 are each 3/8. Seven-eighths and one inch are the same: They’ll take a 3/4 key.

The most frequent error we see in shops is where people gets confused between 1/2 and 7/16. They both use a 3/8 key. Before you order another one, check the shank or head first.

The classic imperial trap is grabbing the wrong nearly-fitting metric key. A 4 mm key slips into a 5/32 socket. A 5 mm key slips into a 3/16 socket. If you tighten them to cap screw torque, both will rounds the corners.

That’s fine if you want to use one or the other, but what about the others? Which near-misses realy work? Which are hazards that could strip the head? The bottom of the page list those.

If none of your keys fit a supposedly imperial screw, the machine might not be imperial at all. Global manufacurers’ post-1990s gear generally are.)

This table is for standard head socket cap screws per ASME B18.3. Smaller keys is used for button head and flat countersunk head screws, as well as set screws of equal nominal size. The imperial version are found in the table below.

A low head cap screw employs a correspondingly reduced socket. They is usually one or two sizes smaller. They will not interchange with this column even though they has the same thread.

Starting at the top, scan down until you find your thread size in the leftmost column, then move across to the column corresponding to the head style of what you have in hand. If it’s a cap head screw, look for the ISO 4762 value in the Cap head key column. This number matches the ISO 4762 from the first table; thus, if you want to quicky reference the offsets, just start there instead.

For a flat head or button head, use a key that’s a size smaller than the matching cap screw size (down to two sizes smaller). For a set screw, use an even-smaller key. You can see the variation in this example: An M8 screw has a 6 mm key for the cap head, a 5 mm key for the flat and button heads, and a 4 mm key for the set screw.

For Socket Screws, the Button head Key Section is for dome-topped screws that meet ISO 7380-1 and 7380-2, including the flange type with the same socket. For the Flat head key section it's 90-degree countersunk screws to DIN 7991 or ISO 10642.

For both the button and flat head keys, the socket sizes match on M3 up to M12. Beyond that, they split apart. There's an M14 flat head screw with a 10 mm key, but there isn't an M14 button head defined by ISO 7380.

The key section for set screws is for headless grub screw per DIN 913 to 916 and ISO 4026 to 4029. The single socket size applies to cup point, flat point, cone point, and dog point all alike.

Now here’s why:

Button heads are stripped by people who think that the cap head key will work. It doesn’t because the socket is actualy smaller.

So often you force on the next smaller size because it’s loose. Flat head and button heads has even shallower sockets which provide less grip for keys than cap heads do. Those styles strip at much lower torques.

Push it all the way down and really turn it hard. Don’t rely on the cap screw torque tables for flat or button heads. They’re rated to about half the seating torque of an equivalent-sized cap screw.

The third type has the smallest and deepest sockets relative to the thread. They are called Set Screw Sockets. Below M3, they’re tiny. They range from 0.7 mm up to 1.3 mm. They are only available as part of precision key set.

Jam a 1.5 mm key into an M2.5 set screw and you’ll ruin it on the spot. Notice how imperial habits gets mixed up at the high end. An M10 set screw requires a 5 mm key. A nearly identical diameter 3/8 inch imperial set screw has a 3/16 inch key. That’s equal to 4.76 mm. It will show up sometimes; other times you can only feel the differance.

Empty cells in the table? Those indicate combinations defined by neither DIN nor ISO standards. An M18 button head screw, for instance, is a custom part from the manufacturer. Don’t guess at its socket; you'll have to measure it.

Another type includes low head cap screws per DIN 7984 and DIN 6912. These have shrunken sockets too, but they aren’t covered here. You should of checked the moddern standards.

The first column tells you the screw size; the next columns is for the head style you're driving, and the key columns provide the key size which reveals the offset relative to the cap head. You see that the standard socket head key repeat from the imperial table above.

For example, the button or flat head take a smaller key than the cap head of the same size. Set screws takes an even smaller key on the same size thread. These sizes is 3/16 for the set screw, 7/32 for the button or flat head, and 5/16 for the cap head on a 3/8-16 thread.

Sizes of Button head key and Flat head key is the same from #0 through 5/8 in., then split at 3/4 in. On that size, the button head require a 7/16 key; the flat head a 1/2 key.

For sizes larger than 3/4 in., there are no standard button heads; flat heads cease at 1-1/2 in. The values for set screw keys is taken from ASME B18.3 Table 14. These apply to all point styles. Cup, flat, oval, cone, or half dog head styles all has the same socket for a specific diameter.

Those numbers on the top of the table aren’t typos; they’re actualy keys available for purchase. According to ASME B18.3, 0.028, 0.035, and 0.050 in keys is real sizes that ship in inch precision sets.

The 0.028 key gets you a #0 set screw; the 0.035 key is shared between #1 and #2 set screw. This is the issue with metric near misses: a 0.9 mm key is 0.0354 in. It’ll sort of turn in a 0.035 socket, but it realy does matter at this size. Even a thousandth of slop tends to round off the corners. To keep the corners from rounding off, should of stick to a dedicated inch microset for your instrument needs.

There are two size-specific traps. On one hand, there is 3/8 flat head and button screws that need a 7/32 key, but many cheaper SAE sets skip this size. You then try to make one fit by forcing a sloppy 3/16 or 1/4 key, which is where those screws gets all mangled.

On the other hand, for set screws, the jumps for 7/8 in and up is odd sized too. They don't follow the cap screw progression, and from 7/8 in up, they jump in odd steps like 1/2, 9/16, and 5/8. Pattern match at your own risk; check with the table.

With these types of heads it’s all about torque discipline. The flat sockets and shallow button heads strips out easy and ASME warns that with short set screws the reduced socket depth can causes the key or socket to fail before reaching normal seating torque. Why? Because short screws further reduce the socket depth.

Next you reach for your socket driver and bit instead of the short arm L-key, which keeps the corners from rounding off. Use a new, sharp key; worn ones focus the load on the smallest point at the edge of the flats. What if a set screw doesn't come loose?

Each metric hex key match its nearest imperial size on the table. Each row pairs a metric hex key with the closest imperial size, both expressed in decimal inches. From this you can determine exact amount of interference or space between the two sizes which appears in the Difference (in) column.

Before you assume you can make a substitution based on a small difference, be sure to check the Substitution verdict.

Because the direction make a difference, the socket needs to be equal to or larger than the key being inserted. For example, a 7/32 socket will hold a 5.5 mm key because it is 0.0022 inches smaller. But a 7/32 key will never go into a 5.5 mm socket at all.

There is four combinations that make sense day-to-day. A 5/32 bit fits into a 4 mm socket. A 5.5 mm bit fits into a 7/32 socket. A 5/64 bit and a 2 mm key is interchangeable in both directions because they are only six ten-thousandths apart. A 5/16 inch bit fits into an 8 mm socket.

You can expect to have roughly two to two and a half thousandths of slop in certain pairing. That’s fine for light seating and removal but not repeated torquing of Grade 8 or class 12.9 screws.

Using a 19 mm bit in a 3/4 socket is as big as you should of go. The difference between the two is only two thousandths, which is why you see 3/4 bits turning up in metric truck tool kits.

And of course, the well-known evil twin pairs live up to their reputations. 4 mm in a 5/32 socket and 3/16 in a 5 mm socket appear to match, yet shear off the corner. The 4 mm is twelve ten-thousandths smaller then the 5/32, and the 3/16 is nine and a half thousandths smaller then the 5 mm. Both sound small until you realize that with force on the key, it just twists and touches the sides only where the corners was, and peens them over.

It is even worse with 3/8 versus 10 mm and 1/4 versus 6 mm. The difference is thirteen thousandths or more, and it is near certain to strip. As a last-ditch rescue operation, you can sometimes tap a next-smaller-diameter key into a partly rounded socket. This is not a correct substitution; it's an extraction trick.

Other metric sizes don’t have an imperial cousin at all. For example, the 7, 12, 17 and 24 mm keys is located between imperial keys. They are sandwiched between imperial keys on either side.

That’s where you can use this as a quick diagnostic test. If your imperial set skips from loose to will-not-enter, that’s probably metric because there’s a gap in the size. And the opposite is also true.

Inch keys with three numbers like 0.028, 0.035, and 0.050 has no metric doubles either (same with imperial 7/64, 9/64, and 7/16). A half thousandth is a big fraction of the flat width at such small sizes.

If you need to replace a key, apply this rule: Always put the stand-in key into the socket, never the other way around. Maximum slop is 0.0025 inches or less. Use this only for taking things apart at low torque levels.

Purchase the right keys to tighten parts to the correct torque. A hex key is cheaper than all the tools, drills-outs and new fasteners required for removing a rounded socket.