Socket Size Chart: Drive Size, Metric and SAE Socket Table (Free PDF)

Find your job in the Best for column, and then read left to find the drive. The size of the drive will match the square anvil on the breaker bar or ratchet or extension, as well as the square hole in its back.

The two columns typically shown (SAE and Metric) are the types of sockets makers stock in that drive. Find your socket size within one of these spans, then that’s the naturaly home for it.

The “Square” drive (mm) is the actual size in millimeters of that inch-based square drive: 6.35 mm for 1/4 in; etc. The ISO designation (mm) is the rounded metric version of this drive's name. This version is found in European catalogues (e.g., Gedore) or ISO 1174, which round off the range to 6.3, 10, 12.5, 20, and 25 mm.

A 12.5 mm drive socket will fit any 1/2 in ratchet, because they is physically the same drive.

This is NOT an absolute breaking limit. This is where The Typical torque wrench range column comes into play. It is a practical capability guide based off the torque wrenches manufacturers create with each drive.

In other words, you’ll have no problem doing over 100 ft-lb with a good 3/8 in drive ratchet. However, don’t cheat by using to much torque beyond what’s listed on the span; if the fastener spec calls for more than the listed span, move up a drive instead.

Why are there overlaps? Because sometimes a 40 ft-lb bolt can be tightened well enough using either a 3/8 in or 1/2 in drive.

Buyers get trapped at the rack for two reasons.

One is that most manufacturers overlap their range of sockets so that a 19 mm is available in 3/8 in, 1/2 in and 3/4 in drives. Torque-wise, these match only where the job calls for them, or you can put a 14 mm 1/4 in drive socket on a rusted M10 chassis bolt, which subject the entire load to a 6.35 mm square.

The other trap: Adapters such as 1/2 in female to 3/8 in male pass along the limitation of the smaller drive; they don't transmit the bigger drive's capacity. Twisting an anvil off is classic behavior with step-up adapters trying to drive big sockets off little ratchet.

The table above outlines square-drive impact and hand tools. Spline drives, 1/4 in hex bit drive and the occasional 5/8 in square drive found on some specialized or older tools is not included.

If you only buy one set, the trades default is 3/8 in drive with a 1/4 in set added for light work and a 1/2 in breaker bar for wheels and suspension. You should of got only one set?

Start at the top of the column labeled with socket size and scan until you find yours. Look to see if those drive columns lists it as commonly stocked (labeled "Common") or less common (likely found alone or in larger sets). If there is a dash, then it's not a good drive for that size.

Lastly, the column labeled Typical Fastener tell you what size it is for, double check to make sure you've got the right row.

A hex head is measured by its socket size, and that’s a width across the flats, not the width of the bolt thread. An M8 bolt has a hex head that’s 13 mm wide across the flats (that’s why you can use a 13 mm socket to fit it). That’s also the same width in inches, as seen in the decimal column, so you can easily check it against a SAE socket on the same rail.

We have more clear verdicts from that sort of comparison in the interchange table below.

Observe what happens to all those doubled-up entries due to the history of standards. By moddern ISO standards, an M10 head is 16 mm but by older DIN standards it’s 17 mm while by JIS standards it’s 14 mm. Ditto for M12 heads (i.e., 18 vs. 19 vs. 17 mm) and M14 heads (i.e., 21 vs. 22 vs. 19 mm).

When you’re working on a car, a metric set missing 15 or 17 or 21 mm just won’t cut it. Even though no current ISO bolt heads measure 15 or 17 mm, they exist as real-sized part.

An additional wrinkle with cheap sets is the even number spacing. So you have a set that jumps from 19 mm, then 21, then 24, skipping 20, 22 and 23. The 22 mm skip hurts most. That size is used alongside 7/8 inch near equivalents and old M14 heads.

Ideally, you want a set that has all the sizes between 8 and 24. That will cover almost every automotive fastener.

The table is for hex socket openings on a square drive. It’s not for spline, hex bit, or E-Torx openings. This table doesn’t show which length or wall thickness of socket to buy either.

To find that, refer to the Spark Plug Table for thin-wall spark plug sizes and the Socket Types Table for standard, deep, or impact socket. It should of been easier to find than this.

The Socket size (in) column is where you’ll find your fractional size. If you’re comparing fractions quickly, use the Decimal (in) column; it’s immediately clear that 0.875 is 7/8 while 0.813 is 13/16.

If you’re determining if a metric socket could substitute for an imperial one, check out mm equivalent. The drive columns functions identically to those in the metric table. Common indicates the socket is typically available in this drive. Less common indicates you’ll only find them single or in bulk-sized set. A dash indicates you should of check in another drive.

This column shows the typical fastener served. It provides an association between the socket and the bolt or nut that uses it. It's based off the dimensions for hex head bolts (ASME B18.2.1) and hex nuts (B18.2.2).

Note that there are two instances of a fork in the road here. For example, a 7/16 in bolt head has a 5/8 in socket but a 11/16 in finished hex nut; conversely, a 9/16 in bolt head has a 13/16 in socket but a 7/8 in nut. Therefore, retain both socket in your set; otherwise, you could have the wrong size at either end of your joint.

In general, SAE sizes step up mostly in 1/16 in increments through 1 in. And thereafter they jumps in mixed or 1/8 in steps. In all catalogues, the fractions don’t get reduced. If it’s marked 5/8 on a socket, that’s what it is. It is not 10/16. So if you’re thinking 10/16, look for the 5/8 stamp.

Some sizes, like 9/32 and 11/32, dropped out of common usage along with points-era ignition work. But they still exist in full runs at 1/4 in drive.

Grabbing nearly matching metric sizes is also the classic rail error. While 19 mm fits onto a 3/4 in head, there’s no 18 mm that will fit anything SAE. Nor does 14 mm even slide over a 9/16 in hex.

All of these are scored out in thousandths of an inch in the below interchange table. Use it to figure out whether something will work before you round off a corner.

Square-drive “hex” sockets: The above table shows all the 6- and 12-point hex openings for inch-size fasteners. Whitworth is not included. Whitworth is a separate socket system in which the number shown is the actual diameter of the bolt, not the width of the head.

This is not a wrench chart. Combination and open-end wrench runs has their own oddities regarding interchange, and they are in our wrench size chart.

This tells you the thread size for the bolt or nut. You read down and see which socket fits its hex.

The ISO socket (mm) is the width across flats of a moddern ISO 4032 nut or an ISO 4014/4017 hex bolt. These is common standards today and this is the first number you would try on something manufactured in the past couple of decades.

Use the square drive from the Drive to reach for column. It has the torque capability that matches fasteners of that size at normal property classes.

The Legacy DIN socket (mm) column is there because there was four sizes of large-headed DIN 931/933 bolts with matching nuts (DIN 934). These were: 17 mm for M10 bolts; 19 mm for M12 bolts; 22 mm for M14 bolts; and 32 mm for M22 bolts.

You'll find these on machinery, trailers and other things designed in Germany prior to ISO harmonisation, and they still carry these larger heads today. They're still available as DIN 934 nut replacements too, hence the use of both kinds of sockets.

The JIS small hex (mm) column refers to Japanese vehicles, where M8, M10, M12, M14 and M16 bolts often sport heads measuring 12, 14, 17, 19 and 22 mm respectively.

The collision zone is where you’re looking. Is it an M10 DIN head or an M12 JIS head? What about an M12 DIN head or M14 JIS head? The latter two could both fits your 19 mm socket. The former two could both fit your 17 mm socket.

So when you have a mystery bolt to measure, you don’t look at the socket that fits and infer its size. Instead, you use calipers to measure the thread diameter. Why? Because if you infer the size based off what socket fits, you’ll buy the wrong replacement bolt.

Flange-head bolts make things even more complicated. There is many different M10 flange bolts used in cars that fit 15 mm sockets. But there are also M8 flange bolts that use 12 mm sockets. And so on. No matter which standard is used, the result is the same.

One exception is structural bolts. Larger heads from the same bolt system is used here, such as high strength structural sets like EN 14399 or something similar.

Sometimes stainless or hot-dip galvanised hardware follows DIN heads (rather than ISO). Before engaging an impact gun in a fastener, you check the head with a caliper if torque is of importance.

This one is a table of socket sizes versus thread sizes. Note that when using a ring spanner or open end, the same width heads apply. The widths are shown on our spanner chart from the wrench end.

You will note that there are no Allen bolts (hex socket-head cap screws) in this table whatsoever. These bolts requires hex keys. We cover the hex keys on our hex key size chart.

The first column is the fastener diameter, then select the socket from the column that matches what you're turning. For the head of a bolt use the Hex bolt socket (in) column. For the finished hex nut on the opposite end use the Hex nut socket (in) column. If the joint use a heavier hex pattern, use the Heavy hex socket (in) column.

The Drive to reach for column indicate which square drive corresponds with common torque levels for that diameter in Grade 8 or Grade 5 hardware.

Bolt and nut columns diverges at two sizes. That's the biggest gotcha about inch hardware. A 7/16 in fastener combines a 5/8 in bolt head, 11/16 in nut. And a 9/16 in fastener is a combo of a 13/16 in head and a 7/8 in nut.

Otherwise, number sizes diverge oppositely. The wider #6-#12 screw are hex machine screw nuts, larger than the corresponding hex screw heads. Elsewhere the head and nut is identical in their shared socket.

Most big A307 hardware is heavy hex. Structural bolts (A325/F3125) is also heavy hex. Pretty much any kind of stud bolt nut for pipe flanges is heavy hex too. Heavy hex has a width across flats of 1.5 times the diameter + 1/8 in.

Your 3/4 in flange nuts won’t fit into the 1-1/8 in socket that the table said it would? They’re heavy hex. They requires a 1-1/4 in socket.

Two other traps exist. Officially, a 1/4 in hex bolt is 7/16 in across its flats. Many imported hardware items, including 1/4 in hex cap screws, has 3/8 in heads. Bring both sizes.

Inch-system hardware is on this table. If you find the head about the same as one of those, it's probably metric, especially on a vehicle built since the 1980s. Before assuming size, check the interchange table.

For lag screws: Use head width of a hex bolt from the table.

For finished hex nuts: Use dimensions from this table.

This does not apply to square-head set screws larger than those in the table.

Whitworth/BSF hardware is not applicable.

Flange-head fasteners with other than standard hex diameters is not applicable.

In the table above, every row is a matched pair of SAE socket to its nearest metric socket.  Exact size (mm) is the converted fraction; Gap (mm) and Gap (thou) are the distance between those two openings. That becomes a shop decision which goes in the Verdict column.

Rule of thumb: If it’s less than roughly 5 thou in the gap, the substitution are invisible in use. If it’s more like 5-10 thou, then you’ll get by fine with a 6-point socket and a good hex. Anything over 10 thou is borrowing trouble, don’t do it.

It’s directional. And as the judgements say, The larger open end fits over the smaller hex, never the other way around. A 13 mm socket slides over a 1/2 in head but not vice-versa (because 12.70 mm is smaller than 13 mm). Rows labeled with one-way only fits one-way as indicated.

The safe pairs is also worth learning by rote; they’re called that because they are so commonly used. You’ve got 3/4 in and 19 mm (2 thou away from each other); 1-1/16 in and 27 mm (half a thou); 1-13/16 in and 46 mm; 5/32 in and 4 mm; 7/32 in and 5.5 mm; 1-3/8 in and 35 mm; 1-1/2 in and 38 mm.

And you remember 13 mm and 1/2 in is the bad pair, loose enough to round off a tight or corroded hex? The only reason 14 mm and 9/16 in isn’t equally dangerous is that it’s barely fitting in one direction.

Contact area causes damage. A slight substitution will work on a new zinc-plated bolt, but it will strip a previous rounded or rusted one because all the corners takes the hit.

Did you run one loose? Full seat and use a 6-point socket. No impact, don’t shock-load with an impact gun. Just hold the load steady.

Why? A socket is closed and has a 6-point form that grips all six flats. So it can absorbs much more slop.

An open-end wrench, however, concentrates its load onto just two flats, which is why it can stand much less slop. And don't apply these judgements to spanners, since we have the wrench size chart to cover that scenario.

Instead of sorting your socket walls by size, use the table to sort them by type. Use the When to grab it column to identify what you’re going to be doing.

Before you pull out the gun, check the impact rating. Before you commit to a length, check Relative reach. Each type in the table are available in the same size runs as listed above in the SAE and metric tables. Size and type is independent choices.

It stands out the quickest in a jumble of drawers. Chrome vanadium steel with a polished chrome finish; that’s a hand socket. An impact rated one have matte black finish over chrome molybdenum steel.

No reputable manufacturer chromes the body of their impact sockets; the colour is just a convention, it doesn’t guarantee it’s an impact one. You can see the walls is much thicker on impact, which explains why an impact socket won’t always fit where the chrome one used to be.

For depth, the general rule of thumb is to get the shortest socket that will clear the stud (additional length eats up clearance and also introduces wobble). Deep sockets is for the opposite case, like when a nut is spun far down a long stud and a shallow socket would bottom out.

A semi deep splits the middle which is why they have come to be common in mechanics sets.

Torque choice: The number of points determine where the socket bears on the bolt. On flats, a 6-point opening take that bite; it shrugs off higher torque and slightly rounded corners as well.

A 12-point doubles the engagement angles and is faster in confined swings. This is also mandatory on true 12-point flange bolts, but it concentrates load closer to the corners of a standard hex. Use 12-point when you need speed (tight confines), but default to 6-point if they're tight, rusty, or especially if impacted.

Not included: Crowfoot attachments; E-Torx female and bit sockets

Bit sockets are a combination of a driver or Torx or hex bit inserted into a socket-shaped body.

A crowfoot is a head for a wrench with a square drive instead of a socket. Bit sockets, E-Torx, and crowfoots follows their own sizes in the hex and Torx key charts. You should of checked the moddern charts for more accuracy.

The Socket size column is where you match the hex of your spark plug. The Inch marking column indicate the SAE stamp. This is because a 5/8 in and a 16 mm socket are actualy the same tool, as is a 13/16 in and 20.8 mm. The Typical plug thread connects the socket to what it's threaded for in the head. And Retention describes how the socket retain the plug during its trip in/out of a deep well.

Note: Most newer vehicles use 14 mm threads. Most older gas-powered tools, like lawn and garden equipment, plus some older engines, still use a 20.8 mm (13/16 in) plug, which together with the 5/8 in (16 mm) covers about all garage jobs. I also have a smaller one for really tight spaces, like 14 mm. Many newer engines use 14 mm via their longer reach plugs, so we are seeing a general move toward smaller plug hexes. This creates a need for slim wall 14 mm tools, too, which are not a novelty anymore.

The 5/8 in (16 mm) size are most common on newer vehicles (14 mm thread).

The hex can be the trick, too; plug wells is cast snugly around the hex and it’s the wall thickness that cracks porcelain and breaks cylinder-head plastic. A regular deep socket that technically fits the hex may wedge into the well.

Extra-deep thin-walled spark plug sockets are made for swallowing the insulator; no rubber insert or magnet is a luxury. A dropped-in plug usually closes up its gap and occasionally cracks invisibly.

The drive is done by hand. Threads are pulled with an impact gun on a plug socket, but plugs seats at low torque in aluminium heads. The socket gets twisted free rather than yanked off because the rubber insert can grip the porcelain hard enough to snap the insulator if you yank it.

Wobble extension or universal joint are normal equipment for angled wells.

Plug hexes met with square-drive sockets on this table.

Outside the realm of a socket chart: Plug selection topics include taper-seat vs. gasket seat, heat range, and reach. Some racing and aviation plugs is sized larger; you confirm against the plug maker’s spec vs. whatever was on the last plug.