Metric sockets are sized in millimeters; SAE sockets are sized in inches, usually fractions such as 1/2 or 9/16. Some sizes are close enough to look alike, but a nearby number is not the same nominal size. A socket that goes over a fastener is not necessarily the right fit.
What metric and SAE markings tell you
A socket marked 13 mm is intended for a fastener with a nominal 13 mm width across its flats. A socket marked 1/2 inch is intended for a nominal half-inch width. In both cases, the marking describes the fastener end of the socket.
This is different from the thread size of a bolt. It is also different from the square connection at the other end of the socket, explained in our socket drive-size guide.
Why 13 mm and 1/2 inch are not the same
One inch is exactly 25.4 mm, as NIST’s length reference explains. Half of that is 12.7 mm, not 13 mm.
The difference is only 0.3 mm across the nominal width. That is small enough for the two sizes to look very similar on a bench, but the measurement has not become identical.
The direction of the difference matters too. A nominal 13 mm opening is larger than a nominal 1/2-inch head; a nominal 1/2-inch opening is smaller than a nominal 13 mm head. These are not two equivalent ways to label one size.
Why “it goes on” does not settle the question
A socket needs some clearance to slide onto a fastener. Manufacturing tolerances, wear, coatings, corrosion, and an already damaged head also affect the fit you feel.
That means a slightly oversized socket may go on without providing the intended fit. Extra movement can let the socket rotate against the head before it takes the load, increasing the risk of slipping or rounding the fastener.
The useful distinction is between being able to install the socket and having the correct size engaged properly. A worn head does not make a nearby metric or SAE size its new intended specification.
What a conversion chart can—and cannot—tell you
A chart can express an inch size in millimeters or identify a nearby stocked size. Those are different results. An exact conversion preserves the measurement; rounding it to the next familiar socket size changes it.
| SAE marking | Exact conversion | Nearby metric size |
|---|---|---|
| 1/2 inch | 12.7 mm | 13 mm |
| 9/16 inch | 14.2875 mm | 14 mm |
| 3/4 inch | 19.05 mm | 19 mm |
These examples explain why the sizes are easy to confuse; they are not substitution recommendations. The gaps are different, so one pair that seems to fit does not establish a general rule for the rest of the set.
Do you need metric, SAE, or both?
The equipment you work on determines which sizes earn their place. A focused kit can suit equipment with a known fastener system. A mixed kit is useful when your work includes different machines, repairs, and replacement hardware.
A metric socket does not need a “metric ratchet.” Metric and SAE sockets can share the same square drive size. For example, both a 13 mm socket and a 1/2-inch socket may connect to a 3/8-inch-drive ratchet while fitting different fastener sizes.
Other socket choices remain separate: 6-point versus 12-point describes the opening profile, and shallow versus deep describes reach and clearance. Choosing a different profile or depth does not correct the wrong nominal size.
Keep similar-looking sizes easy to identify
Separating metric and SAE groups and leaving the markings visible makes selection easier than guessing from appearance. This matters most with a mixed set, where neighboring sockets can look almost identical.
Milwaukee’s 43-piece SHOCKWAVE set (49-66-7009) includes both systems on a 3/8-inch drive. The pictured ToolFellas socket insert gives that specific set dedicated positions in the 48-22-8436 Compact Low-Profile Organizer.
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