If you work on a reasonably modern car, buy metric first and treat SAE as an optional second purchase. Almost every fastener on a car built in the last few decades has a metric head, whoever built it. SAE sockets earn their place on older American vehicles, on trailers, on aftermarket brackets, and on the assorted non-automotive hardware that ends up in a garage. Buying both at once is not wrong, but it doubles what you carry and pay for a set you may open twice a year.
The rest of this page is how to confirm that for your own car before you spend anything, and which near-identical sizes cause the damage people blame on cheap tools.
SAE Is a Confusing Name for a Simple Thing
SAE sockets are sized in fractions of an inch. Metric sockets are sized in millimeters. That is the entire technical difference.
The name causes the confusion. SAE stands for the Society of Automotive Engineers, which makes it sound like the automotive standard. It is not. In tool aisles, SAE has come to mean the inch, or fractional, sizing you see marked 1/4, 3/8, 7/16 and so on. You will also see the same sockets sold as "standard" or "imperial," which are the same thing again.
Both measure across the flats of the fastener, which is the distance between two opposite faces of the hex. A 13 mm socket is 13 mm across those flats plus a small clearance. A 1/2 inch socket is 12.7 mm across the same measurement. That gap is the whole subject of the section further down.
The Changeover, and Why No Single Date Covers Every Car
American manufacturers moved to metric fasteners gradually, not overnight.
General Motors told its suppliers it was converting to metric in 1973, and published accounts of the transition describe metric heads spreading through American models across the following decade while inch threads and inch hardware persisted alongside them. Ford and the rest followed on their own timelines. By the turn of the century, reporting on the changeover describes US-built cars as effectively all metric.
Imported vehicles were metric far earlier. Japanese, Korean and German cars have been metric throughout the period most home mechanics are working on.
What this means in practice: a car from the 1990s onward is a metric car, and a car from the 1970s or early 1980s may be a genuine mixture on the same vehicle. Transition-era cars are the reason experienced mechanics own both sets and the reason a blanket "just buy metric" answer fails some readers. If your car sits in that window, assume mixed until you have checked.
Where an SAE Set Still Earns Drawer Space
Even on an all-metric car, inch hardware turns up around it.
Trailers, hitches and towed equipment are frequently inch-sized in the US market. Aftermarket parts are sized by whoever manufactured them, so a bolt-on bracket, a roof accessory or a replacement bumper mount may not follow the car. Older tools and shop equipment in the garage itself, including bench vises, jack fittings and engine stands, often use inch hardware. Anything from a hardware store shelf is likely inch-sized in the US.
There is also the pragmatic case: if you also work on lawn equipment, a boat, farm machinery or an American motorcycle, the inch set stops being an occasional tool.
The honest test is not the car, it is the list of things you work on. If the answer is "one modern car," metric alone will get you through a year of ordinary jobs. If the answer includes a trailer or anything older than you are, buy both.
The Near-Miss Sizes, and What They Actually Cost You
This is the part that damages fasteners, and it is pure arithmetic. One inch is exactly 25.4 mm, so every inch size converts to a millimeter figure you can compare directly.
| Inch size | In millimeters | Nearest metric | Gap |
|---|---|---|---|
| 3/4 | 19.05 | 19 mm | 0.05 mm |
| 5/16 | 7.94 | 8 mm | 0.06 mm |
| 7/16 | 11.11 | 11 mm | 0.11 mm |
| 5/8 | 15.88 | 16 mm | 0.13 mm |
| 7/8 | 22.23 | 22 mm | 0.23 mm |
| 9/16 | 14.29 | 14 mm | 0.29 mm |
| 1/2 | 12.70 | 13 mm | 0.30 mm |
| 1/4 | 6.35 | 6 mm | 0.35 mm |
| 11/16 | 17.46 | 17 mm | 0.46 mm |
| 3/8 | 9.53 | 10 mm | 0.48 mm |
Two conclusions come straight out of that table.
At the top, 3/4 and 19 mm differ by five hundredths of a millimeter, and 5/16 and 8 mm by six hundredths. Those pairs are close enough that the sockets are functionally interchangeable in the hand, which is why nobody ever notices them.
The trouble is the middle of the table. A 1/2 inch socket on a 13 mm bolt is loose by three tenths of a millimeter. It goes on. It feels like it fits. Then it slips under load and takes the corners off the head, and the reader blames the socket brand.
The rule that follows: a socket that goes on with a wobble is the wrong socket. A correct socket seats with almost no play. If you can rock it, put it back and find the other set.
Settling It for Your Own Car Without Spending Anything
You do not need to trust a general rule when the car is in front of you.
Read the fastener. Many bolt heads are marked, and the fastener supplier's own hardware is consistent across a vehicle. If one accessible bolt takes 10 mm cleanly and refuses 3/8, the car is metric.
Check the vehicle's own service information. The manufacturer's service documentation lists fastener sizes along with the tightening specifications, and it is the only source that is right for your exact vehicle rather than right on average.
Try before you buy. If a friend or a tool library has both sets, ten minutes on your own engine bay answers the question permanently.
Start with the sizes you will actually reach for. On most cars a small handful of metric sizes does the large majority of the work, and a set that covers a wide span of sizes matters less than most box copy suggests.
Buying: What Changes Between Tiers, and What Does Not
Sizing is not a quality feature. A metric set and an inch set from the same product line are the same steel, the same wall thickness and the same tolerances. Nobody makes a better socket by measuring it in millimeters.
What separates a budget set from a premium one is the fit tolerance, the finish, how the markings survive a year of use, and whether the maker replaces a broken socket. Budget sets are genuinely fine for a great deal of home work. Where a loose tolerance hurts most is exactly the near-miss problem above, because a sloppy budget socket in the correct size can behave like a slightly wrong socket in the correct standard.
Combined metric-and-inch sets look like better value per piece. They are also the reason so many drawers are full of sockets nobody has touched, which is a piece-count problem rather than a sizing one.
Where This Stops
Fastener sizing tells you which socket goes on. It tells you nothing about how tight the fastener should be.
Tightening specifications are specific to your vehicle and to the individual fastener, and this site does not publish them. They live in the manufacturer's service information for your exact year, model and engine. For anything on brakes, suspension, steering, the airbag system or the fuel system, the correct figure and the correct procedure matter more than the tool, and that work belongs with a qualified mechanic if you do not have both.
If the job that sent you looking for sockets involves getting a wheel off, the lifting half of it matters more than the socket half. That is covered separately in where to place a jack under a car and in the comparison of jack stands against ramps, and the reason a car is never left on the jack alone is set out in why a floor jack is not a support. For an oil change, car ramps usually beat jacking the car at all.
FAQ
Do I need both metric and SAE sockets?
For one modern car, no. Metric alone covers nearly everything. Add an inch set when you take on a trailer, an older American vehicle, garden or shop equipment, or aftermarket hardware that did not come from the car's manufacturer.
Can I use a 1/2 inch socket on a 13 mm bolt?
You can fit it, and that is the problem. A 1/2 inch socket measures 12.70 mm against a 13 mm fastener, so it sits loose by three tenths of a millimeter and can slip under load and round the head. Use the correct socket.
Are 19 mm and 3/4 inch the same?
Not exactly, but they are within about five hundredths of a millimeter, which is close enough that they interchange in practice. The same is true of 8 mm and 5/16 inch. These are the exceptions, not the rule.
What does SAE mean on a socket set?
In tool retail it means the socket is sized in fractions of an inch, as opposed to millimeters. It is used interchangeably with "standard" and "imperial." The name comes from the Society of Automotive Engineers, which is why it reads as if it should be the automotive default when on modern cars it is not.
My car is from the early 1980s. Which do I need?
Assume both, and verify on the car. That period is the middle of the American changeover to metric, and mixed hardware on a single vehicle was normal.
The Short Version
- Modern car: buy metric. Inch sizes are the second purchase, not the first.
- SAE earns its place on trailers, older American vehicles, aftermarket brackets and shop equipment.
- Transition-era cars are genuinely mixed. Check, do not assume.
- The rounding culprit is usually the near-miss size, most often a 1/2 inch socket on a 13 mm head.
- If it wobbles, it is wrong. Put it back and get the other set.
- Sizing standard is not a quality tier. Metric sockets are not better made than inch ones.
