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Short answer: for any car built in the last few decades, a metric socket set does most of the work. Buy an SAE socket set too if you work on older domestic vehicles, trailers, farm equipment or anything with aftermarket hardware. For most garages, the right answer is a set that includes both systems in standard and deep lengths.
SAE and metric sockets look identical and use the same 1/4-, 3/8- and 1/2-inch drive squares. The only difference is how the opening is measured: fractions of an inch or millimeters. That difference matters because most sizes don't line up, and a socket that's almost right is how bolt heads get rounded.
This guide covers where each system shows up, which sizes overlap, and how to decide what to buy.
Verdict Up Front
- Choose a metric socket set first if you mostly work on cars, motorcycles, appliances or imported equipment.
- Choose an SAE socket set first if you work on older American cars and trucks, trailers, lawn tractors or US-built machinery.
- Buy both if you work on a mix, or if you want one set that never leaves you hunting. Combined sets are the default for good reason.
SAE vs Metric at a Glance
| SAE sockets | Metric sockets | |
|---|---|---|
| Measured in | Fractions of an inch | Millimeters |
| Size steps | 1/32" and 1/16" (e.g., 7/16", 1/2", 9/16") | 1 mm (e.g., 12, 13, 14 mm) |
| Common on | Older US vehicles, trailers, farm and industrial equipment, aftermarket hardware | Modern cars and trucks, motorcycles, imported equipment, most appliances |
| Typical 3/8" drive range | 1/4" to 1" | 6 to 19 mm (some sets to 24 mm) |
| Drive sizes | 1/4", 3/8", 1/2", 3/4" | Same 1/4", 3/8", 1/2", 3/4" |
| Interchangeable sizes | Only a few near-matches (see below) | Only a few near-matches (see below) |
SAE ↔ metric socket converter
1/2" = 12.70 mm. Nearest metric: 13 mm, 0.30 mm larger. Not interchangeable.
All conversions use 1 in = 25.4 mm, with sizes measured across the flats. Swap rule from our socket size chart: under 0.10 mm apart is usable in a pinch on a fastener that isn't torqued hard, 0.10 to 0.15 mm is a gamble, more than that needs the correct socket. Never swap on brake, steering or suspension fasteners. Drive-size ranges are typical of current sets, not a standard.
The Differences, One by One
Measurement System
SAE (from the Society of Automotive Engineers) sockets are sized in inches, so the opening of a 1/2" socket fits a hex that's 1/2" across the flats. Metric sockets are sized in millimeters: a 13 mm socket fits a 13 mm hex. One inch equals exactly 25.4 mm, so a 1/2" socket is 12.70 mm, which is close to 13 mm but not the same.
Which Vehicles Use Which
Modern cars and trucks, including American brands, are overwhelmingly metric. SAE is still common on older domestic vehicles, on trailers and hitches, on a lot of farm and industrial equipment, and on aftermarket accessories. Many vehicles mix both, especially after years of repairs and add-ons.
Size Steps
Metric sockets go up in 1 mm steps: 10, 11, 12, 13 mm. SAE sockets go up in 1/16" steps in the common range, with some 1/32" steps in small sizes: 5/16", 11/32", 3/8". That's why metric sets usually have more sockets in the same range. A gap-free 8 to 19 mm metric run is 12 sockets, while a 5/16" to 3/4" SAE run is 8.
Which Sizes Overlap
Only three common pairs are close enough to swap in a pinch, and even those are best avoided on high-torque fasteners. All conversions below use 1 in = 25.4 mm, rounded to two decimals (5/8" is 15.875 mm, so it sits 0.125 mm under 16 mm). We use the same rule as our socket size chart: under 0.10 mm apart is usable in a pinch, 0.10 to 0.15 mm is close but not ideal, and anything over that needs the right socket.
| SAE | Exact mm | Nearest metric | Difference | Swap? |
|---|---|---|---|---|
| 5/16" | 7.94 | 8 mm | 0.06 mm | Yes, in a pinch |
| 3/8" | 9.53 | 10 mm | 0.48 mm | No |
| 7/16" | 11.11 | 11 mm | 0.11 mm | Close, not ideal |
| 1/2" | 12.70 | 13 mm | 0.30 mm | No |
| 9/16" | 14.29 | 14 mm | 0.29 mm | No |
| 5/8" | 15.88 | 16 mm | 0.13 mm | Close, not ideal |
| 11/16" | 17.46 | 17 mm | 0.46 mm | No |
| 3/4" | 19.05 | 19 mm | 0.05 mm | Yes, in a pinch |
| 13/16" | 20.64 | 21 mm | 0.36 mm | No |
| 7/8" | 22.23 | 22 mm | 0.23 mm | No |
| 15/16" | 23.81 | 24 mm | 0.19 mm | No |
| 1-1/16" | 26.99 | 27 mm | 0.01 mm | Yes, in a pinch |
Our socket size chart has the full list in order, including large sizes.
Why "Close Enough" Rounds Bolts
A socket that's a few tenths of a millimeter too big contacts the fastener closer to its corners than its flats. On a loose bolt you may get away with it. On a tight, rusty or torqued fastener, the socket slips and shaves the corners. Pair that with a 12-point socket and the risk goes up. A 6-point socket in the exact size is the safe combination; more in our 6-point vs 12-point guide.
Set Contents and Cost
Combined SAE and metric sets carry more sockets than single-system sets, so they're bigger and heavier. If you only work on metric vehicles, a metric-only set gives you more metric sizes for the money. If you need both, buying a combined set is usually simpler than two separate kits.
Markings and Color Coding
Most quality sockets are stamped or laser-etched with the size, and many also mark the system, such as "MM" on metric sockets. Some brands color-code: Milwaukee uses ink-filled size labels to separate SAE from metric on its flare nut wrenches, and organizer makers like Ernst recommend red rails for SAE and blue for metric. Whatever you buy, store the two systems apart so you don't grab a 13 mm when you meant 1/2".
Wrenches Follow the Same Rules
Everything here applies to combination wrenches too. A 1/2" wrench on a 13 mm bolt is 0.30 mm loose, and an open end grips only two flats, so it rounds even faster than a socket. If your car is metric, your wrench set should be metric first.
Drive Size Is Not the Same as Socket Size
New buyers sometimes mix these up. The drive size (1/4", 3/8", 1/2", 3/4") is the square hole that fits the ratchet, and it is always measured in inches, even on metric sockets. A "3/8-inch metric socket set" means metric socket sizes on a 3/8-inch drive. That's normal, and it's why your ratchets work with both SAE and metric sockets.
What a Good SAE Socket Set Includes
For an SAE socket wrench set in 3/8" drive, look for every size from 5/16" to 3/4" in standard and deep lengths, ideally to 7/8" or 1". In 1/2" drive, look for 7/16" to at least 1" and up to 1-1/2" for truck and equipment work.
If you already own an SAE standard set, an SAE deep set fills the most common gap.
What a Good Metric Socket Set Includes
In 3/8" drive, a metric socket set should run 8 to 19 mm with no gaps in standard and deep lengths. Starting at 6 mm and running to 21 or 24 mm is better. In 1/2" drive, 10 to 24 mm covers most lug nuts, brakes and suspension. Watch for skipped sizes at 11, 16 and 20 mm, which budget kits often drop.
The Case for Buying Both
If you only buy one set, a combined SAE and metric set is the safest choice. You'll never need to guess which system a fastener uses, and you'll have the SAE sizes for aftermarket hardware even on a metric car. A complete 3/8" drive set like the GearWrench 80550 covers 1/4" to 1" and 6 to 19 mm in standard and deep lengths.
For more combined options, see our best socket set picks, or our dedicated 3/8" drive and 1/2" drive guides. If deep sockets are your gap, our deep socket set guide covers SAE and metric options.
Choose SAE If...
- You work on older American cars, trucks or motorcycles
- You maintain trailers, hitches, lawn tractors or US-built farm and industrial equipment
- Your metric set is complete and you keep meeting aftermarket SAE hardware
Choose Metric If...
- Your vehicles were built in the last few decades, including American brands
- You work on motorcycles, imported equipment or appliances
- You want the most sizes per dollar in the range you'll actually use
Choose Both If...
- You work on a mix of vehicles or equipment
- You're buying your first set and don't want to guess
- You help friends and family with whatever they bring
How to Tell Which System a Fastener Uses
- Check the vehicle or equipment age and origin. Modern vehicles are mostly metric; older domestic and farm equipment is often SAE.
- Look at the bolt head. Metric bolts often carry a property class number such as 8.8 or 10.9. SAE bolts often use radial lines to show grade.
- Try the snuggest socket from each system. The right size slides on with almost no play. If both feel loose, try the next size down.
- Check the service manual. It lists fastener sizes and torque specs for most jobs.
Common Mistakes
- Forcing a near-size. If a socket feels loose, it's the wrong system or size. Find the exact one.
- Assuming American means SAE. Modern American vehicles are mostly metric.
- Ignoring markings. Most sockets are stamped or laser-etched with size and system. Check before you pull.
- Mixing up sets. Keep SAE and metric on separate rails or trays, ideally color-coded. Many organizers use red for SAE and blue for metric.
FAQ
Do I need an SAE socket set if my car is metric?
Usually you still want a few SAE sizes. Even on metric cars, aftermarket parts, hose clamps, trailer hitches and accessories often use SAE fasteners. A combined set covers both, or a small SAE set fills the gap cheaply.
Can I use metric sockets on SAE bolts?
Only on a few near-identical pairs: 8 mm on 5/16 inch, 19 mm on 3/4 inch and 27 mm on 1-1/16 inch are within about 0.06 mm. Other pairs differ by 0.1 to 0.5 mm, which is enough to round a tight fastener.
Are American cars SAE or metric?
Modern American cars and trucks are mostly metric. Older domestic vehicles, many trailers and a lot of farm and industrial equipment still use SAE fasteners, and some vehicles mix both.
What is the difference between an SAE socket wrench set and a metric one?
Only the sizes. SAE sockets are measured in fractions of an inch, like 1/2 or 9/16 inch. Metric sockets are measured in millimeters, like 13 or 14 mm. The drive sizes (1/4, 3/8 and 1/2 inch) are the same for both.
How can I tell if a bolt is SAE or metric?
Check the head markings: SAE bolts often have radial lines indicating grade, while metric bolts show a property class number like 8.8 or 10.9. If there are no markings, try the snuggest-fitting socket from each system.