If you have spent any time turning wrenches, you have run into it: the bolt is metric, your socket is SAE, and the closest thing in the drawer almost fits. That "almost" is where rounded bolt heads and busted knuckles come from. Working across both systems is just part of the job in most trades, so knowing how SAE and metric sizes line up, and where they do not, saves you real time and money.
This guide gives you the full SAE to metric conversion chart for wrench and socket sizes, points out the close calls that will bite you, and walks through the tool sets that cover both systems so you are never stuck guessing.
Shop Wrench and Socket SetsWhy SAE and Metric Both Exist
SAE sizing measures fasteners in inches and fractions of an inch, like 1/2 inch or 9/16 inch. Metric sizing measures in millimeters, like 13mm or 14mm. There is no clean one-to-one conversion between the two, which is exactly why you cannot reliably grab one to do the other's job.
The split goes back decades. The United States built its machinery and vehicles around inch-based fasteners while most of the rest of the world standardized on metric. As global manufacturing and imported vehicles took over, American industry gradually shifted toward metric too, but it never fully replaced the huge installed base of SAE equipment already in the field. The result is the world you actually work in: older American-built vehicles, machinery, and equipment lean heavily SAE, while imported and most modern equipment lean metric.
The catch is that plenty of machines mix both, sometimes on the same assembly. You might find a metric bolt holding a bracket that is fastened to a housing with SAE hardware, because the two parts came from different suppliers. On modern vehicles, brake line and chassis fittings commonly run metric, while older American vehicles lean SAE. That split shows up all over the trades, not just under the hood, which is why a pro who works on a variety of gear cannot pick a side.
SAE to Metric Wrench and Socket Size Chart
Here is the reference you came for. This chart lists common SAE sizes, their exact decimal inch value, the millimeter equivalent, and the closest metric size you would actually reach for. Use it to find the nearest match, but read the next section before you treat a "close" size as a substitute.
| SAE (in.) | Decimal (in.) | mm equivalent | Closest metric wrench | Notes |
|---|---|---|---|---|
| 1/4" | 0.250" | 6.35 mm | 6 mm or 7 mm | Falls between 6mm and 7mm |
| 5/16" | 0.3125" | 7.94 mm | 8 mm | 8mm (0.315") is a very close match |
| 3/8" | 0.375" | 9.53 mm | 10 mm (close) | 10mm (0.394") is larger; do not force |
| 7/16" | 0.4375" | 11.11 mm | 11 mm | 11mm (0.433") is extremely close |
| 1/2" | 0.500" | 12.70 mm | 13 mm | 13mm (0.512") is close |
| 9/16" | 0.5625" | 14.29 mm | 14 mm | 14mm (0.551") is very close |
| 5/8" | 0.625" | 15.88 mm | 16 mm | 16mm (0.630") is close |
| 11/16" | 0.6875" | 17.46 mm | 17 mm or 18 mm | Falls between 17mm and 18mm |
| 3/4" | 0.750" | 19.05 mm | 19 mm | Nearly identical; 19mm is 0.748" |
| 13/16" | 0.8125" | 20.64 mm | 21 mm | 21mm (0.827") is close |
| 7/8" | 0.875" | 22.23 mm | 22 mm | 22mm (0.866") is close |
| 15/16" | 0.9375" | 23.81 mm | 24 mm | 24mm (0.945") is close |
| 1" | 1.000" | 25.40 mm | 25 mm or 26 mm | 1" runs slightly larger than 25mm |
Metric to SAE Conversion Chart
Plenty of the time you are working the other direction: the bolt is metric and you want to know the closest inch size. This chart runs metric wrench and socket sizes from 6mm up to 24mm, with the decimal-inch value and the nearest SAE wrench. Same rule applies, a close size is a backup, not a substitute.
| Metric (mm) | Decimal (in.) | Closest SAE wrench | Notes |
|---|---|---|---|
| 6 mm | 0.236" | 1/4" (close) | 1/4" is 0.250", slightly larger |
| 7 mm | 0.276" | 9/32" or 1/4" | Falls between 1/4" and 9/32" (0.281") |
| 8 mm | 0.315" | 5/16" | 5/16" is 0.3125", a very close match |
| 10 mm | 0.394" | 3/8" (close) | 3/8" is 0.375", slightly smaller; do not force |
| 11 mm | 0.433" | 7/16" | 7/16" is 0.4375", extremely close |
| 12 mm | 0.472" | 15/32" or 1/2" | Between 15/32" (0.469") and 1/2" |
| 13 mm | 0.512" | 1/2" | 1/2" is 0.500", close |
| 14 mm | 0.551" | 9/16" | 9/16" is 0.5625", very close |
| 15 mm | 0.591" | 19/32" or 5/8" | Between 19/32" (0.594") and 5/8" |
| 16 mm | 0.630" | 5/8" | 5/8" is 0.625", close |
| 17 mm | 0.669" | 11/16" | 11/16" is 0.6875", close |
| 18 mm | 0.709" | 23/32" or 11/16" | Between 11/16" and 23/32" (0.719") |
| 19 mm | 0.748" | 3/4" | Nearly identical; 3/4" is 0.750" |
| 21 mm | 0.827" | 13/16" | 13/16" is 0.8125", close |
| 22 mm | 0.866" | 7/8" | 7/8" is 0.875", close |
| 24 mm | 0.945" | 15/16" | 15/16" is 0.9375", close |
How the Conversion Math Actually Works
If you ever need to convert a size the chart does not list, the math is simple. One inch equals exactly 25.4 millimeters, and one millimeter equals about 0.03937 inch. To turn an SAE size into millimeters, take the decimal-inch value and multiply by 25.4. A 1/2 inch wrench is 0.500 inch, so 0.500 times 25.4 gives you 12.7mm. To go the other way, divide a millimeter size by 25.4 to get inches; a 13mm socket is 13 divided by 25.4, or about 0.512 inch.
The reason there is no clean crossover is that the two systems step in different increments. SAE wrenches jump in fractions like sixteenths of an inch, while metric wrenches step in whole millimeters. Those steps rarely land on the same number, so most sizes end up close to a metric neighbor but not exactly on it. That small gap is the whole reason the next section matters.
The Close Calls: Sizes That Look Interchangeable but Are Not
A handful of SAE and metric sizes sit close enough that a wrench will slip on. That does not mean it should. On a snug, high-torque, or rusted fastener, even a couple thousandths of slop lets the tool round off the corners. These are the pairs to respect:
- 5/16 inch (7.94mm) vs. 8mm (8.00mm): almost dead even, with the metric a hair larger. One of the safer swaps, but still not exact.
- 3/8 inch (9.53mm) vs. 10mm (9.98mm): the 10mm is noticeably larger. A 3/8 inch wrench on a 10mm bolt is the classic way to round one off.
- 7/16 inch (11.11mm) vs. 11mm (11.00mm): extremely close, SAE slightly larger, but a loose fit can still cam over under load.
- 1/2 inch (12.70mm) vs. 13mm (13.00mm): close, but the 13mm is larger. Fine loose, risky under load.
- 9/16 inch (14.29mm) vs. 14mm (14.00mm): very close, with SAE slightly larger.
- 5/8 inch (15.88mm) vs. 16mm (16.00mm): close, metric slightly larger. Another one that will round a tight fastener.
- 3/4 inch (19.05mm) vs. 19mm (19.00mm): about the only pair close enough that most pros treat as interchangeable.
- 7/8 inch (22.23mm) vs. 22mm (22.00mm): close, SAE slightly larger, and not worth the risk on anything torqued down.
A close size is an emergency backup, not a plan. The danger is worst on a fastener that is rusted, torqued tight, or made of soft metal, because that is exactly when a hair of slop lets the tool spin and shear off the corners. Once a bolt head is rounded, you are into extractor territory, which is a far bigger job than grabbing the right wrench would have been. If you own both systems, you never have to gamble a bolt head on a two-thousandths guess.
Here is the same information as a quick verdict you can scan before you reach for a close size:
| Size pair | Verdict | Why |
|---|---|---|
| 3/4" and 19mm | Safe swap | Only 0.002" apart; effectively the same size |
| 5/16" and 8mm | Usually OK | About 0.003" apart, metric slightly larger |
| 7/16" and 11mm | Risky | Very close, but slop can cam over under load |
| 5/8" and 16mm | Risky | 16mm is larger by about 0.005"; can round a tight fastener |
| 7/8" and 22mm | Avoid | SAE larger by about 0.009"; not worth it when torqued |
| 9/16" and 14mm | Avoid | SAE larger by about 0.011"; loose enough to round the bolt |
| 1/2" and 13mm | Avoid | 13mm is larger by about 0.012"; a 1/2" will slip and round it |
| 3/8" and 10mm | Avoid | 10mm is larger by about 0.019"; the classic bolt-rounder |
Combination Wrench Sets: SAE, Metric, or Both
A combination wrench is the backbone of any set. The smart move for anyone working across both systems is a reversible ratcheting SAE/metric set, or an SAE set and a metric set that live side by side. Cheap wrenches flex under load and round off fasteners, which is the exact problem you are trying to avoid.
Socket Sets and Ratchets
Sockets are where most people first feel the split, because you are constantly swapping to find the one that seats fully. A dual SAE and metric set solves that in one box. Match the drive size to your work: 1/4 inch for small fasteners, 3/8 inch for general work, 1/2 inch for higher torque.
Milwaukee 48-22-9004 1/4" Drive 50-Piece Ratchet & Socket Set - SAE & Metric
Socket Drive Sizes at a Glance
Drive size is the square hole on the back of the socket that the ratchet fits into, and it decides how much torque the socket can take and how tight a spot it fits. Most pros end up owning all three common drives. Here is what each one is for.
| Drive size | Best for | Typical socket range |
|---|---|---|
| 1/4 inch | Small fasteners, electronics, tight spaces, dash and trim work | Roughly 5/32" to 9/16", or 4mm to 14mm |
| 3/8 inch | General-purpose work, the do-everything drive for most jobs | Roughly 5/16" to 3/4", or 8mm to 19mm |
| 1/2 inch | High-torque work: suspension, brakes, lug nuts, big fasteners | Roughly 3/8" to 1-1/4", or 10mm to 32mm |
Nut Drivers and Hex Keys
Nut drivers handle the small hex-head fasteners electricians, HVAC techs, and equipment mechanics hit constantly, in both SAE and metric. A matched SAE set and metric set means you always have the exact size for sheet metal screws, terminal blocks, and panel hardware.
Hex keys, also called Allen wrenches, are the other place the two systems collide, because the SAE and metric sizes come even closer together than wrench sizes do. A 3mm hex is nearly a 1/8 inch, and using the wrong one strips the recess in a hurry. The chart below lines up the common metric hex sizes with their nearest SAE match. As with everything else here, close is not exact.
| Metric hex (mm) | Decimal (in.) | Closest SAE hex | Notes |
|---|---|---|---|
| 1.5 mm | 0.059" | 1/16" (close) | 1/16" is 0.0625", slightly larger |
| 2 mm | 0.079" | 5/64" | 5/64" is 0.078", nearly identical |
| 2.5 mm | 0.098" | 3/32" (close) | 3/32" is 0.094", slightly smaller |
| 3 mm | 0.118" | 1/8" | 1/8" is 0.125", close |
| 4 mm | 0.157" | 5/32" | 5/32" is 0.156", nearly identical |
| 5 mm | 0.197" | 3/16" | 3/16" is 0.188", close |
| 6 mm | 0.236" | 1/4" | 1/4" is 0.250", close |
| 8 mm | 0.315" | 5/16" | 5/16" is 0.3125", nearly identical |
| 10 mm | 0.394" | 3/8" | 3/8" is 0.375", close |
Adjustable Wrenches: The In-Between Fix
An adjustable wrench sidesteps the SAE-versus-metric question by fitting a range of sizes in both systems. It is not a replacement for the exact wrench on a high-torque or delicate fastener, since the movable jaw can slip, but as a backup for the size you do not have, it earns its spot in the bag.
Which System Your Trade Uses Most
You will not carry every size equally. Which system you reach for most depends on what you work on, and knowing that helps you spend your money where it counts.
- Automotive and diesel: mostly metric on anything built in the last few decades, with SAE on older domestic vehicles, trailers, and plenty of aftermarket parts. Techs who touch a range of model years need deep coverage in both.
- HVAC: a real mix. Equipment fasteners trend metric, but panel screws, set screws, and refrigeration fittings pull you into SAE hex and nut driver sizes constantly.
- Plumbing: leans SAE on domestic fixtures and fittings, though imported valves and pumps bring metric in. Adjustable wrenches earn their keep because fitting sizes are all over the map.
- Construction and framing: structural fasteners and equipment run both ways depending on the manufacturer, and heavy equipment service almost always needs both systems on the truck.
- Appliance and equipment repair: frequently mixes SAE and metric on the same unit, exactly the situation where a dual set saves you a walk back to the toolbox.
The honest answer for most trades is that you will use both often enough that owning only one system will strand you on a job sooner or later.
6-Point vs. 12-Point: Why It Matters on Worn Fasteners
Once you are buying sockets and wrenches in both systems, the number of points in the opening becomes a real decision, especially when you are already dealing with a close-size risk. A 6-point socket contacts the fastener on its flats, spreading force across the strongest part of the head. That grip is what you want on a rusted, tight, or already-worn bolt, because it resists rounding far better.
A 12-point socket has twice the engagement angles, so it slips onto a fastener faster and works better in tight quarters where you cannot swing the wrench far. The trade-off is that it contacts the corners of the head, which is where damage starts. For high-torque or damaged fasteners, reach for 6-point. For fast work on fasteners in good shape, 12-point is a convenience worth having. Many pros keep both.
What to Look For in a Wrench or Socket Set
The set you buy should make the SAE-versus-metric problem disappear, not add to it. A few things separate a set that lasts from one that rounds fasteners and rusts in the box:
- Both systems covered: either a reversible SAE/metric set or a matched SAE set and metric set, so you are never missing the size you need.
- Quality alloy: chrome vanadium steel for hand tools and chrome-molybdenum for impact sockets holds up under torque without flexing or cracking.
- Clear, permanent size markings: laser-etched or stamped sizes that will not wear off, so you grab the right size the first time.
- The right drive size: 1/4 inch for small fasteners and tight spots, 3/8 inch for general work, 1/2 inch for high torque. Many pros own all three.
- A tooth count that fits the work: on ratcheting tools, a higher tooth count (72 or 90) needs less swing arc, which matters in cramped spaces.
Buy the quality once and you stop replacing rounded fasteners, stripped tools, and your own patience.
So Which Should You Buy?
Building from scratch and working on a mix of equipment? Start with a reversible SAE/metric combination wrench set and a dual SAE and metric socket set. Add matched SAE and metric nut driver sets for electrical, HVAC, or appliance work, and keep an adjustable wrench in the bag for the sizes you do not have on you. Buy quality tools that carry both metric and SAE options, and you protect your fasteners while saving yourself the cost and downtime of a rounded bolt.
Key Takeaways
- There is no clean conversion between SAE and metric, so keep both on hand.
- Only 3/4 inch and 19mm are truly interchangeable; the rest will round a fastener under load.
- A reversible SAE/metric combination set and a dual socket set cover most work; an adjustable wrench backs you up.
Frequently Asked Questions
Is 3/8 inch the same as 10mm?
No, but they are close enough to cause trouble. 3/8 inch equals 0.375 inch (9.53mm), while 10mm equals 0.394 inch. The 10mm is slightly larger, so a 3/8 inch wrench can slip and round off a 10mm fastener. Use the correct size, especially on a tight or rusted bolt.
What SAE size is closest to 19mm?
3/4 inch is almost identical to 19mm. 3/4 inch is 0.750 inch and 19mm is 0.748 inch, a difference of only two thousandths. These two are close enough that most people use them interchangeably in a pinch, though the exact size is always the safest choice.
Can I use SAE wrenches on metric bolts?
Sometimes, but it is risky. A few sizes are close enough to work in a pinch, but even a tiny difference can round off a bolt head under high torque or on a rusted fastener. Pros keep both SAE and metric on hand so they never have to force a close-enough size.
Do American vehicles use SAE or metric?
Both, depending on age. Modern vehicles tend to use metric fittings while older American models use SAE sizes. Many vehicles mix the two, sometimes on the same assembly, which is why a full toolbox carries both systems.
What is the difference between SAE and metric wrenches?
SAE wrenches are sized in inches and fractions of an inch (like 1/2 inch), while metric wrenches are sized in millimeters (like 13mm). There is no clean one-to-one conversion between the two systems, so you cannot reliably swap one for the other.
Do I really need both SAE and metric sets?
If you work on a variety of equipment, vehicles, or machinery, yes. Import and modern domestic vehicles lean metric, older American gear leans SAE, and plenty of machines mix both. A reversible SAE/metric combination set covers you either way.
What does an adjustable wrench solve?
An adjustable wrench fits a range of sizes in both systems, so it can bail you out when you do not have the exact SAE or metric size on hand. It is not a replacement for proper wrenches on high-torque or delicate fasteners, since its jaws can slip, but it is a handy backup.