Two measuring systems, one toolbox
Most tools and fasteners you meet are sized in one of two systems. Metric sizes are measured in millimeters. SAE sizes are measured in inches, usually written as fractions such as 3/8 or 9/16. The term “SAE” comes from the Society of Automotive Engineers, and in everyday talk it generally means the inch system, which you may also hear called standard or US customary. Some references use other names, so it helps to think “inch” versus “millimeter.”
Each system has its own sequence of sizes. Because the two sequences were never designed to line up, many sizes land very close to each other without matching. That is where the trouble starts.
What the number means on a wrench or socket
On a wrench or socket, the number marks the distance across the flats of the fastener head it fits, such as the width between two opposite sides of a hex nut. A 10 mm wrench fits a head that measures about 10 millimeters across the flats. A 3/8 inch wrench fits a head about 3/8 of an inch across.
For bolts and screws, the size usually refers to something different: the diameter of the threaded shaft. A metric bolt labeled M8 has a thread diameter of about 8 millimeters, though the head that tightens it may need a different wrench size. This difference is a common source of confusion. The wrench size and the bolt size are not the same number, and they vary by standard, so it is usual to check the head directly with the tool.
Sizes that are close but not equal
Here are some common near-pairs, converted using 1 inch = 25.4 mm:
| Inch size | Approximate millimeters | Nearby metric size |
|---|---|---|
| 7/16 | 11.1 | 11 mm |
| 1/2 | 12.7 | 13 mm |
| 9/16 | 14.3 | 14 mm |
| 3/4 | 19.05 | 19 mm |
The gaps look tiny, a fraction of a millimeter, but a loose fit concentrates force on the corners of the head instead of spreading it across the flats. Under hard turning, the corners can round off. That ruins the fastener and often makes the next step much harder.
A useful rule is that a tool should fit snugly with almost no wobble when slid onto the head. If it rocks or can turn slightly before it grips, try the other system or the next size. Do not hammer a nearby size on to make it fit. If you find that you end up with rounded heads, see the general ideas around stripped screws and damaged heads, since the repair options narrow quickly once corners are lost.
Where each system shows up
Many modern cars, motorcycles and most electronics and imported equipment are metric. Older American vehicles, some US-made equipment and a good deal of household hardware and plumbing in the United States use inch sizes. Mixed situations are common. A vehicle might be mainly metric with an inch-sized fitting somewhere, and a repaired item might have a replacement bolt of the wrong system. Do not assume; check each fastener.
Woodworking hardware in the US is often inch-based, such as lag screws and carriage bolts, while machine screws and many fittings are available in both. Check the packaging and the thread.
Threads are different too
The size of a bolt and the shape of its thread go together. Metric threads are described by diameter and pitch in millimeters, for example M10 x 1.5, where 1.5 is the distance between thread crests. Inch threads are described by diameter and threads per inch, such as 3/8-16. A metric bolt and an inch bolt of similar diameter will generally not thread into each other’s holes properly, even if they start to go in. Forcing them can cross-thread and damage both parts. The topic of thread pitch deserves its own explanation, but the key point here is that the same system must be used for the bolt and its mating hole.
Avoiding mismatched tools
- Buy sets in both systems if you work on a mix of things, and keep them clearly separated or marked. A set that has both is convenient but check that sizes are legible.
- Read the markings. Stamped sizes can wear off or be hard to see, and a quick wipe and a good light helps.
- Try a tool by hand before applying force. If it fits snugly by hand, it will probably work under load.
- Use a set of calipers or a thread gauge when you are unsure. Calipers measure across flats and thread diameter, and a gauge reveals the pitch.
- Keep sizes together, such as in a labeled socket rail or case, so a missing socket is obvious. Good tool organization saves frustration.
- When replacing a fastener, match the system, diameter, pitch and strength grade of the original. Using a weaker bolt in a structural or safety-critical place is unsafe.
Converting and being practical
A bit of arithmetic helps. To convert inches to millimeters, multiply by 25.4. To go the other way, divide by 25.4. For example, 19 mm divided by 25.4 is about 0.748 inch, which is just under 3/4 inch (0.75). Online charts and a pocket reference card make quick work of it, but a conversion tells you only that two sizes are close, not that they are interchangeable.
Many cabinet and furniture items also use specialty sizes, and some hex keys are sold in both inch and metric sets. Keeping the two sets visibly distinct reduces mix-ups. When in doubt, test the fit, and if it is anything less than snug, keep looking.