Home / Tool Kits / How to Measure a Nut for a Socket
HOW-TO GUIDEHow to Measure a Nut for a Socket So It Fits First Time
A nut is measured across the flats, not corner to corner, and that flat to flat figure is the socket size you need. Measure it in millimetres and inches before you decide, because a 13mm nut and a 1/2 inch nut look almost identical until one starts to slip. Below is how to take the measurement properly, what to do when the nut is already chewed up, and how to spot a metric nut from an imperial one.
Best Socket Sets of 2026
Measuring is only half the job, and a set with clear size markings and tight tolerances is what turns the right number into a solid grip.
See Our Top Pick →What the size of a nut actually refers to
Every hex nut has two widths. Across the flats is the distance between two opposite faces, and that is what socket and spanner sizes describe. Across the corners is the distance between opposite points, roughly 15 percent larger, and no tool is marked with it. Measure the corners, grab the socket with that number on it, and it will be far too big and round the nut on the first pull.
One more distinction worth getting straight early. The size printed on a bolt, such as M8 or M10, is the thread diameter, not the head size. An M8 bolt normally takes a 13mm socket, so a fastener described as an M10 tells you something useful but not the socket size.
The reliable method: digital or vernier calipers
Calipers are the fastest way to get an unambiguous answer, and a basic digital pair costs very little.
- Wire brush the nut first. Rust, paint and road grime can add half a millimetre and push you to the wrong size.
- Close the jaws onto two opposite flat faces, keeping them square rather than tilted.
- Read the value in millimetres, then hit the units button and read it again in inches.
- Compare both readings against standard sizes. A figure within about 0.2mm of a common metric size is metric. One that lands on a clean fraction such as 0.4375, which is 7/16, is imperial.
A nut that measures 12.9mm is a 13mm nut. A nut that measures 12.7mm is a 1/2 inch nut. That 0.2mm gap is the entire difference, which is exactly why guessing by eye goes wrong so often.
Quick Tip
If you only have a steel rule, measure across the corners instead, because the points are easier to line up than the flats. Then multiply that figure by 0.866. A nut measuring 15mm across the corners is a 13mm nut. It is not laboratory accurate, but it will narrow you down to one or two candidates instantly.
No calipers to hand? Three workarounds
- Try sockets in order. Start slightly too small and work up. The correct socket slides on with only a whisker of rotational play and sits flush. If it wiggles noticeably, drop a size.
- Use a combination spanner as a gauge. Spanners carry the same across the flats marking, so find the one that fits snugly and read the number off it.
- Look at the bolt. If the thread is visible, measure its diameter. Roughly 8mm of thread is an M8, which takes a 13mm socket on standard fasteners.
Common metric nut sizes worth memorising
Most household and car jobs live inside a short list. Learning these saves an enormous amount of trial and error:
- M5 bolt, 8mm socket
- M6 bolt, 10mm socket
- M8 bolt, 13mm socket
- M10 bolt, 17mm socket
- M12 bolt, 19mm socket
- M14 bolt, 22mm socket
- M16 bolt, 24mm socket
Treat these as the common case rather than a rule. Manufacturers use reduced head sizes to save weight or space, so you will meet M10 bolts with 16mm heads and M12 bolts with 18mm heads, especially on cars and appliances.
Measuring a nut that is already rounded
A damaged nut still measures close to correct across the flats, because it is the corners that have been shaved away, but a standard socket has nothing solid to bite. Measure it anyway to establish the original size, then pick your approach.
If the flats are still recognisable, use a 6 point socket, since it contacts the flats rather than the corners. If the hex is genuinely mangled, use a bolt extractor socket, a spiral fluted type designed to be tapped on so it bites into the remains. Do not drop a size and force a smaller socket on, because that destroys whatever hex is left.
Six point or twelve point once you know the size
The measurement gives you the number, but the socket profile decides how much abuse the nut can take. A 6 point socket spreads load across the flats and is the right choice for anything tight, seized or already slightly worn. A 12 point engages every 30 degrees, which helps where you cannot swing the ratchet far, but it loads the corners harder. For general home work, 6 point is the safer default.
Mistakes that turn a good nut into a bad one
- Using the across the corners figure directly as the socket size.
- Assuming metric because the rest of the machine is metric. Older British and American equipment mixes standards freely.
- Forcing a socket with visible play. If it rocks side to side, it will round the nut under load.
- Measuring over paint, underseal or corrosion and landing a size too large.
- Accepting an imperial socket on a metric nut because it went on. A 1/2 inch socket on a 13mm nut is a rounding job waiting to happen.
Thirty seconds spent measuring saves the much longer job of extracting a fastener you have destroyed.
Do I measure a nut across the flats or the corners?
Across the flats. That figure is what socket and spanner sizes refer to. If you have measured across the corners, multiply by 0.866 to get the equivalent across flats size.
How can I tell if a nut is metric or imperial?
Measure it in both units. Metric nuts land on whole millimetres such as 10, 13, 17 and 19. Imperial nuts land on clean fractions such as 7/16, 1/2 and 9/16. If a reading sits awkwardly between two metric sizes, it is almost certainly imperial.
Can I use an imperial socket on a metric nut in an emergency?
It is a bad idea on anything tight. The closest pairings, such as 13mm and 1/2 inch, differ by about 0.2mm, which is enough slack to round the corners as soon as you apply real torque. For a finger tight nut it may get you out of trouble, but do not rely on it.