Every mixed toolbox eventually produces the same question: the metric socket is missing, so which SAE one is close enough? A plain unit conversion does not answer it. Thirteen millimetres converts to 0.5118 inches, and there is no 0.5118-inch socket in existence — the useful answer is which of the sizes that actually exist is nearest, and by how much you would be off.
That is what this metric to SAE converter from Arb Digital does. It snaps to the real fractional sizes found in a socket set, reports the gap in millimetres and as a percentage, and tells you whether that gap is small enough to be safe or large enough to round a fastener. It is a tool-matching page, not a unit page.
What This Metric to SAE Converter Does
Enter a metric across-flats size and the tool returns the closest SAE fraction, the exact difference, and a verdict. It also reports separately the smallest SAE size that will physically go over the fastener, which is often not the same as the nearest one — if the nearest fraction is undersized, it will not fit at all no matter how close it looks on paper. Reverse the direction and it does the same thing from an SAE size to the nearest standard metric size.
Here is the boundary with the live converters on the site, because there are several and the difference matters. The length converter, cm to inches, meters to feet and the general unit converter all give an exact decimal for any input — mathematically correct, but a number no tool is manufactured in. The feet and inches calculator handles mixed-unit arithmetic, and the decimal to fraction calculator converts any decimal to a fraction with no notion of which fractions are real tool sizes. This page is the only one that snaps to the manufactured set and reports the fit error, which is the thing that decides whether a fastener survives.
How to Use It
- Measure across the flats. Hex sizes are the distance between two opposite faces, never across the corners, which reads about 15% larger.
- Pick the direction that matches what you have and what you are looking for.
- Set the slop tolerance. Tighten it for high torque, aluminium, brass or any fastener you cannot afford to damage.
- Read the verdict before the number. A close-looking difference can still be an undersized tool that simply will not go on.
- Check the smallest-size-that-fits figure when the nearest match is undersized, so you know what your fallback actually costs you in slop.
The Formula / How It's Calculated
The conversion base is exact: NIST states on its SI Units – Length page that the inch, derived from the 1959 yard definition, is exactly equivalent to 25.4 mm. So an SAE size in inches becomes millimetres by multiplying by 25.4, and the difference is SAE mm − metric mm. Expressed as a percentage that is (difference ÷ metric size) × 100. NIST's general unit conversion guidance covers the same relationships and points to the full conversion-factor tables.
The snapping step is what makes this different from a converter. The tool holds a list of the fractional sizes actually supplied in a standard socket set, from 3/16 in up to 1-1/2 in, and a list of standard metric hex sizes, then searches each list for the entry with the smallest absolute difference. It separately searches for the smallest entry that is greater than or equal to the target, which is the smallest tool that will physically pass over the fastener.
Worked example, matching the default. A 13 mm fastener. The nearest SAE fraction is 1/2 in, which is 0.5 × 25.4 = 12.70 mm. That is 0.30 mm under 13 mm, or 2.31% under — so despite being the closest size in the set, it will not go over the fastener at all. The smallest SAE size that does fit is 9/16 in at 14.2875 mm, which is 1.2875 mm oversize, or 9.90% — far too loose for anything under real torque. The honest answer for 13 mm is that there is no acceptable SAE substitute.
The Pairs That Genuinely Interchange, and the Ones That Do Not
A handful of metric and SAE sizes are close enough to swap without much thought. The best is 19 mm and 3/4 inch: 3/4 inch is 19.05 mm, a gap of 0.05 mm, or 0.26%. That is inside normal manufacturing tolerance and the two are effectively the same tool. Next best are 16 mm and 5/8 inch (15.875 mm, 0.125 mm under) and 8 mm and 5/16 inch (7.9375 mm, 0.0625 mm under) — both undersized, so the SAE tool will not quite go on a tight metric fastener, but the gap is small enough that a worn fastener often accepts it.
The famous failures are 10 mm and 13 mm. For 10 mm, 3/8 inch is 9.525 mm — 0.475 mm undersize, will not fit — and 13/32 inch is 10.319 mm, 3.2% oversize. For 13 mm the gap is the one worked through above. These are exactly the two sizes people most often reach for, which is why "close enough" gets a bad reputation. Run any size through this tool before assuming a substitution works; the numbers are not intuitive because the two systems were never designed to align.
Why an Oversize Tool Rounds Fasteners
A hex fastener transmits torque through six flat faces. When the tool is the correct size, load spreads across those flats. When the tool is oversized, it cannot contact the flats squarely — it rocks until it bears on the corners instead, and the corners are the weakest part of the geometry. Under load the corners deform, the tool slips further, and each slip removes more material until the head is round and no tool of any size will grip it.
This is why the direction of the error matters more than its size. An undersized tool simply does not fit, which is annoying but harmless. An oversized tool fits, feels almost right, and destroys the fastener under torque. It is also why six-point sockets tolerate a slightly larger gap than twelve-point ones: six points sit deeper on the flats and have more contact area to give away before they reach the corners. Twelve-point sockets on a slightly oversize fastener are the classic rounding scenario.
Material matters too. Steel fasteners tolerate a little slop; brass, aluminium and the plated fasteners used on electronics and appliances do not, and a 2% gap that is fine on a suspension bolt will strip a brass fitting. Set the tolerance selector to tight whenever the fastener is softer than the tool. If you are working to a torque figure, the bolt torque calculator covers the load side and the torque converter handles Nm to lb-ft.
Across Flats Versus Across Corners
A surprising number of "wrong size" problems are measurement problems. Hex sizes are always specified across the flats — face to opposite face. Measuring across the corners of a regular hexagon gives a value about 15.5% larger, because the corner-to-corner distance is the flats distance divided by the cosine of thirty degrees. A 13 mm hex measures roughly 15 mm across the corners, which is itself a real wrench size, so the mistake produces a plausible wrong answer rather than an obvious one.
Callipers on the flats of a slightly rounded fastener also read low, which pushes you towards a tool that is too small. If a measurement comes out between two standard sizes, it is much more likely that the fastener is worn or that you caught a corner than that a non-standard size was used. Round to the nearest standard size and check the fit before applying torque.
Bolt Diameter Is Not Wrench Size
One more source of confusion: an M8 bolt does not take an 8 mm wrench. The M-number is the thread's nominal outside diameter, while the wrench size is the head's across-flats dimension, and the relationship between them depends on the head standard rather than on arithmetic. An M8 hex head commonly takes 13 mm, and sometimes 12 mm on a flanged or reduced head.
SAE fasteners have the same split: a 5/16-inch bolt normally takes a 1/2-inch wrench. So when someone says "it's a 13", they mean the tool; when they say "it's an M8", they mean the thread. Convert the one you actually have, and never assume that a converted thread size gives you a wrench size. If you are working the other way from a nominal length, the length converter remains the right tool for plain dimensional conversion.
Arb Digital's free tools library covers unit conversion, torque, fasteners and dimensional maths, and our team is happy to talk through anything the tools cannot answer.
Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Using a plain unit conversion as a tool size — 0.5118 inches is arithmetically right and does not exist as a socket.
- Treating the nearest size as the usable size — if the nearest fraction is undersized it will not go on at all.
- Measuring across the corners — that reads about 15.5% high and lands you on a genuinely different standard size.
- Confusing M-number with wrench size — M8 is the thread diameter, and the head usually takes a 13 mm wrench.
- Using a twelve-point socket on an oversize gap — it bears on the corners first and rounds the head fastest.
Related Free Tools From Arb Digital
Use the length converter or unit converter when you want the exact decimal rather than the nearest tool size, cm to inches for a single quick figure, the decimal to fraction calculator to express any decimal in fractional form, the bolt torque calculator for tightening figures, and the torque converter for Nm and lb-ft. Everything else is in the free online tools hub.
Frequently Asked Questions
1/2 inch, which is 12.70 mm — but that is 0.30 mm undersize, so it will not go over a 13 mm fastener. The smallest SAE size that fits is 9/16 inch at 14.2875 mm, which is 9.9% oversize and too loose for real torque.
19 mm and 3/4 inch are the closest pair, differing by only 0.05 mm. 16 mm and 5/8 inch, and 8 mm and 5/16 inch, are close but slightly undersized, so the SAE tool may not go onto a tight metric fastener.
An oversize tool cannot sit flat on the six faces, so it bears on the corners instead. The corners deform under load, the tool slips, and each slip removes more material until nothing grips.
An undersized wrench simply will not fit over the fastener, so it is safe in the sense that it cannot cause damage. Forcing one onto a worn head is a different matter and usually makes the head worse.
No. M8 refers to the thread's nominal outside diameter. The head is usually 13 mm across the flats, and sometimes 12 mm on a flanged or reduced-head design, so the two numbers are unrelated.
Measure across the flats, from one face to the opposite face. Measuring across the corners gives a figure about 15.5% larger, which often coincides with a different real wrench size and produces a convincing wrong answer.
A length converter returns an exact decimal for any input. This tool snaps to the fractional sizes that are actually manufactured and reports how far off the fit is, which is the number that decides whether a fastener survives.
