How this instrument works
No metric bolt size in millimetres equals an inch fraction exactly, so any "nearest SAE wrench" answer is necessarily an approximation with some leftover error. A 13 mm bolt, for instance, sits closest to a 1/2 inch wrench (12.700 mm), but the two are not the same size -- they differ by about 2.3%, close enough that the 1/2 inch wrench grips without slipping, but not an exact match. This page reports both the nearest standard wrench and that wrench's precise millimetre size, so the gap is visible rather than hidden.
Published metric-to-SAE charts do not always agree with each other, and that is worth saying plainly rather than glossing over. A full engineer's or machinist's set, sized in 1/32 inch increments, can have a genuinely closer wrench available for a given metric size than a common retail combination-wrench set, which is typically sized in 1/16 inch increments across 13 wrenches from 1/4 inch up to 1 inch. Two charts built on those two different conventions can recommend a different "nearest" wrench for the same metric bolt. Rather than pick one contested pairing table and present it as the answer, this tool computes the nearest match live against the common 13-wrench, 1/16 inch retail set, and shows the exact millimetre size of whatever it picks so the size of any mismatch is never hidden.
- Enter the bolt or fastener's metric size into Metric size (mm); it opens at 13, one of the most common sizes on cars.
- Read the closest match from Nearest standard SAE wrench, given as an inch fraction like 1/2".
- Check That wrench's exact size (mm) to see precisely how close the match is, rather than assuming it is exact.
- The page opens already matching 13 mm to a 1/2" wrench (12.700 mm), so you can see the roughly 2.3% gap before entering your own size.
- If the mm gap shown feels too large for a tight-tolerance job, reach for a metric wrench of the exact size instead -- this tool finds the closest inch-fraction stand-in, not a perfect substitute.
Worked example -- 13 mm to the nearest SAE wrench
Enter 13 into Metric size (mm), the field's default. Nearest standard SAE wrench reads 1/2", and That wrench's exact size (mm) reads 12.700 -- one of the most common metric sizes on cars sits closest to the 1/2 inch wrench among the 13 standard sizes, with a gap of about 2.3% (0.300 mm) that a socket set can absorb.
Enter 17 instead -- another very common size, found on many spark plugs and lug nuts -- and Nearest standard SAE wrench reads 11/16", with That wrench's exact size (mm) at 17.462. The gap there is smaller, about 2.7% (0.462 mm), showing how the closeness of the match varies from one metric size to the next rather than staying constant.
Questions
Why isn't there an exact SAE match for a metric size?
Because metric millimetres and inch fractions come from two unrelated measurement systems, so no whole-number millimetre size lines up exactly with a sixteenth-of-an-inch wrench. Every "nearest" pairing carries some leftover gap; this page shows that gap explicitly as a millimetre figure rather than implying the match is perfect.
Which 13 SAE sizes does this tool check against?
The standard 1/16-inch-increment combination wrench set: 1/4", 5/16", 3/8", 7/16", 1/2", 9/16", 5/8", 11/16", 3/4", 13/16", 7/8", 15/16", and 1". This is the common retail set most home and shop toolboxes carry, rather than the finer 1/32-inch increments found in some engineer's or machinist's sets.
Why do different metric-to-SAE charts sometimes disagree?
Because they are not always built on the same set of reference wrenches. A chart based on a full 1/32-inch engineer's set can have a genuinely closer size available than a chart based on the common 1/16-inch retail set, so the two can recommend different "nearest" wrenches for an identical metric size. This page is explicit that it checks against the 13 standard 1/16-inch sizes rather than asserting one universal pairing.
Why is 10 mm the size mechanics joke about losing?
It is more a running joke about how often a 10 mm socket goes missing from a toolbox than anything about the size itself. For reference, 10 mm is nearest to a 3/8" wrench (9.525 mm) among the standard 1/16-inch sizes, a gap of about 4.8%.
Should I use the SAE match or buy the exact metric wrench?
For a quick job where the wrench grips without rounding off the fastener, the nearest SAE match shown here is often close enough, especially at gaps under about 3%. For a tight-tolerance fastener, repeated use, or anything prone to rounding off, the exact metric size -- not an inch-fraction stand-in -- is the safer choice.