About the Handiwork Socket & Wrench Size Calculator
The Socket & Wrench Size Calculator looks up the standard wrench or socket size (width across flats) for any metric or SAE hex bolt or nut, from M3 to M48 and 1/4" to 1". It also shows the closest size in the other measurement system, for when you only have a metric set on hand for an SAE bolt or vice versa.
How to use the Handiwork Socket & Wrench Size Calculator
- Choose Metric or SAE / Imperial.
- Select the bolt or nut designation, like M8 or 3/8".
- Read the exact standard wrench size, plus the closest socket size in the other system if that is all you have available.
Where these sizes come from
Metric hex bolt and nut wrench sizes follow ISO 4014 (bolts) and ISO 4032 (nuts), which standardize the width across flats — the distance between two parallel faces of the hex head — for each thread diameter. SAE/imperial hex bolts follow ANSI/ASME B18.2.1, which likewise fixes one wrench size per bolt diameter. Both are look-up standards, not something you can derive from the bolt diameter with a simple formula, which is why this tool works from a size table rather than a calculation.
The M10 / M12 / M14 exception
ISO 272:1982 changed the across-flats size for M10, M12, and M14 from the older DIN 931/933 values of 17 mm, 19 mm, and 22 mm to the current 16 mm, 18 mm, and 21 mm. Both sizes are still in circulation — older hardware, some automotive fasteners, and budget bolts sometimes use the legacy size. If the calculated size feels slightly loose on one of these three, try one millimeter larger before assuming the bolt is a nonstandard size.
Why the cross-system size is only approximate
Metric and SAE sizes were never designed to line up with each other, so the "closest" socket in the other system is rarely an exact match — it is usually off by a few hundredths of a millimeter to a few tenths of a millimeter. A socket that is very slightly too large can round off the corners of the bolt head under load; a socket that is too small will not seat at all. Use the cross-system size only as a temporary substitute, and switch to the correct standard size for anything you plan to torque significantly.
Bolts vs. nuts
Standard hex nuts use the same across-flats size as the hex bolt of the same nominal diameter (an M8 nut takes the same 13 mm socket as an M8 bolt head), so this chart applies to both. Flange bolts, flange nuts, and some structural or heavy hex fasteners can use a different size than the standard hex series — check the fastener’s markings or measure it directly if a standard socket does not fit.
Continue this workflow
Use the adjacent tool when the next step calls for a different input, output, or method.
Assumptions and limitations
- Covers standard ISO 4014/4017 metric hex bolts (M3–M48) and ANSI/ASME B18.2.1 SAE hex bolts (1/4"–1"); it does not cover flange bolts, heavy hex, structural bolts, or non-hex drive types (Allen/hex-key, Torx, square).
- M10, M12, and M14 bolts made to the older DIN 931/933 standard may need a socket one millimeter larger than the current ISO size shown.
- The cross-system "closest socket" figure is a mathematical nearest match, not a certified equivalent — always confirm it seats fully before applying significant torque.
- This is a reference chart, not a substitute for measuring an unmarked or worn fastener directly with calipers.
Sources and standards
These authoritative references were used to verify the method and guidance on this page.
Frequently asked questions
What size socket do I need for an M8 bolt?
A 13 mm socket or wrench fits a standard M8 hex bolt or nut. If you only have SAE sockets, 1/2" (12.7 mm) is the closest match, though it is about 2% undersized and may feel slightly loose.
Why does my M12 bolt need a 19mm socket instead of 18mm?
It is likely made to the older DIN 931/933 standard, which used 19 mm for M12 before ISO 272:1982 changed the current standard to 18 mm. Both sizes exist in the field — try the 1 mm larger size if the calculated one does not seat properly.
Is there an exact metric equivalent for every SAE socket size?
No — metric and SAE sizes are on different measurement systems and only rarely line up exactly. This tool shows the closest standard size and the percentage difference so you know how good (or rough) a substitute it is.
Does this work for Allen keys or Torx bolts?
No, this chart is for standard external hex (6-point) bolt heads and nuts only. Hex-key (Allen), Torx, and square-drive fasteners use entirely different sizing systems.
Do nuts use the same socket size as bolts of the same diameter?
Yes — for standard hex fasteners, a nut and a bolt head of the same nominal diameter (for example, M10 or 3/8") share the same across-flats wrench size, so the same lookup applies to both.