Socket & Wrench Size Converter
Convert socket and wrench sizes between metric and SAE inches, see which substitute fits and how well, and find the wrench or hex key any bolt or nut needs: metric, inch, heavy hex, Whitworth, BSF and BA.
How to Use
- Choose Wrench size to convert a socket or spanner, or Bolt size to find the wrench a fastener needs.
- Wrench size: type it as
13 mm,1/2,1-1/16,0.5 in,1/4 W,5/16 BSFor2BA. A bare number uses the unit switch. - Bolt size: type
M8,M10x1.25,3/8-16,#10or5/16 BSF, then pick the head: hex bolt, hex nut, heavy hex or socket head. - Read the exact size in both units, the best metric and SAE substitutes and how well each fits, and what the size is made for.
- The drawing puts the substitute’s opening over the head to scale, and the ladder shows every nearby size in both systems.
Worked Example
A 1/2" nut and a metric socket set. 1/2" × 25.4 = 12.70 mm across the flats. A 13 mm socket is 13 − 12.70 = +0.30 mm, 2.4% oversize. A good fit is within about 0.2 mm (or 0.8% on big sizes), so 13 mm is loose: it will turn a nut that is not tight, but on a seized one it bears on the corners and can round them off. 12 mm is 0.70 mm too small and will not go on.
A 3/4" nut and a 19 mm socket. 3/4" = 19.05 mm, so 19 mm is 0.05 mm small. That usually still fits, because heads are made at or under their nominal size (an ISO M8 head is allowed to be anywhere from 12.73 to 13.00 mm) and sockets are made slightly over theirs. The 19 mm socket is the right tool for a 3/4" nut in a pinch; a 13 mm on a 1/2" is not as good.
The common mistake: an M10 that will not take a 16 mm. M10 bolts made to the old DIN 933 standard have 17 mm heads; ISO 4017 cut them to 16 mm. Same thread, different wrench, and both are still on sale.
Show Work
Formulas
Why There Are So Many Wrench Sizes
Joseph Whitworth proposed the first national screw thread standard in Britain in 1841, and British spanners have been marked with the bolt they fit ever since rather than the jaw size: a “1/4 W” spanner measures 0.525" across. When British Standard heads were later made a size smaller, a 5/16" BSF bolt ended up taking the 1/4 W spanner, which is why old British tools carry two markings. Small instrument and electrical screws used the numbered British Association (BA) series.
American and British engineers agreed on the Unified inch thread in 1948, after wartime trouble with parts that would not interchange, and their wrenches are marked across flats (AF) in fractions of an inch. Metric hex sizes followed DIN in Germany and then ISO. When ISO 4014 and 4017 replaced DIN 931 and 933, four sizes changed: M10, M12 and M14 heads lost a millimetre and M22 gained two, so a 17 mm and a 16 mm can both be “the M10 wrench”.
The socket set itself took off in 1920, when Snap-on began selling handles and interchangeable sockets with the slogan “five do the work of fifty”. Because a hexagon is 15.5% wider across its corners than across its flats, a socket that is too big rides up onto the corners under load, which is how fasteners get rounded off.
About This Converter
This converter translates socket and spanner sizes between millimetres, SAE fractions and British markings, rates how well each substitute fits, and looks up the wrench or hex key for metric, inch and British fasteners. The tables come from ISO 4017, 4014, 4032 and 4762, the older DIN 933, ASME B18.2.1, B18.2.2 and B18.3, and published Whitworth, BSF and BA spanner jaw sizes.
Fit ratings compare nominal sizes. Real heads are made slightly under nominal and wrench openings slightly over, so a “tight” pair often works; a “loose” one is fine for light work but risky on a seized or high-torque fastener. Everything runs in your browser.
Related tools: Bolt Torque & Preload Calculator, Decimal to Fraction Converter and the thread sizes reference.
Frequently Asked Questions
What metric socket is the same as 1/2 inch?
None exactly. 1/2" is 12.70 mm, so a 13 mm socket is 0.30 mm (2.4%) oversize: it turns a nut that is not tight, but can round off a seized one. 12 mm is 0.70 mm too small and will not go on. For anything stuck, use a real 1/2" socket.
Which metric and SAE sizes can be swapped?
A few pairs are within a tenth or two of a millimetre: 8 mm and 5/16" (0.06 mm apart), 16 mm and 5/8" (0.13), 19 mm and 3/4" (0.05), 24 mm and 15/16" (0.19), and 27 mm and 1-1/16" (0.01). Others have no good partner: 3/8", 9/16" and 11/16" sit between metric sizes, so 14 mm is too small for 9/16" and 15 mm is 0.71 mm loose.
Why does my M10 bolt need a 17 mm wrench when charts say 16?
Because it was made to the old DIN 933 standard. When ISO 4017 replaced it, the heads of M10, M12 and M14 shrank by 1 mm (to 16, 18 and 21 mm) and M22 grew from 32 to 34 mm. Both kinds are still sold, so an M10 can need either 16 or 17 mm. Every other size is the same in both.
What size wrench fits a 3/8 inch bolt?
A standard 3/8" hex bolt or nut takes 9/16". A 3/8" heavy hex nut takes 11/16", and a 3/8" socket head cap screw takes a 5/16" hex key. Size jumps are not always the same for bolt and nut: a 7/16" bolt head is 5/8" but its nut is 11/16", and a 9/16" bolt head is 13/16" with a 7/8" nut.
How do Whitworth spanner sizes work?
British spanners are marked with the bolt diameter, not the jaw size. A 1/4 W spanner has 0.525" (13.34 mm) jaws for a 1/4" Whitworth bolt. Later British Standard heads were made one size smaller, so a 5/16" BSF bolt takes the same 1/4 W spanner, which is why many are stamped “1/4 W 5/16 BS”. BA sizes, used on small electrical parts, are numbered: 2BA is 0.324" (8.23 mm).
How do I use the Socket & Wrench Size Converter?
Simply type or paste your value and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Missing socket
No 3/4" socket? A 19 mm one is 0.05 mm small and usually goes on; a 16 mm works for 5/8" at 0.13 mm loose.
Buying the right wrench
An M10 hex bolt takes 16 mm, but older DIN bolts take 17 mm. Check before ordering a single size.
Classic cars and motorcycles
British bikes and Land Rovers use Whitworth, BSF and BA: 5/16 BSF takes the 1/4 W spanner, 13.34 mm across the jaws.
Hex keys for cap screws
An M6 socket head cap screw takes a 5 mm key; a 1/4" one takes 3/16".
Reading a mystery socket
A socket stamped 1-1/16 is 26.99 mm, within 0.01 mm of a 27 mm: the two are interchangeable.
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