Finding the Metric Socket That Will Fit an Imperial Bolt
Nearly every home workshop ends up with a split tool collection: an imperial set inherited or bought cheap, a metric set bought for a car, and a job in front of you that needs a size from the drawer you do not have. The question is never really "what is this in millimetres" — it is "which of the sockets I own is close enough, and close enough to what".
The conversion is the easy half. Deciding whether the nearest metric size is safe to use on a given fastener is the half that costs you a rounded bolt head, so the table below prints both.
Why This Matters
Wrenches and Sockets
Automotive Work
Machining and Pipe Fittings
Turning a Fractional Marking Into Millimetres
Make the fraction a decimal
Divide the top by the bottom. Sixteenths are the common case: 1/16 is 0.0625, so 3/16 is 0.1875, 5/16 is 0.3125 and 9/16 is 0.5625.
Enter it and read the millimetres
The result appears as you type and carries eight decimal places, which is enough to see the difference between a snug fit and a sloppy one.
Judge the gap, not just the number
Anything under about 0.2 mm of slack is usually safe on a well-torqued fastener. Half a millimetre on a tight or rusted head is how corners get rounded.
The reverse direction covers the other half of the drawer problem: you are holding a metric bolt and want to know which imperial spanner might work. Press the swap arrows and millimetres go in on top, with decimal inches coming out below.
Fractional Spanner Sizes and Their Metric Neighbours
The last column is the tool most people reach for when the right one is missing. The gap tells you whether that is a reasonable idea: positive means the metric tool is loose on the imperial fastener.
| Marking | Decimal inches | Millimetres | Nearest metric (gap) |
|---|---|---|---|
| 1/4" | 0.25 in | 6.35 mm | 6 mm (−0.35 mm) |
| 5/16" | 0.3125 in | 7.94 mm | 8 mm (+0.06 mm) |
| 3/8" | 0.375 in | 9.53 mm | 10 mm (+0.48 mm) |
| 1/2" | 0.5 in | 12.70 mm | 13 mm (+0.30 mm) |
| 9/16" | 0.5625 in | 14.29 mm | 14 mm (−0.29 mm) |
| 3/4" | 0.75 in | 19.05 mm | 19 mm (−0.05 mm) |
Fractions Go In as Decimals, Live
Type 0.5625 and the millimetre figure is there before you finish. Nothing to submit, so you can walk the whole sixteenths scale in under a minute.
Flip It When the Fastener Is Metric
The swap arrows reverse the pair, turning the question round: a 13 mm head becomes 0.5118 in, which is what tells you a 1/2" is too small.
Copy It Into a Parts Order
Each field yields the number with no unit attached, ready to drop into a supplier search box or a cutting list without any tidying.
Down to Micrometres for a Tolerance Band
Fits and clearances on machined work are quoted in microns. Search the unit list for Micrometer and the same value reappears at that scale.
Tool and Fastener Questions
What is 1 inch in millimetres?
25.4 mm, fixed by international agreement in 1959. Everything else on this page follows from it: a sixteenth of an inch is 1.5875 mm, and a thousandth of an inch — a "thou" — is 0.0254 mm.
Can I put a 13 mm socket on a 1/2 inch bolt?
Often, carefully. The bolt measures 12.7 mm across the flats, leaving 0.3 mm of play. On a lightly torqued fastener with a six-point socket that is tolerable; on a rusted one, or with a twelve-point socket, the corners take the whole load and round off.
What is closest to 9/16 inch in metric?
9/16" is 14.29 mm, which falls awkwardly between tools. A 14 mm is 0.29 mm undersize and will not seat; a 15 mm is 0.71 mm oversize and will slip. This is the size worth owning in imperial rather than substituting.
Is a 1/2 inch drive the same as a 1/2 inch socket?
No, and it catches people out. The drive size describes the square hole where the ratchet plugs in — 1/4", 3/8" or 1/2" — and is quoted in inches even on fully metric sets. The socket size, the opening at the other end, is separate and may be metric.
Why does one car use both fastener systems?
Model lines change over decades while individual assemblies do not. A platform designed in imperial keeps those fasteners in the chassis long after a metric engine or a bought-in metric alternator arrives, so the two systems end up bolted to each other.
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