Socket Size Converter

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Tools • mm ↔ inch

Spec

Size

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Choose a size to see equivalents

How to Use the Socket Size Converter

Pick the system you know — choose the sizing standard you already have (e.g. US).
Select your size — pick your known size from the list.
Read every equivalent — all other systems update instantly, with your source highlighted.

Why Use This Socket Size Converter

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Every system side by side

Choose the size you already know and the equivalents in all the other standards appear together, with your source highlighted — so nothing has to be converted twice to be checked.

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Live as you type

Results are recalculated on every keystroke, with no submit step and no page reload, so a value can be adjusted until the answer looks right rather than guessed once.

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100% private

Everything runs in your browser — no account, no uploads, nothing leaves your device.

Understanding the Socket Size Converter

Toolboxes mix metric (mm) and SAE (inch) sockets, and the sizes are close but rarely identical — a 1/2" socket is 12.7 mm, so a 13 mm is the nearest metric size. Using the wrong one can round off a fastener.

This chart lines up SAE and metric socket and wrench sizes so you can grab the right one, or the closest safe substitute, for any bolt or nut.

Formula & Logic

Sockets are sized to the fastener's across-flats dimension, and the practical question is almost always whether a metric socket can substitute for an imperial one or vice versa. Some pairs are close enough to work and others are close enough to be dangerous — a 13 mm socket on a ½-inch fastener has 0.3 mm of slop, which will round the corners of a tight bolt. The general rule is that a difference under about 0.2 mm is usable and anything beyond it risks damage, with six-point sockets far more tolerant than twelve-point.

Socket size = fastener across-flats dimensionImperial to metric: inches × 25.4Usable substitution: difference under ~0.2 mmSix-point grips the flats; twelve-point grips closer to the corners

where:

across flats: the distance between opposite faces of the hex
slop: the gap between socket and fastener — what rounds corners
point count: six-point for tight or corroded fasteners; twelve-point for access

Assumptions: Drive size (¼, ⅜, ½ inch) is a separate specification describing the ratchet fitting and is unrelated to the fastener size. Never use an imperial socket on a metric fastener under high torque if a correct size is available.

Step-by-Step Example: Which Metric Sockets Substitute Safely

Convert three imperial sizes and assess each nearest metric neighbour.

Sizes to check3/8 in, 1/2 in, 3/4 in

3/8 inch: 0.375 × 25.4 = 9.525 mm. Nearest metric is 10 mm — 0.475 mm of slop, too much.
1/2 inch: 0.5 × 25.4 = 12.7 mm. Nearest is 13 mm — 0.3 mm of slop, marginal and risky.
3/4 inch: 0.75 × 25.4 = 19.05 mm. Nearest is 19 mm — only 0.05 mm difference.
That last pair is the well-known safe substitution: 19 mm and 3/4 inch are interchangeable.
Also close: 5/16 in (7.938 mm) against 8 mm — 0.062 mm, usable.
And 9/16 in (14.288 mm) against 14 mm — 0.288 mm, not advisable.

Result19 mm ≈ ¾ in (0.05 mm apart) — but 13 mm on ½ in has 0.3 mm of slop

The 19 mm / ¾ inch pairing is the one genuinely safe substitution in common sizes. The 13 mm on ½ inch case is the one that rounds fasteners, because it feels like it fits until torque is applied and the corners give way.

Socket Size Converter FAQ

Is a 13mm socket the same as 1/2 inch? ▾

Almost — 1/2" = 12.7 mm, so a 13 mm socket is the closest metric size but is slightly larger. They are not perfectly interchangeable on tight fasteners.