Is an M8 bolt the same thing as an 8 mm socket?

No. M8 is the thread diameter — 8 mm — while the standard hex head above it measures 13 mm across the flats. You need a 13 mm socket, not an 8 mm one.

Can I use a 1/2 inch socket on an M8 bolt?

Not reliably. Half an inch is 12.70 mm, which is 0.30 mm smaller than the 13 mm head, so the socket will not seat squarely. The correct tool is a 13 mm socket.

The hex key just spins inside my cap screw — what now?

The internal hex is stripped, and more pressure only polishes it smoother. Switch to the extraction procedure: penetrating oil, a pilot hole drilled in 0.5 mm steps, then a screw extractor.

How tight should I make wheel nuts?

To the figure on the vehicle placard, which for passenger cars normally falls between 80 and 140 N·m. Tighten in a star pattern with a torque wrench and re-check after a short drive.

M8 bolt: 13 mm socket

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A standard M8 hex-head bolt takes a 13 mm socket across the flats. An M8 socket cap screw — the kind with a cylindrical head and a recessed hex — takes a 6 mm hex key instead. The chart below maps every common metric size to the tool that fits it.

The number on the socket

Socket sizes are the distance across the flats of the bolt head, in millimetres for metric hardware. An M8 hex head measures 13 mm across those flats, so the socket and the head share one number.

Two systems side by side

Metric and fractional inch tools overlap on paper and collide in the toolbox. 5/16 in is 7.94 mm — a hair under 8 mm — while 1/2 in is 12.70 mm, a full 0.30 mm under the 13 mm an M8 head needs.

When the bolt fights back

Rounded flats and stripped hexes call for an ordered response: oil, impact, pilot hole, extractor. Each step trades a little of the bolt for a chance to save the thread it sits in.

The number on the socket is the distance across the bolt's flats — for an M8 hex head, that distance is 13 mm.

M8 names the thread, not the tool: 13 mm socket on the hex head, 6 mm key in the cap screw.

What size socket or hex key fits an M8 bolt?

A hex-head M8 bolt takes a 13 mm socket; an M8 cap screw takes a 6 mm hex key.

The M8 marking describes the thread — a nominal diameter of 8 mm — and says nothing about the head. Under ISO 4017, the standard hex head on an M8 bolt measures 13 mm across the flats, so a 13 mm socket, ring spanner or combination wrench is the correct tool.

If the fastener is a socket cap screw under ISO 4762 — a cylindrical head with a recessed hex — the tool drops one size below the thread: M8 takes a 6 mm hex key. That key sits in every standard metric folding set, which carries 1.5, 2, 2.5, 3, 4, 5, 6, 8 and 10 mm.

A typical 3/8 in-drive socket set covers 8–19 mm, so the 13 mm socket you need sits in the middle of the most common set sold. The smaller 1/4 in drive (6.35 mm square) suits light work; 1/2 in and larger drives handle suspension and wheel hardware.

Six cards worth pinning above the bench

Each card is one size rule pulled from the chart — thread, head and tool in a single glance.

The key terms of this guide, drawn to one scale

The master size chart

Metric bolt sizes and the tools that fit them, per ISO 4017 and ISO 4762
Bolt size	Hex-head socket (across flats)	Cap-screw hex key
M6	10 mm	5 mm
M8	13 mm	6 mm
M10	17 mm	8 mm
M12	19 mm	10 mm
M16	24 mm	—

Metric bolt sizes and the tools that fit them, per ISO 4017 and ISO 4762

Asked at the counter

Is an M8 bolt the same thing as an 8 mm socket?: No. M8 is the thread diameter — 8 mm — while the standard hex head above it measures 13 mm across the flats. You need a 13 mm socket, not an 8 mm one.
Can I use a 1/2 inch socket on an M8 bolt?: Not reliably. Half an inch is 12.70 mm, which is 0.30 mm smaller than the 13 mm head, so the socket will not seat squarely. The correct tool is a 13 mm socket.
The hex key just spins inside my cap screw — what now?: The internal hex is stripped, and more pressure only polishes it smoother. Switch to the extraction procedure: penetrating oil, a pilot hole drilled in 0.5 mm steps, then a screw extractor.
How tight should I make wheel nuts?: To the figure on the vehicle placard, which for passenger cars normally falls between 80 and 140 N·m. Tighten in a star pattern with a torque wrench and re-check after a short drive.