Bolt and screw size guide: metric charts, spanner sizes and thread basics

A metric fastener designation reads diameter × pitch × length: M6 × 1.0 × 20 is a 6 mm nominal diameter thread with a 1.0 mm pitch, 20 mm long. Drop the pitch — M6 × 20 — and the standard coarse pitch is implied, which is what you'll get unless you ask otherwise. Length is measured from under the head, with one exception that catches everyone: countersunk screws and bolts are measured overall, head included, because the head sits within the material.

This guide decodes the metric system properly — one chart covering pitch, spanner and hex key sizes, clearance holes and tapping drills for the sizes that matter — then covers head markings and grades, wood screw gauge numbers, and the imperial and legacy threads that still turn up in UK workshops and vintage equipment.

Metric bolt size chart

Six numbers per size answer ninety per cent of workshop questions: what pitch it is, what turns it, what hole it passes through and what hole you tap for it.

Spanner sizes apply to hex-head bolts and hex nuts; the hex key column covers socket-head cap screws and, one size down as a rule of thumb, grub screws. Clearance holes here are the medium fit — the general-purpose choice; close and free fits exist either side. Tapping drills are for the standard coarse pitch and follow a rule worth memorising: tapping drill = diameter minus pitch. M6 with a 1.0 pitch taps at 5.0 mm, M8 at 6.8 — the rule holds all the way up the chart, and a jobber drill set plus taps and dies turns the table into finished threads.

How is bolt length measured?

From under the head to the tip — the length states how much fastener is available to do work, so the head doesn't count. The exception is countersunk screws, measured overall including the head, because a countersunk head disappears into the material and contributes to the working length. Button, pan, hex and cap heads: under the head. Countersunk: total. Ordering by the wrong convention leaves an assembly one head-height short, which is exactly the sort of error that surfaces at the end of a build rather than the start.

What do the markings on a bolt head mean?

The numbers stamped on a hex head are the property class, and they're load-bearing information in both senses. The common grades are 8.8, 10.9 and 12.9: the first number is the tensile strength in hundreds of N/mm², the second the yield-to-tensile ratio — so an 8.8 bolt offers 800 N/mm² tensile strength and yields at 80% of it. Grade 8.8 is the general engineering default; 10.9 and 12.9 buy more strength with less ductility, and matter anywhere torque specifications live. Unmarked mild-steel fasteners (typically 4.8 or lower) belong in undemanding joints only.

Stainless fasteners are marked by alloy instead: A2 is the general-purpose stainless for corrosion resistance indoors and out, A4 adds molybdenum for marine and chemical exposure. Note that standard A2/A4 fasteners are roughly comparable to mid-grade steel for strength — corrosion resistance, not strength, is what the premium buys. Where fasteners must stay put under vibration, the answer is usually chemistry rather than muscle: threadlockers, spring or serrated washers, or nyloc nuts each suit different service and disassembly needs.

What about wood screws and gauge numbers?

Traditional wood screws use gauge numbers rather than millimetres, and the two systems now coexist on packaging. The useful equivalences: No.4 ≈ 3.0 mm, No.6 ≈ 3.5 mm, No.8 ≈ 4.0 mm, No.10 ≈ 5.0 mm, No.12 ≈ 5.5 mm — with length quoted overall for countersunk patterns, as above. Self-tapping and self-drilling screws for sheet metal carry their own gauge series on the same numbering logic. The pilot-hole rule of thumb in timber mirrors the tapping-drill rule in metal: drill for the core of the screw, not the thread — roughly the shank diameter minus twice the thread depth, or simply the size that lets the core slide and the thread bite.

Which imperial and legacy threads still matter?

Unified threads dominate American equipment: UNC (coarse) and UNF (fine), designated diameter–TPI, so 1/4-20 UNC is a quarter-inch thread at 20 turns per inch. They are not interchangeable with metric look-alikes — 1/4" and M6 will start on each other's nuts and then bind, wrecking both threads, and a thread gauge costs less than the first assembly ruined by guessing.

The distinctly British survivor is BA, the small-fastener series that built a century of UK electronics and instruments: 2BA (4.7 mm) and 4BA (3.6 mm) turn up constantly in vintage radio, test equipment and classic vehicles. Restoration work either sources BA fasteners or re-taps to the nearest metric — and knowing which decision was made last time is half the job. For everything current, the metric chart above and the screw and bolt kits that follow it are the sensible default, with standoffs and spacers carrying the M3/M4 end of the system into enclosure and PCB work.

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Frequently asked questions

What does M6 mean on a bolt?

A metric thread with a 6 mm nominal outside diameter. Written M6 × 1.0 × 20, the second figure is the thread pitch in millimetres and the third the length — measured under the head, or overall for countersunk types.

What size spanner fits an M8 bolt?

13 mm across flats for a standard hex head. The full run: M5 takes 8 mm, M6 takes 10 mm, M8 takes 13 mm, M10 takes 16 mm (17 mm on older DIN-standard bolts) and M12 takes 18 mm (19 mm DIN).

What drill do I use to tap an M6 thread?

5.0 mm for the standard coarse pitch. The general rule is tapping drill = diameter minus pitch, so M6 × 1.0 taps at 5.0 mm, M8 × 1.25 at 6.8 mm (rounded to the stock drill size), and M10 × 1.5 at 8.5 mm.

What's the difference between 8.8 and 10.9 bolts?

Strength grade. 8.8 offers 800 N/mm² tensile strength yielding at 80% of it; 10.9 raises that to 1,000 N/mm² at 90%. Higher grades suit torque-critical and highly loaded joints but are less ductile — match the grade the design specifies.

Why won't my 17 mm spanner fit this M10 bolt?

Because it's an ISO-standard bolt with a 16 mm head. M10 heads come in two sizes — 16 mm under the current ISO standard, 17 mm under the older DIN standard — and both are still sold. The threads are identical; only the hex differs.

What size is a No.8 wood screw in metric?

Approximately 4.0 mm shank diameter. The common conversions: No.4 ≈ 3.0 mm, No.6 ≈ 3.5 mm, No.8 ≈ 4.0 mm, No.10 ≈ 5.0 mm — with lengths on countersunk wood screws quoted overall, including the head.

Stock the sizes that fit

From M3 machine screws to threaded rod, with the nuts, washers, taps and gauges that make the chart above real.