What Size Is an M6 Screw?

An M6 screw has a 6mm nominal diameter and a 1mm thread pitch as standard, making it one of the most common fixings in UK homes, from flat-pack furniture to door handles. “M6” tells you the diameter, not the length, the head type, or what tool fits it, so this guide covers the rest: how to check a fixing you already have, which type to buy, what to drill, and roughly what it costs.

What “M6” actually means

The “M” stands for metric, and the number that follows is the outer thread diameter in millimetres, measured at the widest point of the thread. An M6 screw or bolt has a 6mm outer diameter and, unless it’s specifically sold as “fine,” a 1mm thread pitch (the distance between one thread peak and the next), confirmed against get-it-made.co.uk’s ISO 724:2023 thread data. A fine-pitch M6 exists too, at 0.75mm, but it’s rare outside precision engineering and not something you’ll find on a Screwfix shelf.

Metric sizing runs in a fairly predictable sequence, and it helps to see M6 next to its neighbours, particularly if you’ve arrived here after being redirected from an M8 search. Nobody remembers M5 exists until they need one on a Sunday evening with the shops shut.

How to check a screw you already have is M6

Lay the screw flat and measure straight across the threads at their widest point, not down the length and not across the narrower core underneath the threads. A ruler is close enough for DIY purposes, though a set of digital vernier callipers (Toolstation and Screwfix both stock inexpensive ones) removes the guesswork, since eyeballing a 6mm shaft against a 5mm or 7mm one from an angle is exactly the kind of measurement that goes wrong on the second attempt, not the first.

There’s a quicker shortcut for anything with a hexagonal bolt head: if a 10mm spanner or socket sits flush on the head with no play, you’re holding an M6 hex bolt. It only works for hex heads, though. Socket caps, pan heads, and countersunk screws need the calliper method or a hex key test instead (see the next section).

M6 bolt vs socket screw vs machine screw vs coach bolt

The “M6” figure only fixes the diameter. What you actually buy depends on the head, and each head type expects a different tool.

Both the AF (spanner) and Allen key figures are confirmed against Monster Bolts’ metric hex and socket size chart, which matches the ISO 4017/DIN 933 hex bolt standard and the ISO 4762/DIN 912 socket cap standard referenced by engineering hardware suppliers. The mistake worth naming here: buying a bag of M6 socket cap screws for a job that actually specified hex bolts, then finding a 5mm Allen key doesn’t touch a hex head, and a 10mm spanner doesn’t fit down into a recessed socket. Check the head type against the instructions, not just the size, before you buy. Fitting alcove shelf brackets is a common example where the wrong head type turns a ten-minute job into a second trip to the shop.

What size hole to drill for M6

The hole size for an M6 fixing depends entirely on whether it needs to pass through freely (clearance) or cut its own thread as it goes in (tapping).

These figures come from the AmesWeb metric clearance hole chart (built to ASME B18.2.8) cross-checked against Accu’s UK tapping and clearance guide. The common mistake is drilling the 5mm tapping size when the bolt actually needs to pass straight through, then wondering why it won’t push through cleanly, or the reverse: drilling a clearance-sized hole into metal that was meant to be tapped, leaving nothing for the thread to bite into. And if you’re screwing into timber, chipboard, or MDF rather than metal or a pre-tapped fixing, an M6 tapping drill isn’t the right reference at all. Wood and chipboard screws are sized separately, which is exactly the situation covered in stopping a squeaking chipboard floor, where the wrong pilot hole is the usual culprit.

Is M6 the same as a 1/4-inch screw?

No. A 1/4-inch UNC bolt measures 6.35mm across, close to M6’s 6mm but not identical, and the thread pitch is genuinely different: 1.27mm on a standard 1/4-20 UNC thread against M6’s 1mm, according to a side-by-side comparison of the two standards. An M6 bolt can start threading into a 1/4-20 nut and feel fine for a turn or two before the mismatched pitch jams it solid, stripping the thread in the process. If you’re replacing a fixing on older imperial-sized furniture or fittings, the safe move is to match the original standard exactly rather than assume “close enough” on a 0.35mm difference.

What M6 fixings cost in the UK

M6 fixings are cheap relative to almost any other part of a DIY job, typically a pound or two for a small pack rising to a few pounds for stainless or specialist finishes.

Prices from Screwfix’s M6 range, checked 25 August 2026 and worth re-checking before you buy, since fastener pricing moves. For general DIY use where you don’t yet know the exact length you’ll need, a mixed assortment box tends to work out cheaper per fixing than buying single lengths one job at a time, and it means one trip to a workbench rather than three trips to Screwfix.

FAQs

What is the diameter of an M6 screw in mm? An M6 screw has a nominal outer thread diameter of 6mm, measured across the widest point of the thread.

What Allen key fits an M6 bolt? A 5mm Allen key fits an M6 socket cap screw. A standard M6 hex-head bolt needs a 10mm spanner or socket instead, not an Allen key.

What drill bit do I use for an M6 screw? It depends on the job: 6.6mm for a standard clearance hole that lets the bolt pass through freely, or 5.0mm for a tapping hole that cuts a matching thread into metal.

Is M6 the same as 1/4 inch? No. A 1/4-inch UNC bolt is 6.35mm with a 1.27mm thread pitch, while M6 is 6mm with a 1mm pitch, and the two aren’t interchangeable.

What’s the difference between M6 and M8? M8 has an 8mm outer diameter and a 1.25mm pitch, a full 2mm wider than M6, and is typically used for heavier fixings like gate hinges rather than furniture assembly.

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