I've purchased some blinds from a UK company and their instructional video says to use a "Size 6 Drill Bit"... except I can't find an agreed definition on what size this is.

Number Maniacs says: #6 drill bit = 0.2040 inches (slightly larger than 13/64, but smaller than 7/32)

Amazon UK says: #6 drill bit = (9/64") = 0.140625 inches

And I own a drill bit that has "6.0" written on it, but it also has "15/64" which is 0.234375 inches

It seems numbers can often refer to mm. So "6.0" might be 6mm but it also seems there's a "size 6" that is equivalent to 5.1816mm.

Can anyone help explain what size I need to use?


  • 1
    Maybe it's a Whitworth size? :D (poking fun at 180 year old standards ;) On a serious note, have you contacted the vendor to see which of the "size 6" standards they're referencing? Additionally, if you're drilling a pilot hole for a screw, simply select a bit that's the same size as (or slightly smaller than) the shaft of the screw, ignoring the threads.
    – FreeMan
    Jun 19, 2023 at 17:55
  • 1
    Usually numbered bits are HSS straight shanks. Another site also said 0.204 inches. Those drill bits at amazon might be for wood screws and are using screw size.
    – crip659
    Jun 19, 2023 at 17:56
  • @crip659 Thanks, but why does my "6.0" drill bit have a 0.234375 size on it, then? So confusing!
    – Chuck
    Jun 19, 2023 at 17:58
  • 1
    @Chuck 6mm is roughly 0.23".
    – TripeHound
    Jun 19, 2023 at 18:02
  • Usually bits will have a number, a letter or the size on them. Unusual for more than one, two size markings are odd.
    – crip659
    Jun 19, 2023 at 18:03

3 Answers 3


Your 6.0 bit is evidently a 6 millimeter bit.

A #6 drill is not 6mm. It is 5.18mm or 0.204 inches.

"Number and letter" sized drills are a specific series of drill sizes (that are not handy fractions of an inch nor convenient round millimeter numbers) that are typically useful in machining operations. So a when a drill is number 6 drill bit, it is NOT 6mm nor is it a drill bit the size of a #6 machine screw, which is an entirely different system. A common number drill set would be 1-60, with 61-80 as an add-on if you need tiny holes.

Have a link to a handy drill chart. If it link rots, search for drill size chart.

enter image description here

You may also find "tap drill sizes" for #6 machine screw threads, and pilot drill sizes for #6 wood screws (which are a different size than #6 machine screws...) those are not #6 drill size.

  • Thanks, I used a "5" bit (ie. it was actually 5mm and it worked fine).
    – Chuck
    Jun 19, 2023 at 18:49

The UK doesn't use 'size number' for drill bits any more, they've been sold in mm for decades. I'd blame the scriptwriter for not knowing that; the actress, of course just read the script.
(Wood) screws can still occasionally be specified by 'number' but drill bits always by mm. This is now so universally accepted that they don't always even write 'mm' on the bit, just the number.

enter image description here

You have a bit of guesswork which you can do by eye. Hold the plug alongside the drill bit. Do they look the same size [at the fat end, but ignoring the 'collar'], give or take?
The plugs in the video are red - red is usually [but not always] for 6mm plugs i.e. they need a 6mm hole. The hole for a rawlplug does not have to be absolutely accurate - the plug itself is what makes the structure work, as it expands & traps into the brickwork.

All you need is that the plug will tap reasonably snugly into the hole. If it puts up too much of a fight, just waggle the drillbit in the hole & try again.
If you accidentally make the hole too big, fill the hole with No More Nails or similar, stuff the plug in & wipe off the excess, then come back tomorrow once it's dry. We are not working to space-ship tolerances here, nor are we holding up the Clifton suspension bridge;)

btw, from your calculations - you forgot to take the very last step…
"15/64" which is 0.234375 inches = 5.953125mm

  • In other words 15/64 is within a hair's breadth (literally) of 6mm. Drilling into most materials with hand-held drills, your holes will be oversized by more than that anyway, so to all intents and purposes 15/64" = 6mm
    – Chris H
    Jun 20, 2023 at 10:52
  • And even on the chance that you end up with a hole that's slightly too small, the fastener is probably capable of hogging it out to the necessary size.
    – Huesmann
    Jun 20, 2023 at 12:33

I hate instructional videos so much. They are too imprecise, and get ruined further by actors and TV producers. UL agrees - they must approve instructions and will not approve videos.

Anyway, the video says "use a #3 as a pilot hole, then follow with #6".

Except... Every drill size numbering system I know of, #3 is larger than #6.

The only thing that makes sense is 3mm and 6mm.

That feels right. 1/8” (3mm) drill are my standard "go to" for pilot holes.

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