If your bits aren't marked with sizes, here's how to select a bit: Hold the screw to the light or a white background, with the bit you plan to use in front of it. You should see screw threads sticking out past the bit, but the bit should hide the rest of the screw. When you find the right size bit, store it with the box of screws so you know which one it is.
'6' is a #6-size screw. Screw gauges are a measure of the head size and shaft size, and are roughly linear but not quite a 1:1 relationship (a #8 screw is a little less than twice the diameter of a #4 screw). There isn't a good system for converting gauge to a calibrated measurement, so you're best off consulting a table like this:Screws have three basic measurements: gauge, threads per inch, and shaft length in inches. So, you may also see a measurement like 6-32 x 1 1/2'. This means it's a #6 diameter, with 32 threads per inch (almost double the normal thread count as a standard wood screw) and an inch and a half long. When the middle number is absent (6 x 1 1/2'), the screw has the 'normal' number of threads per inch for that size and type of screw (for #6 wood screws that's 18).
![Chart Chart](/uploads/1/2/5/3/125371800/957064370.png)
You might find this table useful as well. Gauge Metric equiv. Pilot hole Clearance hole Masonry plug size-3 2.5mm 1.0mm 3.0mm Yellow ( 5mm hole)4 3.0mm 1.5mm 3.5mm Yellow ( 5mm hole)6 3.5mm 2.0mm 4.0mm Red ( 6mm hole)8 4.0mm 2.5mm 4.5mm Red ( 6mm hole)10 5.0mm 3.0mm 5.5mm Brown ( 7mm hole)12 5.5mm 3.5mm 6.0mm Brown ( 7mm hole)14 6.5mm 4.0mm 7.0mm Blue (10mm hole)-See also and the original,.Disclaimer: I wrote the table after more than 30 years in trade (builder/carpenter). Great table @handyman; particularly useful to get info from someone who knows both UK and US standards. I hope you don't mind me table-ifying it as per Steven's comment.
(Although sorry about the wife/mortgage;-)). Steven, as per johnny why's request, I have added back the link to handyman's site, but here in the answer and not, as you'd suggested, in his profile.
Note that I am not, nor do I know or am in any way connected with handyman, so there should be no danger of inappropriate self-promotion. The linked-to page is completely pertinent to, and enhances the answer, so I hope that's OK.–Apr 1 '18 at 17:50.