# Understanding US Screw Sizes

I'm going to be building a workbench, so I've been 'screw shopping.' I need some explanation on the sizing of screws when they are advertised as (example) 6 x 3/4". I know that 3/4" is the length of the screw, but what does the 6 mean?

"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: http://hingedummy.info/screwinfopage2.htm

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).

• Good answer. Only thing I could add is that some "normal" thread screws can come in both fine and coarse thread varieties, so you could have "6 x 1 1/2" coarse" or "6 1 1/2" fine". Apr 23, 2012 at 16:19
• Also, I usually see screw gauges for sizes less than 1/4". Above that I see fractions. Apr 23, 2012 at 21:06
• I agree with everything except the "normal" thread count for a wood screw most are much coarser or less threads per inch. Oct 23, 2017 at 19:30

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 as an image here and the original table here.

Disclaimer: I wrote the table after more than 30 years in trade (builder/carpenter).

• Answers that link-only are generally frowned upon and will likely be removed. It looks like you have some good info here, could you include it directly in the answer? You can include the link in your profile if you want to promote your site. Jan 28, 2015 at 2:12
• hi, great info! can you re-add the link? :) Aug 2, 2017 at 19:05
• 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.
– tkp
Apr 1, 2018 at 17:50
• 2 questions — can you extend the table up to 10mm metric? And, does the metric equiv refer to the major diameter or the shank? Aug 25, 2018 at 3:21
• Detly, I'm afraid I don't see screws over 14g/6.5mm gauge here. Maybe someone in the US can add them (assuming that is where they are available)? Aug 26, 2018 at 7:52

The sites mentioned in previous answers are nice, but are not comprehensive. There are several different ways of measuring the diameter which is critical to understand based on the application. For example, for placing a screw in a tapped hole, the important diameter is the major diameter (the largest diameter around the shank including the threads). A chart with those measurements is given here: http://www.engineersedge.com/screw_threads_chart.htm along with explanations of the different measurements: https://en.wikipedia.org/wiki/Screw_thread#Diameters

Back in the last century when we were not sure of a screw's gauge, a reliable rule of thumb was to measure the diameter of the countersunk head in inches. Deduct from that measurement 1/16", and then count the number of 1/32" remaining. So a screw head with a diameter of 1/4" was a 6 gauge, 5/16" was an 8 gauge, 3/8" was a 10 gauge and so on.

McMaster-Carr also has a good description of a lot of information concerning the many kinds of threaded fasteners available. http://www.mcmaster.com/#about-machine-screws/=h8mmn6

• Good summary, but they omit PoziDrive and Hex-insert for the heads. Apr 26, 2012 at 6:56

# 6 refers to the size of the screw - diameter, threads, etc

http://www.wlfuller.com/html/wood_screw_chart.html

http://www.csgnetwork.com/screwinfo.html

The way I was shown was to measure across the screw head in1/16th's double it and take away 2. ie. if it is 5/16th's across double to 10 - 2 = 8