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Home ImprovementUnderstanding US Screw Sizes
[+83] [7] Calvin Allen
[2012-04-23 15:58:06]
[ screws ]
[ https://diy.stackexchange.com/questions/13824/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?

[+78] [2012-04-23 16:11:34] KeithS [ACCEPTED]

"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).


(3) 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". - Tester101
(1) Also, I usually see screw gauges for sizes less than 1/4". Above that I see fractions. - Jay Bazuzi
I agree with everything except the "normal" thread count for a wood screw most are much coarser or less threads per inch. - Ed Beal
But what does #6 actually mean? At hardware shops in the US, legacy unit screw sizes seem to go down to about ⅛" diameter (via lots of complicated fractions), but smaller ones have a hash and a numeral, e.g. #6. Is the 6 shorthand for another fraction, or is it part of yet another confusing and loosely related system? - Michael Scheper
(2) @MichaelScheper #6 is called the thread gauge. It's an arbitrary system, which has no helpful meaning. Just know that bigger gauge means a thicker screw. Gauges go from #0 to #12 in screws, says en.wikipedia.org/wiki/Unified_Thread_Standard The american wire system unhelpfully goes the opposite way (16 gauge wire is thinner than 12 gauge wire), as do ear piercings. Wire gauges at least mean something--it's the number of times that wire fits in 0.5 inches. So 16 gauge wire is 1/16 x 1/2 inch, or 1/32 inch in diameter. - Zachary Vance
Good but the screw gauge number no longer has anything to do with head size. It is only a measure of the threaded shaft. - Bob Stein
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[+21] [2015-01-27 23:09:44] handyman

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 [1] and the original table here [2].

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

[1] https://i.sstatic.net/hU5Sx.jpg
[2] https://handycrowd.com/screws-explained-gauge-size-pilot-holes-and-wall-plugs/

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. - Steven
hi, great info! can you re-add the link? :) - johny why
(1) 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
2 questions — can you extend the table up to 10mm metric? And, does the metric equiv refer to the major diameter or the shank? - detly
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)? - handyman
p.s. here the outside of the thread diameter is measured when talking about metric screws... - handyman
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[+5] [2013-05-14 19:27:36] Peter D

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


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[+4] [2014-03-18 01:06:17] John Davidson

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.


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[+2] [2012-04-24 04:30:15] whatsisname

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. - staticsan
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[+1] [2012-04-23 16:08:01] Jason

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


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[0] [2014-03-18 22:37:32] John Bignell

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


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