Understanding US Screw Sizes

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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?

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"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).

2Good 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 2012-04-23T16:19:57.840

Fantastic explanation, thanks!Calvin Allen 2012-04-23T16:21:24.323

1Also, I usually see screw gauges for sizes less than 1/4". Above that I see fractions.Jay Bazuzi 2012-04-23T21:06:06.000

I agree with everything except the "normal" thread count for a wood screw most are much coarser or less threads per inch.Ed Beal 2017-10-23T19:30:19.437

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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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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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You might find this table useful as well...

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.Steven 2015-01-28T02:12:58.133

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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 2012-04-26T06:56:05.480

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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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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