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