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Hi @prvz27,
The gauge of a screw is the thickness of the shank. The shank is the un-threaded section of the screw. The higher the gauge the thicker the screw. A thicker gauge indicates better shearing resistance capabilities in a high torque situation. Commonly, in terms of number, the gauge will come first then the length of the screw will follow.
Let's take a look at Buildex 8-10 x 40mm Climacoat Countersunk Ribbed Head Treated Pine Screws. The numbers are 8-10 x 40mm, so obviously the 40mm is the length. But why are there two numbers for the gauge 8-10? It's because those numbers indicate the minimum cut away from threads 8 and the actually shank gauge 10. Not only does the 8-10 indicate the thickness of the shank it also gives you an indication of the coarseness of the thread.
Bolts work a bit differently and generally have their thickness followed by their length. You'll notice both metric (millimetre) and imperial (inch) versions available. Whether you use a metric or imperial bolt will likely only be relevant if you are tightening them into pre-existing hardware or matching them to others. It is irrelevant if you are just bolting two pieces of timber together. If you ever run into a bolt description similar to M3 x 1.25 x 30, they are indicating it is a Metric 3mm shank x 1.25mm between threads x 30mm long.
Finally, there are different thread pitches (the angle of the thread). You might have run into this issue when you have a nut and bolt that look the same sizes but then cross-thread when you go to tighten them up. Those thread types are the inch-based Unified coarse/fine (UNC/UNF) and metric coarse/fine.
In regards to what gauge and thickness to use in which situation, think of it as the more force on the joint that the screw is holding the larger length/gauge you should use. Obviously, this has exceptions and at times two smaller screws are better than one longer screw. Bolts can be used over screws when there is not great joint strength and you wish to clamp it together. A bolt squeezes two structures together and provides superior holding strength over screws.
I hope this helps and hasn't confused you even more.
Please feel free to let me know if you need further information or had questions.
Mitchell
