A gage number like "6" is the diameter and the length is the length of the screw that goes into the wood not inclusive of the head unless it's a flat head. The smaller the gage number the smaller the screw diameter (opposite of nails and wire diameter).
Machine screws are often found in sizes of: 0, 1, 2, 3, 4, 5, 6, 8, 10, 12, 14—the larger the number, the larger the screw.
Gauge refers to the diameter of the drywall screw. In most cases, you will be using either #6 or #8 drywall screws. Keep in mind that as gauge numbers increase, the screw becomes larger in diameter. So, a #6 drywall screw would be thinner than a #8 drywall screw.
6 (3.5mm) x 25mm Slotted Countersunk Wood Screws (DIN 97) - Stainless Steel (A2)
M6 threads are 6mm metric screws. The standard metric rack screw is called M6 x 1. The 'M' means it's metric. The '6' is the outside diameter measured in millimeters, and the '1' is the distance between adjacent threads, also in millimeters.
Screw Size Number #6. Thread Diameter . 138" (3.46mm)
What Drill Bit Size Corresponds to a #6 Screw? For a #6 screw, the recommended pilot hole sizes are 5/64″ for softwoods and 3/32″ for hardwoods.
The first number is the diameter. The bigger the number the bigger the screw. The second number is the number of threads per inch. Check your state and local codes before starting any project. Follow all safety precautions.
No. 8 (4.2mm) Thread Size.
No. 10 (5mm) x 75mm Pozi Countersunk Wood Screws (DIN 7997Z) - A2 Stainless Steel: Accu.co.uk: Precision Screws.
Measuring Screw Length
For the vast majority of machine screws, the nominal length is measured from below the head to the tip of the screw. For countersunk fasteners, the length of the screw is generally measured from the top of the head to the tip of the screw.
The first number in a metric bolt specification refers to the bolt's nominal outer diameter. For example, a bolt specified as "M6-1.0 x 30" fits a 6.0-mm diameter hole.