A gauge number for screws in the imperial system corresponds to a certain fraction of an inch in diameter. To figure out the gauge number for a certain diameter, or vice versa, you have to look at a gauge guide to match up the "#" of the gauge with a fraction of an inch. You can find these guides online. https://medium.com/jaycon-systems/screws-how-theyre-classified-and-how-to-know-what-kind-to-use-898f729b33c7 Measure the width of a thread in fractions of an inch to get the diameter. Use a ruler or measuring tape to measure from one side of a thread to the other using the nearest fraction of an inch. This diameter for screws in the imperial system is represented by either a gauge number or in fractions of an inch. [5] X Research source Take into account the type of screw head when you are measuring, because different screw heads rest differently on surfaces. To get the length of round-headed screws that aren’t countersunk, start measuring from the flat underside of the screw head. [3] X Expert Source Gino Colucci

For a countersunk screw with a rounded head, also called an oval countersunk, you start the measurement where the oval top and the countersunk half meet in the middle. In other words, where the oval top would rest on the surface. For example, a countersunk screw with a flat head will rest flush with whatever it is embedded into, so start the measurement at the top of the screw head. [2] X Expert Source Gino Colucci For instance, a flat-headed countersunk screw will rest flush with a surface. Measure from the top of the flat head to the tip of the screw to get the length.Measure the width of a thread in mms to get the diameter. Use a ruler or measuring tape to measure from one side of a thread to the other in mms. This is how the diameter is represented for screws in the metric system. [9] X Research source Measure in inches from the tip to where the screw head rests to get the length. Wherever the screw head would rest when it is fully embedded in something is where you start the measurement. Use a ruler or measuring tape to measure from here to the tip of the screw. [1] X Research source

Measure in mms from where the screw head sits to the tip to get the length. Start measuring from wherever the screw head would rest on the surface when it is fully screwed in. Use a measuring tape or ruler to measure from here to the tip of the screw. [8] X Research source Measure the distance from one thread to the next in mms to get the pitch. Screws use pitch as a measurement in the metric system instead of thread spacing. Use a measuring tape or ruler to measure the distance from one thread to the next in mms to get this final measurement. [10] X Research source Count the number of threads in 1 inch to get the thread spacing value. Lay the screw next to a ruler or measuring tape and hold it steady. Count the number of threads in the space of an inch to get the thread spacing for screws in the imperial system. [6] X Research source