By using a few simple measuring tools and referencing the metric screw dimension charts, you can size a metric screw according to its diameter, length, thread pitch, head diameter, head height, drive width, drive depth and length. Begin by measuring the primary dimensions, diameter and thread pitch. Then size the remaining secondary dimensions. Finally, check the metric screw charts to verify your results.
Hold your metric screw so that you can apply a caliper to the bottom of it. Measure in millimetres the bottom of the screw with your calipers. The result is the screw diameter.You can use a bolt gauge as well to measure the diameter. Place the screw in the marked hole in the gauge that appears closest to the screw diameter. If the screw fits in the hole you have found its diameter. If not check the next size smaller or larger size hole until the screw fits.
Pinch the head of your screw and hold it horizontally. Use your ruler or caliper and measure from under the head of the screw to its end. The result in millimetres is the screw length. Flat head, oval head and flat undercut machine screws are exceptions. Their length is measured from the top of the head to the end of the screw.
Find the thread pitch of your metric screw. Metric screw threads are identified by thread pitch as opposed to US screws which are identified by threads per inch. Thread pitch is known by the size of a screw's threads relative to it's diameter. All metric screws are made with a large or course thread pitch or a smaller fine thread pitch. Locate the metric thread pitch indicator section on your bolt gauge. Apply your screw to the numbered pitch section that seems to match. You may have to try this a couple of times until the threads fit in the correct thread pitch section.
Use your caliper and size the head dimensions of your screw. The head dimensions of a metric screw will vary depending on what type of screw you want to size. The primary head dimensions are the head height and width. Measure the height from the bottom of the head to the top, and measure the width across the top of the head. Metric screw heads vary in shape including hex, flat, round, truss and oval, to name a few. Some metric screws also have a have a Phillips, slotted, hex or torx drive machined into the centre of the head used for tightening. You can measure the length, width, and depth of the drive with your caliper and ruler.
Navigate to a fastener distributor or manufacturer's website such as the resource website below. There you will find size charts for all metric screws. If you know a screw's diameter and thread pitch, you will be able to fill in the remaining dimensions from the charts. Or, use the charts to verify the dimensions you recorded with your caliper, ruler or bolt gauge.
Diameter, thread pitch, and length are the primary metric screw dimensions. Obtain a bolt gauge at most hardware stores and fastener distributor companies.