How to Measure Screw Size

If you have some loose screws and want to know if they’re the right size for a project, or if you need to get some duplicate screws of the same size, you can find out how big they are with a few simple measurements. Before you start, make sure you know whether you need to take the measurements in imperial or metric units. To find the length of the screw, line it up with a ruler and measure from the tip of the screw to the head. If the screw has a rounded head, measure to the flat part, where the screw will lie flush with the surface that it’s going to be screwed into. Measure the length in inches if you’re using imperial units, and in millimeters if you need metric units. To get the diameter of a flat-tipped screw, put the end of the screw against your ruler or measuring tape. Measure the width across the tip in either fractions of an inch or in millimeters. If you’re measuring in fractions of an inch, you may need to use a screw gauge chart to convert the measurement into a standard gauge size. For example, a 1/16-inch screw has a gauge of #0. On the other hand, if you’re using the metric system, the diameter of the screw is usually listed in millimeters. This means that a 5 mm screw would be labeled as 5.0 on the packaging. The final common measurement for screws has to do with the number and spacing of the threads, or the thread pitch. In the imperial system, you’d measure this by calculating the number of threads per inch. Line the screw up next to a ruler or measuring tape and count how many threads you see in the space of an inch. The most common pitch numbers are between 35 and 40 threads per inch. If you’re doing a metric measurement, check the distance between two threads instead. For example, a typical screw with a 2 mm diameter will have a pitch of 0.4 mm between each thread.