Screw Diameters – How to Read Them Accurately

Knowing how to read screw dimensions accurately is a critical skill for DIY enthusiasts, mechanics, and professionals alike. Accurate measurements help ensure that the fastener matches the needs of the project by matching its head diameter, shaft diameter, and thread pitch (the distance between each screw crest/peak). A wrong-sized screw can damage or weaken the materials being joined, while a screw too big will not fit into the hole.

In imperial measurement, screw size is defined by the gauge number that corresponds with its major diameter: for example, a #8 screw has a diameter of 1/4″ and is marked as such. Below that, screws get a little trickier, as they have a smaller diameter and are listed with a series of numbers (starting with #12 and going up to #0000, which is super tiny).

When measuring the minor diameter, it is important to measure from the base of the thread, rather than the tip, so that you know that you’re getting a accurate reading. The minor diameter is essential because it determines how well the screw will engage the threaded portion of the material being screwed into, which in turn impacts the strength of the connection between components. This is why it is important to use precise tooling such as calipers, which can provide measurements down to the nearest thousandth of an inch or millimeter. To learn how to measure for these different measurements, check out our article on Screw Diameters.