Screws and bolts are measure by the outside of the threading. Think: to make a hole for a 1/2 inch bolt, you just need to use a 1/2 inch drill bit. (Nuts, then, are harder to measure.)
https://www.boltdepot.com/fastener-information/Measuring/Measure-Diameter.aspx
tags: screws, nuts, bolts, tools, build