A few months ago, I was designing a small custom bracket on my 3D printer. I needed a screw, measured the hole, and ordered what I thought was a matching 1/2-inch bolt.
When it arrived, the bolt was just a hair too wide. My hole was 12 mm flat, but the 1/2-inch bolt was 12.7 mm. That half-millimeter gap ruined the entire afternoon print.
Remember this: one inch is exactly 25.4 millimeters. Our length converter displays four decimal places so you never miscalculate engineering tolerances again.