You Toolmongers have been holding out on me. How come no one told me — or, perhaps worse, why didn’t I discover before — that there are simple formulas for determining the nominal diameters and clearance holes of (Unified Thread Standard) machine screw sizes 0–14*? All those years I spent looking up that little table, or trying to find that plastic gauge thingy…
Specifically, the nominal diameter in inches for a size N machine screw is: .060″ + (N × .013″), and the clearance hole is: (N +4)/64. For example, a #6 machine screw has a nominal diameter of .060″ + 6 × .013″ = .138″, and its clearance hole is 10/64 = .156″
If you work with really small machine screws like #00 and #000, the formulas also work if you set N to -1 and -2 for screw sizes 00 and 000, respectively.
What other clever, or quick, or back-of-the-envelope calculations should I know about?
*and, yes, I know machine screw sizes 12 & 14 are rare; the next “standard” size above #10 is usually ¼-20.