So as a little nod to everyone who likes to make stuff, here's what some research, checking needles from my sewing kit (so I got interested, so sue me) with calipers (because Prym and Simplicity both tend to round needle diameters to the nearest .10), and extrapolating led me to. This might be more useful for model-builders and miniature-makers than for seamst(ers)(resses) anyway, since at least it tells you what size hole a given needle will poke.
For the much too interested: Basically American needle sizes and European are arbitrary number sequences.
PLEASE NOTE: FOR HAND SEWING ONLY. MACHINE NEEDLES ARE COMPLETELY DIFFERENT.
No warranty implied! If you end up using the wrong needled to sew your zombie together and the first time he flips a guy off his hand flies backwards over his head, it's your problem (and his) not mine.