As a prelude to buying a pair of shoes (a hateful task), I checked my existing collection for a size reminder. I run from 7 to 10. Helpful it is not. It's a conspiracy to save the high street I tell you.
Shame we aren't all on Paris Points (Size = [last length in cm] x 1.5)
Suspect your feet are about 28.5cm long, which is about a 9/(43) and American sizes are n+1 (unless they are for women, when they are n+1.5)
I wish there were real sizes that related to real measurements!