Question

What percentage of Europe did the Nazis invade during WWII/The Holocaust?

Answer 1

About 35%-40%

Explanation:

The Deutsches Reich at it's zenith in 1941-1942 would've had I averaged about 35% to 40% of Europe.

Of course, you only say "Nazi occupied", thus meaning we count out all of Hitler's allies. In the map, you can see the areas in black and lighter black to be either a) German controlled (proper Germany), or b) Under German occupation (Northern France, Norway, Benelux, Ukraine, The Baltic, a bit of mainland Russia, Serbia, and Thessaloniki (which is in Greece.)) Also some of the Aegean islets.