Hah! Funny story...i was on my first flight in UK Airspace on VATSIM and EGLL_N_GND was like "Excellence 891, readback correct, QNH 1013, departing runway 09R, advise push and start". I had to call him back and ask what "Que and EX" was because that's what it sounded like. He said "just press B on your flight sim, its the same as what the american's call altimeter" i thanked him, and sure enough, on my flight sim guage, it said "MB 101302 ALTIM 29.92" Some gauges show millibars, some dont, but it's the same ting, just different format. US uses Inches of Mercury and Europe usues Millibars. I like QNH better because it sounds cooler. I'm a ATC at Los Angeles and i just wanna convert my altimeter so badly sometimes, especially for the French pilots who fly back and forth between Paris, they always readback wrong. "Uh...roger sir, QNH 2997" "Air France 28 Heavy, negative sir, Altimeter 29.97" "Oh okay, altimeter, rgr thanks for AFR28"
So, he only difference between the two is format, just press b and you're set.