Any 6-cylinder BMW owner who would like considerable time and money expended unnecessarily need not consider this basic suggestion -- check all engine belt sizes on your Bimmer, buy them, and store them in your trunk. Do not trust the word of dealer parts countermen, factory shop or parts manuals, or independent cross-reference parts books.
If your belts are dirty and you can't read them, clean off the grease with a small amount of kerosene or varsol on a rag. You'll probably have to crank the engine over a few times to find all the printing, but it'll be worth it. Why? Because there have been pulley diameter changes for increased cooling. Alternator belt sizes could be 12.5 x 1025, 12.5 x 1050 or 12.5 x 1060. For those with hydraulic power steering, you might need a 9.5 x 1125, 9.5 x 1075 or 9.5 x 900. More than likely, the air conditioning compressor belt is a 12.5 x 800; but don't trust me, go look at 'em all!
Again, don't let anyone talk you into a belt with a "Don't worry, it'll fit." When you find out it doesn't, it won't do you a bit of good. Accept a belt only if the correct size is imprinted on it.
For a maximum of about $10 for all three, you will have bought an inexpensive "insurance policy."