Most suppliers and many home stores can calculate this for you. The have a chart some cases a program. The information they would need the size of your garage, the pitch of the roof, if and what kind of dormers you have, the type of roof (ie gabble, hip). They would also need the overhang.
Or using basic geometry you would figure the square foot area of each portion of the roof and the enter the total into a estimator like this.
Most of the time ball parking is ok. You can usually return extra (although it is heavy to handle and you want to avoid surplus) You also don't want to be stuck without enough. If the store has a lot of stock on the tile you want. You can go back and get more if you don't get enough. Don't just go by the package square footage as it may not take in account the starter row or hips and valleys.
I hope I helped without confusing the issue for you.