It is very easy to measure square feet for any rectangular area.
Let's say you want to tile two walls. One is behind the bathtub. It is 6 feet long, and goes from the top of the bath to the ceiling, 5 feet high. We multiply 6 ft x 5 ft = (6x5) ft x ft, or 30 square feet. (Yes, it works to multiply units of measure a foot times a foot is a square foot.
Let's say another wall is 10 feet long and 8 feet high. That is 10 x 8 = 80 square feet.
What if a wall is not rectangular? Say part is from floor to ceiling, and part is from bathtub to ceiling. Then break it up into rectangles. Measure each rectangle, and multiply to calculate the square feet of each section of the wall. Add up the size of each section (in square feet) and you get the total size of the wall.
Take a deep breath, we're almost there!
If a wall has a fraction of a foot, the multiplication still works. Say you have a short wall, maybe on one side of a door, that is 2 feet 6 inches wide and 8 feet high. 6 inches is 1/2 of a foot. So the wall is 2 1/2 feet x 8 feet, or 20 square feet.
Working this way, you can measure each wall, and add it all up.
Be sure to buy perhaps 3 extra square feet of tile. Good if you make mistakes, or if something breaks later, say, when you're putting in a towel rack.