Measure the length of one of the walls that you are going to tile. By way of example, assume this measures 7 feet.
Measure the width of the same wall that you previously measured. By way of example, assume this side measures 4 feet.
Multiply the two sides together to calculate square feet. In the example, 7 times 4 equals 28 feet squared.
Repeat for any other walls or floor that you want to replace.