Measure the length of the room in feet, using a measuring tape. Write the number down so you do not forget it.
Measure the width of the room. Multiply this number by the number you arrived at for the room's length. This will be the square footage of the room. For example, if the length is 20 feet and the width is 15 feet, the area of the room -- and its ceiling -- is 300 square feet.
Check the labels on paint cans. They will indicate how many square feet they will cover. Note that this is only one paint coat and you will probably need more than one coat. If your room is 300 square feet and the paint label states it covers 200 to 400 feet, purchase two cans. If you plan to paint three coats, purchase three cans.