To find the surface area of any space, multiply the length of that space by its width. To find the area of a room for carpeting, you need a measuring tape and a calculator.
Create a diagram for reference
Draw a diagram of the rooms you plan to carpet. The diagram does not need to be to scale. Label the rooms.
Measure each room
Use the measuring tape to measure the length and width of each room in your diagram. If the room is not a rectangle, divide it into rectangles, and take the measurement of each rectangle. Write down the measurements on your diagram using arrows to show the distance and direction covered by the measurement. Keep your units consistent. If you are measuring in feet, convert the measurement to inches so you can more easily multiply the values.
Multiply length by width
For each room or rectangular section within a room, multiply the length by the width to get area of that room in square feet. Add up all the square footage totals.
Calculate the carpet needed
Use the sum of the areas you found as a starting estimate for the area of carpet you will need. Devise a plan for eliminating as many seams as possible with the width of the carpet roll in mind. When estimating how much carpet to purchase, always overestimate so that no section of carpet falls short.