Hardwood flooring is sold by the square foot  in most cases it comes in boxes with the square footage of the contents displayed on the front. To figure out how many square feet you need, simply measure your floors. If the floors are irregularly shaped  as many floors are  divide them into measurable rectangles and add the areas of all the rectangles.
Measuring a Rectangular Room
You figure the area of a rectangle by multiplying its width by its length, so if you want to buy flooring for a single rectangular room, you would only need to measure the length and width of the room. Use a tape measure, and convert inches to decimals to make the calculation easier. For example, if the room is 20 feet 6 inches long, express that as 20.5 feet.
Sample Flooring Calculation
Suppose your living room is 25 feet, 7 inches long and 16 feet, 9 inches wide. Converting to decimal notation, that's 25 7/12 feet by 16 9/12 feet, or 25.58 feet by 16.75 feet. The area of the floor is 428.5 square feet. If you choose a product that comes in boxes that each contain 18 square feet  which is a good average  you need 24 boxes.
But Wait...
Most measurements are approximate, and you'll waste some of the flooring when installing it  that's inevitable. Consequently, most flooring pros add a 10 percent overage onto their materials estimate as insurance. Adding this overage ups the square footage to 471.5 and the number of boxes you need to 26.2. Round up partial boxes to the next whole number. In this case you need 27 boxes. Don't worry  you won't have much left over.
Multiple Rooms and Irregular Floors
The general strategy for handling multiple rooms, Lshaped rooms and other irregularities is to divide the area you need to measure into a series of rectangles, measure each rectangle, calculate its area and add the results to come up with a final square footage. This strategy usually works even when the walls are triangular or curved  as long as you can reduce most if the floor to rectangles, the 10 percent overage usually takes care of measurement inaccuracies.
WholeHouse Sample Calculation
A typical onestory house has a living room , kitchen, hallway and several adjoining rooms. If the house has a second story, it also has a stairway as well as upstairs rooms that must be considered. As an example, consider a house with a 15by20 foot living room, a 3by15foot hallway and two 10by15foot adjoining rooms. At the top of the 20step staircase, which has 1 1/2by3foot treads, is a 5by5foot landing with two adjoining bedrooms, one 10 by 10 and the other 10 by 15 square feet.
The areas to be covered with flooring  not including the stairs  are 300, 45, 150, 150, 25, 100 and 150 square feet. The total area is 920 square feet  1,012 square feet after adding the 10 percent overage. If each box of flooring contains 18 square feet, the project requires 57 boxes of flooring. Multiply the cost of each box by 57 to estimate the total cost of the flooring. Don't forget to add in the cost of the 20 stair treads, as well as risers and nosing.
Tip

Not good with math? Use a calculator available on one of many flooring supply sites or on the World Floor Covering Association site instead.