Calculate the area of your floor and add about 10 - 15%. This is due to material fitting and cutoffs. It is also good to have an extra package of flooring stored at home, should you in the future need to replace a plank in your floor at some point in the future due to damage.
Example of how to calculate the floor area of a room: If your room is 5 meters long and 4 meters wide, then calculate 5m2 x 4m2 = 20m2. Add 15% extra material, 20m2 x 1.15 = 23m2. You therefore need to order 23m2 of wooden flooring to cover your room of 20m2.