1. A method of measuring a mean-to-target (MTT) based on pattern area measurements, the method comprising: providing a design pattern; measuring a plurality of design pattern measurements for calculating an area of the design pattern based on a shape of the design pattern; calculating a series of calculation measurements by continuously substituting a same variation into the design pattern measurements, and calculating a series of calculation areas corresponding respectively to the calculation measurements to generate a database including the calculation measurements and the calculation areas; forming an actual pattern using the design pattern; measuring an area of the actual pattern; selecting a calculation area corresponding to the area of the actual pattern from the database and selecting calculation measurements corresponding to the calculation area; and calculating a difference between the design pattern measurements and the calculation measurements and setting the difference as an MTT.