Every object you create is measured in drawing units. Before you start to draw, you must decide what one drawing unit will represent based on what you plan to draw. Then you create your drawing at actual size with that convention.
If you start a drawing in one system of measurement (imperial or metric) and then want to switch to the other system, use to scale the model geometry by the appropriate conversion factor to obtain correct distances and dimensions.
For example, to convert a drawing created in inches to centimeters, you scale the model geometry by a factor of 2.54. To convert from centimeters to inches, the scale factor is 1/2.54 or about 0.3937.