A method of forming a photomask using a calibration pattern that may exactly transfer a desired pattern to a substrate. The method includes providing one-dimensional calibration design patterns each having first design measures and providing two-dimensional calibration design patterns each having second design measures; obtaining one-dimensional calibration measured patterns using the one-dimensional calibration design patterns and obtaining two-dimensional calibration measured patterns using the two-dimensional calibration design patterns; obtaining first measured measures of the one-dimensional calibration measured patterns and obtaining second measured measures of the two-dimensional calibration measured patterns; establishing a correlation between the first measured measures and the second measured measures; and converting a main measured measure of a main pattern into a corresponding one of the first measured measures using the correlation.