A new method was developed to quantify corneal irregular astigmatism using computed corneal topography. Refractive powers on a mire ring projected on a toric plane (regular cornea) can be approximated to follow a sine curve. The discrepancy between the approximated sine curve and actual refractive powers was calculated and employed as a parameter of corneal irregular astigmatism. Artificial precision models of a sphere and a toric plane showed similar parameters and the least amount of irregular astigmatism (0.02-0.05D). Consecutive measurements on normal human corneas displayed a reproducible irregular astigmatism range of 0.10-0.25D. Irregular astigatism of the eyes with pterygium was significantly greater than that of normal human controls even from early stages of the disease. Surgery for removal of the pterygium normalized the amount of regular astigmatism regardless of the size of the pterygium, but irregular astigmatism of the eyes with large pterygium (the apex reaching within the central 2 mm cornea) remained at a significantly higher than normal level. It was concluded that the current method enables a quantitative analysis of corneal irregular astigmatism, independent by the amount of regular antigmatism.