Javal's rule

{{technical|date=September 2015}}

Javal's rule is a mathematical formula used to estimate astigmatism based on keratometry readings.Schwartz, S. Geometric and Visual Optics. pg 219-221. McGraw-Hill. 2002. The estimate is useful for high cylinder measures, generally over -2.00 diopters.

The estimate is found by multiplying the difference in power between the two meridians by 1.25 and factoring in the average lenticular astigmatism, which is -0.50 x 090.