Javal's rule and Grosvenor's simplification of it are commonly used formulas for predicting spectacle astigmatism from keratometric measurements. We assessed the accuracy of these two rules. Spectacle astigmatism was estimated using both rules from measurements of corneal astigmatism on 100 eyes of 100 subjects. These estimates were then compared to the subjectively determined spectacle astigmatism. Grosvenor's simplification of Javal's rule gave slightly more accurate assessments than the original rule. However, only 66% of results gave estimates within 0.50 D, and 7% differed by more than 1.00 D. This can be compared to previous reports on the accuracy of autorefractors, where approximately 95% of cylinder results were within 0.50 D of the spectacle astigmatism. These results indicate that using Javal's rule or Grosvenor's simplification of it to determine spectacle astigmatism from corneal cylinder readings is of limited clinical value.