Leaf area estimation is an important biometrical observation one has to do for comparing plant growth in field and greenhouse experiments. Determining the individual leaf area (LA) of watermelon (Citrillus lanatus Thunb.) involves measurements of leaf parameters such as length (L) and width (W), or some combinations of these parameters. Two-year investigation was carried out during 2006 (on six cultivars) and 2007 (on one grafted cultivar) under open-field conditions, respectively, to test whether a model could be developed to estimate leaf area of watermelon across cultivars. Regression analysis of LA vs. L and W revealed several models that could be used for estimating the area of individual watermelon leaves. A linear model having LW as the independent variable provided the most accurate estimate (highest r2, smallest MSE, and the smallest PRESS) of LA in watermelon. Validation of the model having LW of leaves measured in the 2007 experiment coming from other grafted cultivar of watermelon showed that the correlation between calculated and measured watermelon leaf areas was very high. Therefore, this model can estimate accurately and in large quantities the leaf area of watermelon plants in many experimental comparisons without the use of any expensive instruments.