<p>This study evaluated different models and proposed a reliable and accurate model using non-destructive measurements of leaf length (L) and/or width (W) for estimating the leaf area of <em>Tithonia diversifolia</em> (TD). Rapid, reliable and objective estimation of the leaf area is essential for numerous studies in agronomy and plant physiology, however, it is done usually by the availability of sophisticated and expensive electronic integrate methods. Allometric equations were developed by measuring W, L and LA of 92 leaves of TD, by a linear regression analysis. The current leaf area was measured using <em>ImageJ</em> software. With the observed and estimated information, a Pearson correlation analysis was performed. Pearson correlation coefficients oscillated between 0.93-0.94 for the best mathematical models. Equations which used as variable the product L*W presented strong relationships with the observed leaf area, manifested in high determination coefficients (R<sup>2</sup> = 0.89). Therefore, the use of the product of L*W as the independent variable was found to be accurate to predict the TD leaf area. In conclusion, a lineal model was developed to predict the leaf area for the <em>T. diversifolia</em> (y=0.755+0.438(W*L).</p>