The authors aimed to estimate the area of the leaf blade of different plants based on linear and dry weight measures, calibrated with the free software ImageJ. The leaves of cotton plants (Gossypium hirsutum L) var. IMACD 6001, cashew (Anacardium occidentale L) var. FAGA 10, soybean (Glycine max L) var. M-SOY 7639RR, and corn (Zea mays L) var. Penta TL were collected from the lower third, middle and top in different phenological phases. It was found that images with a resolution of at least 7.2 megapixels can be used to estimate the actual leaf blade area, processed with the ImageJ software till 324cm^sup 2^. They observed a good agreement in the estimation of the leaf blade area based on linear and dry mass measures and, therefore, it is recommended to estimate leaf area leaves with damaged pieces or measures from several samples of leaves collected at the field based on the dry weight of the leaves, as it is operationally viable.