leaf area estimate may contribute to understand the relationship of interference between weeds and crops. the objective of this research was to obtain a mathematical equation to estimate sida cordifolia and sida rhombifolia leaf area based on linear measures of leaf blade. correlation studies were conducted between real leaf area (af) and dimensional leaf blade parameters such as leaf length (c) and maximum leaf width (l). around 200 leaf blades of each species were analyzed, collected from several agro-ecosystems at s？o paulo state university, in jaboticabal, sp, brazil. the statistical models evaluated were: linear y = a + bx; simple linear y = bx; geometric y = axb; and exponential y = abx. all the evaluated models can be used for s. cordifolia and s. rhombifolia leaf area estimation. for practical purposes, the simple linear regression model is suggested using the c*l multiplication product and taking the linear coefficient equal to zero. thus, an estimate of s. cordifolia leaf area can be obtained using the equation af = 0.7878*(c*l) with a determination coefficient of 0.9307 and s. rhombifolia leaf area by using the equation af = 0.6423*(c*l) with a determination coefficient of 0.9711.