The knowledge of plant leaf area is important to evaluate the vegetal growth. The objective of this research was to determine a mathematical equation to estimate successfully alfalfa (Medicago sativa) leaf area by measuring linear dimensions of the leaf blade of the plant leaflets. The leaves were collected from cultivar ‘Crioula’ cropped at field conditions in Jaboticabal, SP,Brazil. The leaflets length, width and leaf blade area were determined electronically. Linear, geometric and exponential equations were tested, considering the length and width separately and the length × width relation. The lateral leaflets showed similar dimensions and they were different to the central leaflet. So, it was opted to analyze the lateral leaflets together. All equations described satisfactorily the alfalfa leaf area, but it was opted for the linear equation passing through the origin due to its better practical application, showing high significance and normal distribution of residuals. This equation was validated with coefficient of correlation of 0.96, showing high significance (P < 0.01). The alfalfa leaf area can be estimated by the equation LA = 0,691156 × LWct + 0,3652754 × LWlt, where LWct is the central leaflet length × width and LWlt is the sum of the left and right lateral leaflets length × width.