This study aimed to design an accurate and practical system of leaf area determination using a smartphone. A software application for leaf area computation was developed using OpenCV (Open Source Computer Vision) library. OpenCV software was tested to estimate the accuracy of leaf area calculation. Leaf area calculations were undertaken using three different image resolutions to compare their accuracy. The results of the software calculations were then compared with the results of the laboratory leaf area meter to identify any errors. The results showed that higher image resolutions improved accuracy by reducing errors. Low image resolution data indicated an error range between approximately 9.4% to 15.4% , while higher image resolution gave an error range between 1.2% to 9.1%. It was concluded that the OpenCV algorithm gave fast and adequate accuracy for leaf area calculation, and that the the smartphone mobile application system was practical for field use.