The knot area ratio (KAR) is well known to show good correlation with the strength of wood boards. However up to now no system was able to compute it automatically in order to use it in an industrial application. This paper introduces a new method which allows matching the knots from the 4 faces of wood boards. This method is based on the detection of knots by an optical scanner. The knots matching computation is validated through a complete study and tested over 451 structural lumbers (Douglas-fir (Pseudotsuga menziesii) and Spruce (Picea excelsa)). The correlation between the module of rupture and the KAR is also investigated on these samples, as well as the impact on the settings of grading machine (EN 14081). The study shows that coupling a scanner with a standard MOE measurement can significantly improve the grading results.