Purpose: During microscopic discectomy, knowing how much disc remains can be difficult if the ruptured disc has migrated upward or downward because of a limited microscopic field resulting from minimal laminotomy. The purpose of this study is to formulate and validate a primary linear equation that can predict the amount of herniated disc to be removed prior to surgery. Methods: Sixty patients (35 males and 25 females) who had radiculopathy by extruded, ruptured, or migrated disc from May 2010 to December 2012.Each patient underwent microscopic hemilaminotomy and discectomy on lumbar levels. Volumes of herniated discs were ascertained using MRI and a picture analysis communication system. The surgically removed part of the herniated disc was weighed using an electronic scale in the operation theater. A distribution chart was constructed using measured weights (Y axis) and volumes (X axis). Using a regression analysis model, the primary linear equation was calculated. A prospective study was conducted on 30 patients (22 males and eight females) to statistically validate the equation using the Wilcoxon matched pairs sign rank test. Results: The primary linear equation was Y=1.220X, where Y=predictable weight of removed disc and X=measured volume of herniated disc on MRI. The predictable weight of removed disc closely correlation with the measured volume of herniated disc on MRI (R2=0.818). Validation of this equation was excellent (Wilcoxon matched pairs signed rank test by Null hypothesis). Conclusions: The derived primary linear equation can predict the amount of disc to be removed prior to surgery