EARLY detection of a minor tumor size change is difficult using traditional visual and clinical examination measurements. Although they are time-consuming, the general manual methods are the standard approach for quantitative measurement of human tumor and used in several hospitals. Magnetic Resonance Imaging (MRI) techniques are used to measure the human tumor volume manually or automatically. Three mathematical manual methods are used to calculate different tumor sizes. The study group consists of twenty-two patients with different tumor types (meningioma, glioblastoma, abscess, metastases, and glioma). All  patients were scanned and all the MRI data acquired at 1.5 Tesla with diagnosis histological confirmation.The results of this study show that the triangle mathematical method (method -3) is more accurate than the other compared manual methods. The correlation shows a linear relationship between the mean values of method-1 and method-2 has R2 = 0.7975 and the P-value = 0.026 indicates the significance difference between  method-1  and method-2. Moreover, the correlation between method-1 and method-3 has R2 = 0.9367 and the P-value = 0.003 that indicates high significancebetween them. The comparative study of the three manual methods demonstrated that triangle manual method (method-3) has significant approach to be used in medical diagnosis. Additionally, method-3 can indicate which part of the tumor tissue is totally damaged depending on the image parameters.Therefore, the triangle manual method can be considered as an accurate step in the diagnostic process.