In the case of computed tomography (CT) scan, the measurement of effective dose is very essential. A patient size-dependent factor will be used to estimate patient dose from scanner output indices (e.g. Computed tomography dose index, CTDIvol) for patients of different sizes. The size dependent factor shall be used over a range of patient sizes, and extends to large patients as well as small patients.This research performed the estimation of effective dose during CT scan of brain of patients by using PMMA 32 cm reference phantom for treatment planning. The effective dose has been calculated for different patients after CT scan of head or brain. To estimate the effective dose different parameters like anterior posterior (AP), lateral (LAT), AP+LAT dimension, effective diameter, dose length product (DLP) and size specific dose estimate (SSDE) have been calculated. The calculated value of effective dose was in the range of (346-587) mSv. The relations of effective diameter with AP, LAT, AP+LAT dimension, SSDE and age of the patients have been analyzed.