Our previous study revealed that a dominant factor influencing the electromagnetic absorption in a human body for far field exposure at GHz bands is the body surface area. Based on this finding, we discussed the correlation between absorption cross section and body surface area of human for far-field exposure at 2 GHz. We employed an FDTD algorithm for a large-scale SAR (specific absorption rate) calculation with anatomically based Japanese adult and child models As a result, we found that good correlation is observed between body surface area and absorption cross section for realistic human body. This relation was also found to be valid for homogeneous human models and cuboids with the same height, weight and body surface area as those of the realistic human models. Based on the finding, we derived a formula to predict the whole-body average SAR from height and weight of a human body with a given incident power density of electromagnetic waves. The uncertainty of whole-body average SARs at 2 GHz was discussed based on this formula.