The AASHTO Pavement Design Guide provides one of the most commonly used methods of rigid pavement design. One of the drawbacks identified by researchers is, when k (modulus of subgrade reaction) is unavailable and has to be estimated from E (modulus of elasticity), the k-E relationship used in the design gives too high k. This could result in a thin slab thickness in pavement design. This research evaluates five k-E relationships, together with the one used in the AASHTO design guide, to asses their relative performance in calculating slab thickness using the AASHTO thickness design equation. The results of this study concluded that the k-E relationship by AASHTO method is not recommended because of the large errors involved. Among the six k-E relationships evaluated, the equivalent k-E models based on k-E relationship outperformed the other k-E relationships in its ability to estimate k values that best approximate field measured values.