The aim of this research was to develop an equation to predict the compressive strength of concrete in terms of ultrasonic pulse velocity (UPV) and concrete mix design parameters. A wide range of concrete samples were prepared with varying water-cement ratio and density. The variation of density was achieved by using lightweight recycled coarse aggregate, and high density nickel slag aggregate as fine aggregate. The influences of water-cement ratio and density on the compressive strength were evaluated. In addition, the relationships between density and water-cement ratio with the UPV were also studied. A statistical analysis was then conducted, and a simplified equation was proposed to non-destructively predict the compressive strength of concrete in terms of UPV, water-cement ratio and density. Finally, the equation was validated by comparing with the experimental data collected from various published literature. The results show that the proposed equation could predict the compressive strengths of this study and those from previous literature with an overall coefficient of determination (R2) of 0.88.