Techniques to generate megabar (100 GPa) pressure are critical for the study of materials under extreme conditions. However, kJ-class lasers are typically needed to generate such extreme pressures. Here, we present a method to reach megabar pressure on a benchtop using only tens of mJ a few millijoules of laser pulse energy. The core of our method is the utilization of multiple laser rings of different diameters and time delays to additively excite a focusing shock wave. Using high-speed imaging and a streak camera, we measured the shock trajectory and inferred pressure, temperature and density using hydrocode simulation. Our shock compression efficiency, informed by hydrocode, has achieved 23 GPa/mJ. If scaled up, our method may enable high-energy-density experiments on a benchtop.