Formation density is calculated from acoustic logging measurements. This technique does not require a radioactive source and in fact it may offer better precision particularly in rugose boreholes. In at least some embodiments, the technique exploits an observed relationship between a transmission coefficient T(pb, Vc, pmVm) and a breakdown resistance μB(pb, Vc, Vs) to enable determination of the formation density pb from measurements of the formation's compressional wave velocity Vc, the formation's shear wave velocity Vs, and the borehole fluid's acoustic impedance pmVm. The desired measurements can be acquired by attaching or integrating a fluid cell with an acoustic logging tool that measures acoustic wave propagation velocities. Real-time density logs can be obtained from both wireline and logging-while-drilling implementations.