Simultaneous switching noise can significantly impact the performance of high-speed systems and needs to be accurately analyzed. The simulation program with integrated circuit emphasis (SPICE) transient simulation with transistor circuits, packages, and boards is the most accurate method but may require a huge computer resource and a long simulation time. Hence, an estimation method of the simultaneous switching noise from the frequency-domain impedance of power distribution networks was presented in this paper. The estimation method can quickly but still accurately calculate the simultaneous switching noise. The estimation method is validated by SPICE transient simulations with transistor models for both the terminated and unterminated systems. In high-speed systems, the difference between the estimation and the worst noise of the transient simulations is less than 20%.