We propose a method that utilizes the time dependence of P‐wave displacement amplitudes to estimate the final magnitude () for earthquake early warning (EEW) before the arrival of the peak amplitude. A relation between and P‐wave displacement amplitude is employed for the method. Its value is set as a function of time from the P arrival, and is determined using a K‐NET dataset recorded in Japan from a scaling relation between and the time dependence of P‐wave displacement. A test to check the performance of the proposed equation demonstrates in a statistical sense that this technique enables us to estimate more rapidly than conventional methods without loss of accuracy. We conclude that the approach proposed in this article effectively gains a longer lead time as well as reduces the blind zone for EEW.