Scaling relations between the predominant period of P-wave arrivals and earthquake magnitude are explored using datasets from the Pacific Northwest and Japan, and compared with previous observations in southern California (Allen and Kanamori, 2003). We find the same scaling for events in all three geologically diverse regions. The sensitivity of the predominant period observation to magnitude can be optimized using various frequency bands for different magnitude ranges and in different regions. The ability to estimate the magnitude using the first few seconds of the P wave offers a methodology for earthquake early warning. The accuracy of magnitude estimates increases with the number of stations reporting predominant period observations. The most significant improvements in the magnitude estimate occur when the number of reporting stations increased from one to four. As in southern California, we find that the average absolute magnitude error is ∼0.5 magnitude units when the closest four stations to the epicenter are used.