usps calculate time to delivery USPS provides various tools to help customers calculate delivery times for their packages. The USPS Time in Transit tool allows users to estimate the delivery window based on the package’s size, weight, and destination. By entering the origin and destination ZIP codes, customers can receive an estimated delivery date for services like Priority Mail, First-Class Mail, and Media Mail. The time it takes for a package to reach its destination depends on the shipping method selected and the distance between the sender and recipient.