Science and Security of the Four‑Digit ATM PIN

A four‑digit PIN provides 10,000 possible permutations, making a random guess a one‑in‑10,000 chance. Combined with the common banking rule that disables a card after three failed attempts, the risk of unauthorized access is minimal. Cognitive psychologist George A. Miller identified that short‑term memory typically holds five to nine discrete items, meaning a four‑digit number fits comfortably within this limit. This cognitive fit explains why users can remember their PIN without writing it down, enhancing both convenience and security.