Your Short-Term Memory Holds Only 7 Items
George Miller found we can hold about 7 items in working memory — hence 7-digit phone numbers.
Psychologist George Miller's famous 1956 paper established that human working memory can hold roughly 7 items (plus or minus 2) at once. This is why phone numbers are typically 7 digits long. Modern research suggests the true limit may be closer to 4 "chunks" of information.