The nominal standard thickness of a credit card is 0.76 millimeters.
Most conventional bank cards use the ID-1 format defined by ISO/IEC 7810. That format specifies the familiar credit-card footprint—85.60 by 53.98 millimeters—and gives a nominal thickness of 0.76 millimeters. The figure is designed to let cards work reliably in ATMs, payment terminals, imprinters, and wallets.
The standard also allows manufacturing tolerance: the permitted thickness is approximately 0.68 to 0.84 millimeters. Thus, 0.76 mm is the canonical answer for trivia, while individual cards may measure slightly more or less depending on materials, printing, embossing, and embedded electronics.
A frequent mistake is treating 0.76 mm as an absolute thickness for every payment card. Metal cards, novelty cards, laminated credentials, and cards with unusual construction can differ. The number refers to the standard nominal plastic-card specification, not a universal physical law.