Leonardo da Vinci sketched a mechanical calculator based on gear wheels.
Leonardo’s notebooks contain drawings of geared mechanisms that have been interpreted as an early calculating machine. The system used interacting wheels to represent and transfer numerical values, much as later mechanical calculators used toothed gears and carrying mechanisms. It was a design study rather than a documented commercial calculator.
The importance of the sketch lies in its mechanical logic. Rotating one wheel could transmit motion to another, allowing numerical operations to be represented physically. Leonardo frequently explored such systems while studying clocks, hoists, machines, and military equipment.
It is misleading to call Leonardo the inventor of the modern electronic calculator. His device was conceptual, and historians debate how fully the surviving drawings specify its operation. The safest description is an early mechanical calculating-machine design using gears, not a proven working computer.