What Leonardo da Vinci instrument used gears to add numbers mechanically?

Leonardo da Vinci’s calculating instrument was a mechanical calculator.

Leonardo drew a geared calculating device in the Codex Madrid I, completed near the end of the 15th century. The mechanism appears designed to add numbers through the controlled movement of toothed wheels. A carry mechanism could transfer a completed unit from one place value to the next, an essential feature of mechanical arithmetic.

The device is often called Leonardo’s calculator or calculating machine. It was not an electronic computer and was not known to have been built or used in his lifetime. The surviving drawing is also not a complete operating manual, so modern reconstructions necessarily interpret some details.

Leonardo’s work belongs to a longer history of geared calculation. Earlier devices, including the ancient Antikythera mechanism, used gears for astronomical purposes, while later machines by Blaise Pascal and Gottfried Wilhelm Leibniz became documented calculating instruments. Leonardo’s design is significant because it shows Renaissance experimentation with automating numerical operations.