1950s Antares P2 running without shell | Odhner mechanical pinwheel calculator clone | made in Italy

A pinwheel calculator is a class of mechanical calculator described as early as 1685, and popular in the 19th and 20th century, calculating via wheels whose number of teeth were adjustable. These wheels, also called pinwheels, could be set by using a side lever which could expose anywhere from 0 to 9 teeth, and therefore when coupled to a counter they could, at each rotation, add a number from 0 to 9 to the result. By linking these wheels with carry mechanisms a new kind of calculator engine was invented. Turn the wheels one way and one performs an addition, the other way a subtraction.

The teeth are moved by a second wheel located inside the first and moved by a side lever.

As part of a redesign of the arithmometer, they reduced by an order of magnitude the cost and the size of mechanical calculators on which one could easily do the four basic operations (add, subtract, multiply and divide).

Pinwheel calculators became extremely popular with the success of Thomas' Arithmometer (manufactured 1850s) and Odhner Arithmometer (manufactured 1890s).