Ri (理) is a unit of length equal to approximately 3.9 kilometers. As a measure of length, it is often used to measure distances, for example between cities, and is the standard measure for this purpose, similar to the English mile or the German kilometer. Today, the metric system is generally used in Japan, but in novels, the ri is still frequently used to indicate distance.
It is part of the Japanese Shakkanhô system, which was used to measure lengths, areas, volumes, weights and money.