The uncia (plural: unciae) was a Roman unit of length, weight, and volume. It survived as the Byzantine liquid ounce (Greek: οὐγγία, oungía) and the origin of the English inch, ounce, and fluid ounce.

The Roman inch was equal to 1⁄12 of a Roman foot (pes), which was standardized under Agrippa to about 0.97 inches or 24.6 millimeters.^{[citation needed]}

The Roman ounce was 1⁄12 of a Roman pound.