The unit described on this page was in use prior to modern methods of precision measurement. It was based on a standard which is no longer available, and which is not capable of being calibrated against modern measurement units. Therefore, the equivalence to modern SI units or to current United States customary units can only be considered approximate.
The libra (often translated as pound) was a pre-metric unit of mass or weight in Spain, and was carried to the Spanish colonies and used there.
The standard value of the libra was considered to be the libra of Avila or Castilla, at 460 g (1.01 lb); however, a number of other values were used in the various provinces of Spain: