bits per inch (bpi)
Bits per inch (bpi) is an international unit for specifying the storage density of magnetic storage media.
The bpi value is mainly used for magnetic tapes and in tape drives and indicates the amount of data that can be stored on one inch of a magnetic track. It is also referred to as track density. As an alternative to the bpi value, the value Characters per Inch( cpi) is also frequently specified.
The storage density for magnetic tapes has values between 800 bpi and 6,250 bpi, for magnetic cards it is 75 bpi and 210 bpi.