Terabyte (TB): Originally 1,099,511,627,776 (240) bytes, although communications and secondary storage systems developers started using the term to mean 1,000,000,000,000 (1012) bytes. To reduce confusion, we now use the term tebibyte (TiB) for 240 bytes, defining terabyte (TB) to mean 1012 bytes. The figure below shows the full range of decimal and binary values and names.
DecimalAbbreviationValueBinary termAbbreviationValue% Larger
kilobyteKB10001kibibyteKiB2102%
megabyteMB10002mebibyteMiB2205%
gigabyteGB10003gibibyteGiB2307%
terabyteTB10004tebibyteTiB24010%
petabytePB10005pebibytePiB25013%
exabyteEB10006exbibyteEiB26015%
zettabyteZB10007zebibyteZiB27018%
yottabyteYB10008yobibyteYiB28021%
ronnabyteRB10009robibyteRiB29024%

And,

1 byte = 8bits

For example:

What is the actual number that would show up on your computer when the hard drive manufacturer labeled it as 18TB?

18 * 1012 = 18,000,000,000,000 bytes

18,000,000,000,000 bytes / 240 = 16.37 TiB

So, the system would show 16.37 TiB actual size.