TB is a terabyte counted in decimal terms and TiB is a tebibyte counted in units of 1024, and the IEC defined the second suffix precisely so the two could be told apart. They describe different quantities.
Two prefixes, two arithmetics, one letter of difference in the way they are written.
What each one counts
TB, the terabyte, uses the decimal prefix in its ordinary sense: tera means ten to the twelfth, as it does in every other field that uses metric prefixes. Storage makers quote capacity this way, and so do the standards bodies that define the prefixes.
TiB, the tebibyte, uses the binary prefix: two to the fortieth. The IEC introduced the binary prefixes — kibi, mebi, gibi, tebi — because computing had been using the decimal names for binary quantities for decades and the two had become impossible to distinguish in writing.
The difference is small at the kilobyte and unmissable at the terabyte, because each step up multiplies it again.
Why the confusion has survived so long
Because the binary prefixes were adopted unevenly. Some operating systems display binary quantities with the binary suffix, which is unambiguous. Some display binary quantities with the decimal suffix, which is where nearly all of the argument comes from. And network rates ignore the whole question by being counted in decimal bits per second.
So the same file can be described three ways by three tools on one machine, all of them internally consistent.
Where each convention turns up
Decimal, with the decimal suffix: drive capacity as the maker publishes it, memory card capacity, and every link rate anywhere in networking.
Binary, with the binary suffix: file managers and storage tools that have adopted the IEC prefixes, and much of the documentation written since they did.
Binary, with the decimal suffix: a large amount of installed software, and the reason people believe capacity has gone missing.
Memory is a case of its own worth knowing: main memory is manufactured and sold in binary quantities, so a module described in GB really is a binary quantity, and there the loose usage happens to be harmless.
How to keep two sources comparable
Quote each figure in the unit its source used, and put the suffix beside it every time. A page that reports a maker’s capacity in the maker’s unit and a machine’s reading in the machine’s unit is telling the truth twice; a page that converts one into the other has published a figure nobody measured.
That is the rule this site follows for capacity, and it is the same discipline that applies to a class figure in wireless: report what was published, name what it counts, and do not perform arithmetic on somebody else’s specification and present the result as theirs.
The questions that come up before an order
Which suffix should I use when writing about capacity?
The one the source used. If a drive maker publishes a figure in TB, quote it in TB; if a machine reports in TiB, say TiB. Converting between them to make two sources agree produces a figure neither source published, which is the thing worth avoiding.
Do the same suffixes exist for smaller units?
Yes, throughout the range: kibi, mebi, gibi and tebi correspond to kilo, mega, giga and tera, written KiB, MiB, GiB and TiB. The difference between the two conventions grows with each step up, which is why it is barely noticed at the small end and obvious at the large one.
Last reviewed 10 September 2026