What makes your 1TB SSD show up as 910 GB

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If you wonder why every time you buy a hard drive of 10TB, which is reported as 9.1TB or a 250GB SSD is reported as 232GB in your OS you need to understand the difference between TB and TiB.


A Brief Primer to SI and Binary Units

Humans understand and calculate in Decimal system , which means we use 10 Digits. It means that every Unit we have (defined by SI) is a unit of powers of 10. A Kilo is 10^3 ( 1,000) a Mega is 10^6 ( 1,000,000) and so on.

TLDR :
-- Manufacturers use SI Decimal unit (TB), your OS Reports IEC Binary Units (TiB).
-- A TB and GB is shorter than GiB and TiB so you see the disparity.
-- You see a difference of about 7% on GB and 9% on TB.

Now Computers in contrast use a Binary system - they use only 2 Digits. Their Units would have to be in the powers of 2. if you wonder why Numbers like 64 ,128, 256, 512, 1024 and 2048 keep appearing everywhere- Pen drive capacities, SSD capacities, RAM and Cache Etc this is the reason. They are all powers of 2.

So for a computer a Kilobyte would have to have 2^10= 1,024 bytes, but SI unit would have 10^3= 1,000 bytes. How would you distinuguish your Kilobyte of 1,024 from the Kilobyte of 1,000 ? This is where IEC stepped in and created alternate units .


Enter Binary IEC Units - Kibibyte(KiB) , mebibyte(MiB) and gibibyte(GiB)

So IEC introduced the Binary units, and hoped that everybody in Computers would start using these units instead of SI units and the confusion would end

1 KiB = 2^10 bytes
1 MiB = 2^20 bytes
1 GiB = 2^30 Bytes
1TiB = 2^40 Bytes

Now you would expect that all the Hard drives, tapes , memory cards etc would start reporting their device sizes in IEC units and the confusion would end . But this didn't happen.


Vendors still use TB as it makes their drives sound bigger !

The disparity in TB and TiB makes your hard drive 9% Bigger when you talk of its capacity in terms of TB instead of TiB. ( 9.1 TiB become 10 TB, 14.56 TiB becomes 16TB). Obviously somebody in marketing department would find it more more appealing to go with TB instead of TiB to show the sheer capacity this single marvel of Technology could hold. So Vendors still stick to TB and GB since this makes their drives sound bigger.

But your Operating system still keeps showing you TiB and GiB ( though Windows OS for exmaple Labels it as GB)

Screenshot below shows that windows reports 249620852736 bytes to be Equal to 232GB. It should be labelled as 232GiB. If it were GB, it would have to be 249 GB ( in line with what the manufacture sold us - a 250GB drive)

Windows Reporting Capacity in GiB , labelled as GB


Quick Conversion between TB to TiB and GB to GiB.

Its only logical to use the Binary Units when talking about computer storage, but till everyone does that, you have to covert between units. The difference is most prononuced in bigger capacities and we always are asked about TB and GB capacitiy disparity and use the folloiwng values to convert.

- 1TiB = 1.099 TB
- 1GiB = 1.073 TB

Using these you can see that how the popular capacities change in binary units

- 128 GB Flash Drive real capacity is 119 GiB
- 1TB SSD Drive real capacity is 910 TiB
- 4TB HDD real capacity is 3.64 TiB
- 8TB HDD real capacity is 7.27 TiB
- 10TB HDD real capacity is 9.10 TiB

Eventaully when you store data , it will be stored as binary and only binary units will be relevant.

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