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Hard drive Capacity - Fomatted Size vs. Advertised Size

A quick guide to cover the difference in the size you'll see when comparing the size of a hard drive in Windows to it's advertised size on the box.


Another question we get quite frequently is customers wondering why the new hard drive or USB stick that they have bought doesn’t give them the amount of space it was advertised as having.

If you buy a new 1TB drive it will format at around 930GB, so it seems like you’re losing a lot of space. You’re actually not, though. This is down to how a Gigabyte is counted differently in advertising maths compared to how a computer actually understands it.

A Gigabyte is commonly referred to as being 1,000 Megabytes, meaning a 1 Terabyte drive would be considered to be 1,000 Gigabytes, to a total of 1,000,000 Megabytes. This isn’t how a computer understands a Gigabyte however.

A computer calculates a Gigabyte as being 1,024 Megabytes (a Megabyte similarly is calculated as 1024 kilobytes and not 1000) meaning that when this is added up it comes to what sounds like a significantly smaller number, but the total number of bytes on the drive will be the same.

It’s similar in practice to measuring a distance in either centimetres or inches. The total distance will have a different number, but the actual distance will be the same it’s just that in this case both the different measurements confusingly have the same name. I’ve included a quick list of common drive sizes below, with the size you will actually see.


It’s worth noting that some manufacturers, such as Kingston, have stopped using this numbering style for their solid state drives (though not their USB sticks) so a 120GB Kingston SSD will format at 120GB, instead of being sold as a 128GB drive.