There’s marketing specification (“8GB” on the box) and technical specification (“7.4GB” in the OS).
As @dynamicduo explains, this is not unique to Apple and the iPhone… but applies almost universally in all realms of capacities for hard drives, flash drives, whatever.
And it’s the stupidest, most maddening thing… I still don’t understand how it’s legal.
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Marketing relies on the base 10 with ancient greek prefixes (giga = billion)
where 8 GB = 8×10^9 = 8,000,000,000 bytes
.
Computers and their operating systems rely on base 2 numbers and the same prefixes
where 8 GB = 8×2^30 = 8,589,934,592 bytes
.
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The disconnect is that marketing says you have 8 billion bytes (base 10). But when your OS – on the iPhone in this case, but true everywhere – looks at that 8 billion and converts that to base 2 metrics, it sees it as 7.4 GB.
Like this: 8,000,000,000 bytes รท 2^30 = 7.4505 GB