Product specifications use the decimal system
Manufacturers of storage devices measure storage capacity using the decimal system (base 10). That's why the product packaging and online specifications include a statement such as this:
1GB = 1 billion bytes, and 1TB = 1 trillion bytes; actual formatted capacity less.
Some of the storage capacity of any storage device, including of your iPhone, iPad, iPod, and Mac, is used by its formatting. Other factors, such as the operating system installed on the device, use additional storage space.
iOS and macOS use the decimal system
The operating system of your iPhone, iPad, iPod touch, and Mac reports storage capacity using the decimal system (base 10), which calculates 1GB as 1 billion bytes. This is the same measurement system used on the product packaging and specifications.
iOS 10 and earlier, Mac OS X Leopard and earlier, Microsoft Windows, and watchOS use the binary system (base 2), which calculates 1GB as 1,073,741,824 bytes. This difference between the decimal and binary systems of measurement is why the reported storage capacity differs from the storage capacity on the product packaging or specifications.
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The Disk Utility app might also show slight differences in storage capacity, particularly for solid-state drives (SSDs) and flash storage, because of the additional space used by the EFI partition, restore partition, wear-leveling blocks, write-buffer area, metadata, spare blocks, grown bad blocks, and factory bad blocks.
For information about managing storage space on your devices, see: