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 Mac, iPhone, iPad, and iPod, is used by its formatting. Other factors, such as the operating system installed on the device, use additional storage space.
iOS uses the binary system
The operating system of your iPhone, iPad, or iPod reports storage capacity using the binary system (base 2), which calculates 1GB as 1,073,741,824 bytes. For example, the About screen in iOS shows that a 32GB (32 billion bytes) iPhone has about 28GB (28 billion bytes) of storage. This difference between the decimal and binary systems of measurement is why the storage capacity reported by iOS differs from the storage capacity on the product packaging or specifications.
Nothing is missing: the storage capacity is the same no matter which system (decimal or binary) is used.
macOS uses the decimal system
The operating system of your 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.
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: