One prefix is decimal and the other is binary
One megabyte (MB) is 1,000,000 bytes. One mebibyte (MiB) is 1,048,576 bytes. The numbers are close at small sizes, so people often use the labels casually, but the difference grows with large drives and downloads.
The same pattern applies to GB versus GiB and TB versus TiB. Decimal units use powers of 1,000; binary units use powers of 1,024 and have the IEC names kibibyte, mebibyte, and gibibyte.
Why a drive appears smaller after you connect it
Manufacturers commonly advertise storage in decimal gigabytes or terabytes. Some operating systems report capacity using binary calculations while displaying a familiar decimal-looking label. The underlying byte count has not vanished.
Formatting and system partitions can use some capacity too, but the large headline difference is often the unit conversion. Compare bytes with bytes before assuming the hardware is faulty.
Write the unit you actually mean
Use MB and GB when following network speeds, product specifications, and decimal storage conventions. Use MiB and GiB when a binary value matters in technical documentation, memory planning, or system tools.
For ordinary users, include the byte value or explain the convention once. Precision is useful; turning every file-size conversation into a terminology argument is not.
Practical checklist
- Check whether the source uses decimal or binary units.
- Compare the underlying byte count when possible.
- Use MiB or GiB when binary precision matters.
- Do not confuse megabytes with megabits.
- Document the unit convention in technical limits.