Bits and bytes: what do MB, MiB, GB and GiB mean?

A byte (B) contains eight bits (b). Internet connection rates are commonly shown in bits per second, whereas file and memory sizes are shown in bytes. The prefixes matter: decimal kB, MB and GB are not the same as binary KiB, MiB and GiB.

Decimal and binary prefixes

In the SI decimal system, 1 kB is 1,000 B, 1 MB is 1,000,000 B, and 1 GB is 1,000,000,000 B. Binary units use powers of 1,024: 1 KiB is 1,024 B, 1 MiB is 1,048,576 B, and 1 GiB is 1,073,741,824 B. TB and TiB, and PB and PiB, likewise differ. Older software sometimes used “KB” to mean 1,024 bytes, so an unexplained abbreviation can be ambiguous. The NIST table of binary prefixes provides the exact terms and values.

Why a drive appears smaller

A drive sold as 1 TB represents about 1,000,000,000,000 bytes. Expressed in binary units, the same count is roughly 931 GiB. Operating system displays may label units differently, and file systems or recovery partitions can reduce usable space further. This alone does not show that bytes are missing. Compare the byte count and the display unit before suspecting a defective device. Likewise, 500 GiB is about 7.4 percent more storage than 500 GB, although the labels look very similar.

Working through a transfer example

A 100 MB file contains 100,000,000 bytes in decimal notation, or about 95.4 MiB. On a 100 Mbit/s connection, its ideal transfer time is eight seconds: multiply the bytes by eight to get bits, then divide by 100 million bits each second. Real transfers usually take longer because of protocol overhead, latency, congestion, server limits and other traffic. A lower-case b denotes bits; an upper-case B denotes bytes. Confusing them creates an eightfold error even before considering decimal and binary prefixes.

How to report an exact size

When comparing a tariff, download progress bar or storage specification, state the original number of bytes if accuracy matters. Convert to the selected unit explicitly and say whether it is decimal or binary. Avoid adding values expressed in MB and MiB without conversion. If a specification says only “500 GB” but its arithmetic appears to use 1,024 as the multiplier, check the product documentation rather than assuming a convention. The same discipline helps when reporting memory use, backup sizes or network transfer rates.