Bytes, bits, MB and MiB: why your drive has “less” space

One byte is eight bits

A bit is the smallest unit of information: a 0 or a 1. A byte groups eight bits and is enough to represent one basic text character. File sizes are measured in bytes (B), while connection speeds are measured in bits per second (bit/s). That one-letter difference, B versus b, multiplies or divides by eight.

1,000 or 1,024: two prefix systems

The International System defines kilo as 1,000, mega as a million and giga as a billion. But computers work in binary, and for years “kilobyte” was used for 1,024 bytes. To end the ambiguity, binary prefixes were created: kibibyte (KiB, 1,024 bytes), mebibyte (MiB, 1,048,576 bytes) and gibibyte (GiB).

The gap grows with size: between kB and KiB it's 2.4 %, but between TB and TiB it's almost 10 %.

The mystery of the 1 TB drive that shows 931 GB

Manufacturers advertise capacity in decimal units: 1 TB is a trillion bytes. Windows divides by 1,024 three times and labels the result “GB”: 931.32. No space is missing; it's the same amount in a different unit. macOS and iOS, by contrast, use decimal units and show 1 TB.

Some space really is reserved for the file system and, on phones, for the operating system, but that's separate from the unit difference.

Connection megabits versus download megabytes

With a 300 Mb (megabit) per second connection, your browser will download at most about 37.5 MB (megabytes) per second — a little less in practice, because of protocol and network overhead. Some examples at 100 Mbit/s:

• A 4 MB photo: about a third of a second.
• A 1 GB video: about 80 seconds.
• A 50 GB game: just over an hour.

How to really save space

On most devices, photos and videos take up the most room. Reducing photos to the width you'll actually view them at (say, 1920 pixels) and saving them as WebP at quality 75–80 usually cuts their size five to ten times with no visible difference on screen.