Data size converter

Two ways to count a kilobyte

Computers count in powers of two, people in powers of ten, and storage units ended up with both. Decimal (SI) units go up by 1000. Binary units, properly called kibibytes, mebibytes and so on, go up by 1024. The gap grows with every step:

Drive and memory card makers, network speeds and macOS use decimal units. Windows File Explorer uses binary units but still writes KB, MB and GB, and memory (RAM) is binary everywhere: 16 GB of RAM is 16 GiB. The KiB, MiB and GiB names were standardized in 1998 to end the confusion, but the old labels stuck.

Why a 1 TB drive shows 931 GB

A drive sold as 1 TB holds 1,000,000,000,000 bytes. Windows divides that by 1,073,741,824 (1024 × 1024 × 1024) to show gigabytes, which gives 931.32. Nothing is missing: the same bytes are counted in a bigger unit. A little more goes to the file system and, on the drive Windows is installed on, to a recovery partition.

To check a drive in Octoolo, switch Binary on, choose B as the unit and type the drive's size in bytes, 1000000000000 for 1 TB: the GiB line reads 931.3226 and the TiB line 0.9095. The drive’s Properties window in Windows shows both figures, the exact byte count and the rounded size, which makes the difference easy to see on your own disks.

Bits are not bytes

Internet and network speeds are quoted in bits per second, with a small b: Mbps or Mb/s. A byte is 8 bits, so divide by 8 before converting:

• A 100 Mbps connection moves at most 12.5 MB of data per second.
• A 5 GB download at that speed takes at least 5,000 ÷ 12.5 = 400 seconds, 6 minutes and 40 seconds; real downloads are slower.
• A 1 Gbps connection tops out at 125 MB per second.

The same capital B, small b care matters on spec sheets: a camera recording video at 100 Mb/s fills 12.5 MB of card every second, about 750 MB a minute. Uploads work the same way: a 10 Mbps upload sends 1.25 MB a second, so a 1 GB video needs at least 800 seconds, over 13 minutes.

The data size converter works in bytes only. Do the divide-by-8 step in the calculator first, then convert the bytes here.

Will it fit?

Comparing sizes goes wrong when the two numbers come from different systems. Windows shows both file sizes and free space in binary units, so comparing those two with each other is safe. Mixing a file size from Windows with a card or drive size from its box is where the 7% to 10% gap catches people.

A few examples: a 4.7 GB DVD holds 4.38 GB as Windows counts it, a 64 GB memory card shows about 59.6 GB, and a file Windows lists as 8 GB is 8.59 billion bytes, too big for a card sold as 8 GB. Cards formatted as FAT32 also refuse any single file over 4 GB, whatever the free space.

When space is tight, the video compressor shrinks the largest files.