Why Your 1 TB Drive Shows 931 GB: MB vs MiB Explained

You buy a 1 TB drive, plug it into a Windows PC, and it reports 931 GB. A 256 GB phone shows less free space than you expected. Nothing is broken and nobody cheated you. The two numbers use different definitions of a gigabyte.

Two ways to count

Computers work in powers of 2, so early on, a “kilobyte” came to mean 1,024 bytes because it was close to 1,000. As drives grew, that small difference compounded. Today there are two official sets of units:

Drive makers use decimal units. Windows counts in binary units but labels them GB. So a drive with 10¹² bytes is 10¹² ÷ 1,073,741,824 ≈ 931.3 GiB, which Windows shows as “931 GB”.

Common drive sizes

The gap grows with each step up. A binary unit is 2.4% bigger than its decimal namesake at the kilobyte level, 7.4% bigger at the gigabyte level and almost 10% bigger at the terabyte level, so the number your computer shows falls further behind as drives grow. Check any value with the File Size Converter.

On top of this, the operating system, recovery partitions and file system formatting take some space, which is why a phone’s usable storage is lower still.

Which systems use which?

• Decimal: drive and phone makers, macOS (since 2009), iOS, and most Linux desktop tools.
• Binary: Windows File Explorer, and RAM sizes everywhere (16 GB of RAM really is 16 GiB).

Bits vs bytes: internet speed

One more trap: internet speeds are measured in bits per second, while file sizes are in bytes. There are 8 bits in a byte, so a 100 Mbps connection downloads at best 12.5 MB per second. A 1 GB file takes at least 80 seconds, before any network overhead. The File Size Converter converts bits and bytes too.