Why Your 1 TB Drive Only Shows 931 GB

You buy a 1 TB drive, plug it in, and Windows reports 931 GB. Nothing is broken, nothing was stolen, and the drive is exactly the size it says on the box. Two different definitions of the word gigabyte are quietly disagreeing with each other.

Two definitions of the same word

Drive manufacturers use the metric system, where prefixes mean what they mean everywhere else in science. A kilo is a thousand. So one terabyte is 1,000,000,000,000 bytes — a clean power of ten.

Computers address memory in binary, so it has always been convenient for software to round to powers of two. The nearest power of two to a thousand is 1,024, so a "kilobyte" became 1,024 bytes, a "megabyte" 1,024², and so on.

Both are internally consistent. The problem is that they use the same words. Divide the manufacturer's number by the operating system's definition and you get the shortfall:

The gap grows at every step

This is the part that surprises people. The discrepancy is not a fixed 7% — it compounds, because each level multiplies by 1,024 instead of 1,000.

At the kilobyte scale nobody noticed. At terabyte scale you are looking at 69 GB apparently unaccounted for, and at petabyte scale — the world of data centres — the 12.6% gap is a budget line.

What actually shows which

Windows is the main source of the confusion: it does binary arithmetic and then prints the decimal label. macOS switched to decimal years ago, which is why the same external drive can read 931 GB on a PC and 1 TB on a Mac without either being wrong.

The prefixes that were supposed to fix this

In 1998 the International Electrotechnical Commission introduced separate binary prefixes so the two systems could stop sharing vocabulary: kibibyte (KiB), mebibyte (MiB), gibibyte (GiB), tebibyte (TiB). Under that scheme your drive holds 1 TB, which equals 931 GiB, and both statements are precisely true.

Adoption has been partial. Linux tooling and storage engineers use them routinely; consumer software largely ignored them. Nearly three decades on, "GB" still means one thing on a box and another in a file manager.

The other missing space

Unit definitions explain most of the shortfall but not all of it. A brand-new drive also loses capacity to:

• The filesystem itself — NTFS, APFS and ext4 all reserve space for metadata, the journal and the file table. Typically well under 1%.
• Recovery and EFI partitions on a pre-built machine, often 10–30 GB that never appear as usable space.
• SSD over-provisioning — flash drives hold back spare blocks for wear levelling and to replace failed cells. This is invisible by design and is part of why the drive lasts.

If the arithmetic above accounts for the gap you are seeing, the drive is fine. If a 1 TB drive is showing 800 GB, look for a hidden partition rather than a defect.

The same trap with internet speeds

A related unit collision catches people every time they download something. Connection speeds are quoted in bits per second; file sizes are in bytes. There are eight bits in a byte, so a 500 Mbps line tops out around 62.5 MB/s — before any real-world overhead. Watch the capitalisation: Mb is megabits, MB is megabytes.