Storage Capacity Converter

Storage Capacity: Why a 1 TB Drive Shows Only 931 GB

Two prefix systems

This is the root of the classic "my drive shrank" complaint. Two incompatible unit sets exist:

K = 1000 is the SI rule (every SI prefix is a power of 10), while Ki = 1024 is the IEC name invented specifically for binary addressing. KiB is read "kibibyte".

The arithmetic behind 1 TB showing 931 GB

Vendors label capacity in decimal: 1 TB = 10¹² bytes = 1,000,000,000,000 bytes. Operating systems count in binary (and often misuse the "GB" name):

1,000,000,000,000 ÷ 1024³ = 1,000,000,000,000 ÷ 1,073,741,824 ≈ 931.32

So the OS shows about 931 GiB (mislabeled "931 GB"). No bytes are lost — the same byte count is merely counted two ways.

The ratio is constant at about 6.9% (that is 1 − 1000³/1024³) because it is a fixed fraction. The larger the drive, the larger the absolute "missing" amount, but the percentage stays put. If a drive is missing noticeably more than this, suspect filesystem overhead, hidden partitions, or false labeling.

Real usable space is smaller still: the filesystem reserves metadata, there may be an EFI partition, and SSDs often set aside over-provisioning (OP) for wear leveling. So a 1 TB drive showing 931 GiB, then a few GB less of actual file space, is all normal.

Bit versus Byte: an 8× gap

Case decides meaning and the gap is 8×. Network bandwidth uses bits (Mbps); storage uses bytes (MB). Therefore:

100 Mbps broadband = 100 ÷ 8 = 12.5 MB/s actual download speed

That is the standard answer to "why does my 100M line download at only 12 MB/s". For download-time math, convert bandwidth to byte rate first, then divide by file size.

You never reach the theoretical figure: protocol overhead (TCP/IP headers, handshakes), Wi-Fi half-duplex and interference, the server's upload cap, and cross-ISP peering all discount it. On wired links, 70%–90% of theory is normal.

Usage caveats

• Do not compare KB and KiB results as if equal. When comparing two drives or two cloud plans, confirm which convention each side uses. A cloud "GB" may be GiB or decimal, so the real space per price can differ by 7%.
• Transfer rate and capacity are different dimensions. "MB/s" is a rate; "MB" is an amount — never swap them just because both say MB.
• Watch MB versus Mb. One is megabyte, the other megabit. A typo in case makes a reader wrong by 8×.
• Video/audio size estimates vary widely. Same bitrate but different codecs (H.264 vs H.265) change quality per bitrate a lot, and per-quality size by up to 50%. "MB per minute" estimating is rough.

The tool covers bit, B, KB, MB, GB, TB, PB and shows both decimal (1000) and binary (1024) results side by side, so vendor labeling and OS display line up directly. Everything is computed in your browser tab — there is no upload step.