Byte Converter

KB vs KiB — the binary / decimal split, the storage industry's lawsuit history, and why it still matters

Computer storage has two coexisting unit systems that look almost identical and differ by ~7%. The split has been the subject of class-action lawsuits against drive manufacturers, design changes in macOS, and a still-unfinished standardization effort that started in 1998. This guide breaks down the SI / IEC distinction, the practical numbers you need to remember, the operating system that displays each variant, and the network-vs-storage convention split that bites every backend engineer.

The two systems — SI vs IEC

The drift is exponential: KB and KiB differ by 2.4%, MB and MiB by 4.9%, GB and GiB by 7.4%, TB and TiB by ~10%. The bigger the unit, the bigger the gap. A 1 TB drive (1,000,000,000,000 bytes) is reported as ~931 GiB by Windows.

Who shows what

The lawsuit history — why drive vendors switched

Through the 1990s, hard-drive vendors freely mixed decimal and binary in marketing. After multiple US class-action suits (notably Cho v. Seagate, 2007), the industry standardized on decimal SI for capacity claims and added explicit fine print. This is also why Apple changed Finder in macOS 10.6 (2009) to display decimal — to match what users see on the box.

Windows still shows binary with "GB" labels (technically wrong by IEC standards) for backward compatibility. Microsoft has acknowledged the issue but has never changed it.

Bits vs bytes — network bandwidth confusion

Networking has always used bits, storage has always used bytes. 1 byte = 8 bits, so a "1 Gbps" connection delivers 125 MB/s in decimal terms — never 1 GB/s. The distinction matters every time someone says "I have a 1 Gig connection but my download speed is only 100 MB/s":

• 1 Gbps (Gigabit per second) = 10⁹ bits/s = 125 × 10⁶ bytes/s = ~125 MB/s decimal.
• 1 GB/s (Gigabyte per second) = 10⁹ bytes/s = 8 × 10⁹ bits/s = 8 Gbps.

The "MB" in "100 MB/s download speed" you see in browsers is decimal MB (10⁶ bytes). The "MB" in "1 GB RAM" is binary MiB (2²⁰ bytes). Same letters, different meanings, no warning.

Common storage benchmarks — actual byte counts

Programming language conventions

Common byte-conversion mistakes

• Calling KiB "KB" in user-facing copy. Use the actual unit; users notice when their 256 GB phone shows 238 GB available.
• Mixing units in dashboards. If your CDN reports decimal GB and your origin reports binary GiB, a "100% match" looks like 7% drift.
• Storing sizes as "5 MB" strings instead of integers. Always store bytes; format on display.
• Confusing megabits and megabytes when reading speeds. A "100 Mbps" link delivers 12.5 MB/s, not 100.
• Forgetting that drive size includes filesystem overhead. A 1 TB drive provides 1 TB of raw capacity; ext4/NTFS/APFS each consume 0.5–3% for journaling, indexes, and reserved blocks.
• Calculating monthly bandwidth in GB but pricing in GiB. Cloud invoices and your monitoring graphs disagree by 7%; the gap compounds at terabyte scale.