Decimal (GB) vs binary (GiB) — the two counting systems
Data sizes are counted two different ways, and mixing them up is the single biggest source of confusion in storage:
- Decimal (SI): each step is ×1,000. 1 KB = 1,000 bytes, 1 MB = 10⁶, 1 GB = 10⁹, 1 TB = 10¹² bytes. Used by drive manufacturers, networks and macOS.
- Binary (IEC): each step is ×1,024. 1 KiB = 1,024 bytes, 1 MiB = 2²⁰, 1 GiB = 2³⁰ = 1,073,741,824 bytes, 1 TiB = 2⁴⁰. Used by RAM sizing and by Windows (which labels the values "KB/MB/GB").
Why a “1 TB” drive shows ~931 GB
The manufacturer counts in decimal: 1 TB = 1,000,000,000,000 bytes — and that is genuinely what the drive stores. Windows then divides those bytes by 1,024³ (a binary tebibyte) and displays 931.32, but labels it “GB”. No capacity is lost; the operating system is simply reporting in GiB while using the decimal unit’s name. The gap compounds with size: a KiB is 2.4% bigger than a KB, a GiB is 7.4% bigger than a GB, and a TiB is 10% bigger than a TB.
Handy approximations
- TB (decimal) → “Windows GB” (GiB): multiply by 931 (2 TB drive ≈ 1,863 GiB)
- GB → GiB: multiply by 0.931 · GiB → GB: multiply by 1.074
- Bits vs bytes: divide a network speed in Mbps by 8 to get MB/s
- 1 GB ≈ 250 smartphone photos ≈ 200 songs ≈ 30 minutes of HD video
Worked example
1 GB to MB: 1 × 10⁹ ÷ 10⁶ = 1,000 MB exactly. The same gigabyte is 976.56 MiB or 0.9313 GiB in binary units.