Kibibytes vs Kilobytes
理解する Binary and Decimal Storage Units
Learn the DifferenceBuy a "500 GB" hard drive, and your computer shows only 465 GB available. Where did the missing space go? Nowhere—it's a units confusion. Hard drive manufacturers use decimal gigabytes (1,000,000,000 bytes), while operating systems display binary gibibytes (1,073,741,824 bytes). 理解する this distinction prevents confusion and false expectations.
Complete Units Comparison
| Decimal (SI) | Bytes | Binary (IEC) | Bytes | Difference |
|---|---|---|---|---|
| 1 KB (kilobyte) | 1,000 | 1 KiB (kibibyte) | 1,024 | 2.4% |
| 1 MB (megabyte) | 1,000,000 | 1 MiB (mebibyte) | 1,048,576 | 4.9% |
| 1 GB (gigabyte) | 1,000,000,000 | 1 GiB (gibibyte) | 1,073,741,824 | 7.4% |
| 1 TB (terabyte) | 10¹² | 1 TiB (tebibyte) | 2⁴⁰ | 10% |
| 1 PB (petabyte) | 10¹⁵ | 1 PiB (pebibyte) | 2⁵⁰ | 12.6% |
Why Two Systems?
The Binary System (Powers of 2)
Computers naturally work in binary (0s and 1s). Memory addresses, CPU registers, and storage blocks are designed around powers of 2:
- 2¹⁰ = 1,024 (convenient for memory addressing)
- 2²⁰ = 1,048,576 (fits neatly in binary architecture)
For decades, "kilobyte" informally meant 1,024 bytes in computing contexts.
The Decimal System (Powers of 10)
The metric prefix "kilo-" officially means 1,000. Hard drive manufacturers began using the decimal definition (1 KB = 1,000 bytes) because:
- It matches SI (International System of Units) standards
- Larger numbers make products look better (500 GB sounds better than 465 GiB)
Where Each System Is Used
Decimal (KB, MB, GB, TB)
- Hard drive and SSD capacities
- USB flash drive labels
- Network data transfer rates
- Cloud storage quotas
- File download sizes (web)
Binary (KiB, MiB, GiB, TiB)
- RAM specifications
- File sizes in Windows Explorer
- Linux utilities (most)
- Programming/technical contexts
- Memory allocation
Mixed/Unclear
- macOS (switched to decimal in 10.6)
- iOS (decimal)
- Android (varies)
The "Missing Space" Calculation
When your 1 TB drive shows less capacity in Windows:
Step by Step
- 1 TB drive = 1,000,000,000,000 bytes (manufacturer's spec)
- Windows displays in GiB: 1,000,000,000,000 ÷ 1,073,741,824
- Result: 931.32 GiB (displayed as "931 GB" in Windows)
Common Discrepancies
| Advertised | Actual Bytes | Shown in Windows | "Missing" |
|---|---|---|---|
| 250 GB | 250,000,000,000 | 232.8 GiB | 7% |
| 500 GB | 500,000,000,000 | 465.7 GiB | 7% |
| 1 TB | 1,000,000,000,000 | 931.3 GiB | 7% |
| 2 TB | 2,000,000,000,000 | 1,862.6 GiB | 7% |
RAM vs Storage
RAM is truly measured in binary units because memory chips are organized in powers of 2:
- 4 GB RAM = 4 GiB = 4,294,967,296 bytes
- 8 GB RAM = 8 GiB = 8,589,934,592 bytes
- 16 GB RAM = 16 GiB = 17,179,869,184 bytes
RAM manufacturers and operating systems agree on binary measurement, so there's no discrepancy for memory.
Conversion Formulas
Decimal to Binary
- GB to GiB: Divide by 1.074
- TB to TiB: Divide by 1.100
Binary to Decimal
- GiB to GB: Multiply by 1.074
- TiB to TB: Multiply by 1.100
Examples
- 500 GB ÷ 1.074 = 465.6 GiB
- 2 TiB × 1.100 = 2.2 TB
まとめ
The kibibyte (KiB) and kilobyte (KB) confusion stems from computing's binary nature colliding with the decimal メートル法. When precision matters, use the IEC binary units (KiB, MiB, GiB, TiB) for powers of 1,024 and SI decimal units (KB, MB, GB, TB) for powers of 1,000. For everyday use, just remember: storage drives show about 7% less than advertised when displayed in your operating system.