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Binary vs Decimal File Sizes: MB, MiB & TB
Published May 1, 2026
Binary vs Decimal File Sizes — MB, MiB, and Marketing TB
When you buy a 2 TB hard drive, plug it in, and Windows reports 1.81 TB, nothing is missing. It is a labelling difference — storage vendors use decimal units, most operating systems historically displayed binary units, and they use the same abbreviations for different values.
The two systems
| System | Base | 1 "kilobyte" | Prefix names |
|---|---|---|---|
| Decimal (SI) | 1,000 | 1,000 bytes | KB, MB, GB, TB |
| Binary (IEC) | 1,024 | 1,024 bytes | KiB, MiB, GiB, TiB |
The binary system uses base-2 powers because computer memory is addressed in powers of two. The decimal system aligns with the International System of Units.
How the gap grows with scale
| Advertised (decimal) | Actual bytes | Binary display | "Missing" |
|---|---|---|---|
| 500 GB | 500,000,000,000 | 465.7 GiB | 34.3 GiB |
| 1 TB | 1,000,000,000,000 | 931.3 GiB | 68.7 GiB |
| 2 TB | 2,000,000,000,000 | 1,819.6 GiB | 180.4 GiB |
| 4 TB | 4,000,000,000,000 | 3,637.9 GiB | 362.1 GiB |
The percentage difference is always roughly 7.4% at the GB level and grows at larger scales. A 4 TB drive shows as ~3.64 TiB in an OS that uses binary display.
Why operating systems still confuse this
Windows historically displayed binary sizes and labelled them "GB" (incorrect). macOS switched to decimal in 2009, matching the manufacturer's labelling. Linux varies by tool. This means the same file can show different "sizes" depending on which tool you use — the byte count is always identical, only the display unit changes.
RAM is always binary
RAM is addressed in binary units because memory controllers work in powers of two. A "16 GB" RAM stick contains exactly 16 × 1,024³ = 17,179,869,184 bytes — it is actually 16 GiB. In this context, manufacturers and operating systems both use binary, so there is no discrepancy.
Storage overhead beyond the label
Even after accounting for the labelling difference, formatted capacity is slightly less than raw capacity:
- File system overhead: Partition tables, journal, and file allocation tables consume space
- Firmware reserved sectors: Drives keep spare sectors for bad block remapping
- OS page file / hibernation: Windows may pre-allocate space for the page file
Together these can add another 1–5% reduction from labelled capacity.
Convert precisely between all units with the File Size Converter and learn the binary number system in What Is Binary?.