MB vs MiB: What's the Difference?

4 min read•June 1, 2025•Reviewed: June 1, 2026

✓Reviewed by Apex Conversion Editorial Team · Last reviewed June 1, 2026

MB and MiB both describe roughly a million bytes of data — but they're not the same. MB (megabyte) is the SI decimal unit: exactly 1,000,000 bytes. MiB (mebibyte) is the IEC binary unit: exactly 1,048,576 bytes (2²⁰). The 4.9% difference between them creates persistent confusion in storage, memory, and software specifications.

Understanding when each definition is used helps you read storage specs accurately, understand why drives show less space than advertised, and interpret RAM specifications correctly.

Why Two Definitions Exist

Early computer memory was organized in powers of 2, because binary addressing naturally produces those sizes. 2¹⁰ = 1,024 bytes is close to 1,000, and early engineers informally called it a 'kilobyte.' This approximation worked at small scales but grew messier as storage expanded to megabytes and gigabytes.

In 1998, the International Electrotechnical Commission (IEC) introduced unambiguous binary prefixes: kibi- (2¹⁰), mebi- (2²⁰), gibi- (2³⁰), tebi- (2⁴⁰). These became the KiB, MiB, GiB, TiB notation. The original KB, MB, GB prefixes were redefined to mean strictly decimal powers of 10, per SI convention. Adoption of the IEC standard has been slow — most consumers still see 'GB' used for both.

The Exact Sizes

Decimal (SI):
  1 KB  = 1,000 bytes         (10³)
  1 MB  = 1,000,000 bytes     (10⁶)
  1 GB  = 1,000,000,000 bytes (10⁹)

Binary (IEC):
  1 KiB = 1,024 bytes         (2¹⁰)
  1 MiB = 1,048,576 bytes     (2²⁰)
  1 GiB = 1,073,741,824 bytes (2³⁰)

Gaps:
  1 MiB is 4.858% larger than 1 MB
  1 GiB is 7.374% larger than 1 GB
  1 TiB is 9.951% larger than 1 TB

Who Uses Which

Storage manufacturers (hard drives, SSDs, flash drives) use decimal SI: a '500 GB' SSD has 500,000,000,000 bytes. This maximizes the number on the box. Cloud storage providers (Google Drive, Dropbox, AWS S3) also use decimal GB.

RAM is specified in binary: a '16 GB' RAM module actually contains 16 GiB = 17,179,869,184 bytes. This persists because memory addressing is inherently binary. Linux typically shows storage in GiB/MiB with the correct IEC labels. Older Windows used GiB values but labeled them GB. Modern Windows (10 and later) is moving toward more explicit labeling.

Frequently Asked Questions

Is 1 MB 1000 KB or 1024 KB?

In decimal (SI): 1 MB = 1,000 KB. In binary (IEC): 1 MiB = 1,024 KiB. Storage manufacturers use decimal; RAM is measured in binary. The label 'MB' alone is ambiguous — context determines which definition applies.

Why does my file manager show sizes differently than expected?

Different operating systems use different definitions. macOS (since 10.6 Snow Leopard) uses decimal GB. Linux typically uses binary GiB but labels them correctly with IEC notation. Older Windows uses binary values but labels them GB, causing apparent discrepancies with manufacturer specs.

Should I use MB or MiB in technical writing?

For precision, use MiB when you mean 1,048,576 bytes and MB when you mean 1,000,000 bytes. In technical documentation (software, networking, OS specs), using IEC notation (MiB, GiB) eliminates ambiguity. In consumer-facing writing, MB/GB are standard even if not precise.

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