ToolMint
converters6 min readMay 12, 2026

MB vs MiB – Why Your Hard Drive Shows Less Storage Than Advertised

If you have ever bought a hard drive, USB stick, or SSD and noticed that it shows less storage than advertised when you plug it in, you have encountered the MB vs MiB problem. It is one of the most persistent sources of confusion in computing — and it is not a scam. The hard drive manufacturer and your operating system are both technically correct; they are just using different standards for what 'kilo', 'mega', and 'giga' mean.

The Two Standards: SI Decimal vs IEC Binary

In everyday language, 'kilo' means 1,000 (one thousand). The International System of Units (SI) applies this consistently to file sizes: 1 KB = 1,000 bytes, 1 MB = 1,000,000 bytes, 1 GB = 1,000,000,000 bytes, 1 TB = 1,000,000,000,000 bytes. But computers work in binary — powers of 2. The nearest power of 2 to 1,000 is 1,024 (2¹⁰). Early computer engineers reused 'kilo' to mean 1,024, 'mega' to mean 1,024², and 'giga' to mean 1,024³. This informal standard was codified by the IEC in 1998 with new prefixes: 1 KiB (kibibyte) = 1,024 bytes, 1 MiB (mebibyte) = 1,048,576 bytes, 1 GiB (gibibyte) = 1,073,741,824 bytes. The confusion persists because Windows reports file sizes in IEC binary units (GiB) but labels them as GB.

Why a 1 TB Drive Shows as ~931 GB in Windows

A 1 TB hard drive contains exactly 1,000,000,000,000 bytes (SI decimal). When Windows reports its capacity, it divides by 1,073,741,824 (1 GiB) instead of 1,000,000,000 (1 GB): 1,000,000,000,000 ÷ 1,073,741,824 = 931.32 GiB Windows then labels this as '931 GB' — using the GB label for what is actually GiB. So your drive is exactly the size advertised; Windows is just using binary units and labeling them with the decimal name. macOS switched to SI decimal in macOS 10.6 (Snow Leopard), so a 1 TB drive on a Mac correctly shows as ~1 TB. Linux uses binary units by default but some tools show both. Understanding which standard your tool is using removes 90% of the apparent 'missing storage' confusion.

Conversion Reference: MB to MiB, GB to GiB

The conversion factor between SI and IEC units increases at each prefix level: 1 MB = 0.953674 MiB (÷ 1.048576) 1 GB = 0.931323 GiB (÷ 1.073741824) 1 TB = 0.909495 TiB (÷ 1.099511628) Practical examples: a file labeled '500 MB' in SI is 476.8 MiB. A folder showing as '8 GB' in Windows (binary) is actually 8 GiB = 8.589 GB in SI. For RAM, 16 GB of RAM always means 16 GiB (16,384 MiB) because RAM is produced in binary-aligned capacities — manufacturers use 'GB' loosely to mean GiB for RAM, unlike hard drives.

Try the tools mentioned in this guide

File Size ConverterConvert between 12 file size units including SI decimal (KB/MB/GB) and IEC binary (KiB/MiB/GiB) side by side.Length ConverterConvert between 12 length units — km to miles, meters to feet, cm to inches — with an all-units comparison table.Random Number GeneratorGenerate cryptographically secure random numbers, roll dice, flip coins, or pick items from a custom list.

Frequently Asked Questions

What is the difference between MB and MiB?
1 MB (megabyte, SI) = 1,000,000 bytes. 1 MiB (mebibyte, IEC) = 1,048,576 bytes. The difference is about 4.9%. Windows uses MiB but labels it MB.
Why does a 1 TB hard drive show as 931 GB in Windows?
Hard drives are sized in SI decimal (1 TB = 10¹² bytes). Windows reports in IEC binary (divides by 1,073,741,824 per GiB) and labels the result as GB. 1,000,000,000,000 ÷ 1,073,741,824 = 931.32 — no storage is missing.
Does macOS show the correct storage size?
Yes. Since macOS Snow Leopard (10.6), Apple uses SI decimal units. A 1 TB drive shows as approximately 1 TB in macOS Finder.
How do I convert GB to GiB?
Divide GB by 1.073741824. Example: 500 GB ÷ 1.073741824 ≈ 465.66 GiB. Or use the File Size Converter above which shows both SI and IEC values simultaneously.

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