BytesToMbps

Megabytes (MB) vs Mebibytes (MiB)

Ever bought a 1TB hard drive only to find it has 931GB of space? Here is the technical reason why.

The Great Storage Deception

When you buy a hard drive or SSD, manufacturers use the International System of Units (SI). In this system, 'Kilo' means exactly 1,000.

So, 1 Kilobyte (KB) is 1,000 Bytes, and 1 Gigabyte (GB) is 1,000,000,000 Bytes.

However, Windows operating systems calculate storage using binary mathematics (base 2), which is the International Electrotechnical Commission (IEC) standard. In binary, a 'Kibi' means 1,024.

Therefore, 1 Kibibyte (KiB) is 1,024 Bytes, and 1 Gibibyte (GiB) is 1,073,741,824 Bytes.

This discrepancy is why your 1 Terabyte (TB) hard drive only shows up as ~931 Gigabytes in Windows (which is actually 931 Gibibytes). To calculate your exact download time for these large files, use our Download Time Calculator.

SI vs IEC Comparison Table

SI Unit (Base-10)IEC Unit (Base-2)Difference in Size
1 KB (1,000 Bytes)1 KiB (1,024 Bytes)2.4%
1 MB (1,000,000 Bytes)1 MiB (1,048,576 Bytes)4.8%
1 GB (1,000,000,000 Bytes)1 GiB (1,073,741,824 Bytes)7.3%
1 TB (1,000,000,000,000 Bytes)1 TiB (1,099,511,627,776 Bytes)9.9%

Frequently Asked Questions

Why does my 1TB hard drive only have 931GB?
Manufacturers sell drives using decimal math (1TB = 1,000,000,000,000 bytes). Windows reads them using binary math (1 TiB = 1,099,511,627,776 bytes). When Windows converts the raw bytes, it results in ~931 GB (which is technically 931 GiB).
Is macOS affected by this storage difference?
No, since OS X 10.6 (Snow Leopard), Apple's macOS calculates storage using the same base-10 (SI) metric system as drive manufacturers. A 1TB drive will show exactly as 1TB on a Mac.
Did the hard drive manufacturer lie about the size?
No, they are legally and technically correct in using standard SI prefixes (Kilo = 1000). The confusion is solely caused by Microsoft Windows using base-2 math but displaying base-10 labels.
What is the difference between MB and MiB?
A Megabyte (MB) is exactly 1,000,000 bytes. A Mebibyte (MiB) is 1,048,576 bytes. The 'i' in MiB stands for 'binary', indicating it uses base-2 computing.
How much usable space is on a 2TB drive?
On Windows, a 2TB drive will show approximately 1,862 GB (or 1.81 TB) of usable free space before formatting overhead.
Will I lose even more space after formatting?
Yes, formatting a drive creates file system structures (like NTFS or FAT32) which reserve a small percentage of your drive's capacity for file indexing and metadata.