Decimal vs binary units
There are two ways to count storage. The decimal (SI) system goes up in steps of 1,000, just like kilometers and kilograms. The binary system goes up in steps of 1,024 (2¹⁰), which is how computer memory is addressed. Since 1998 the binary units have their own names (kibibyte, mebibyte, gibibyte) but Windows still shows them with the decimal labels, which causes endless confusion.
The formula
decimal: 1 KB = 1,000 B, 1 MB = 10⁶ B, 1 GB = 10⁹ B, 1 TB = 10¹² B
binary: 1 KiB = 1,024 B, 1 MiB = 1,024² B, 1 GiB = 1,024³ B, 1 TiB = 1,024⁴ B
bits = bytes × 8
Example: a 1 TB drive
| Step | Result |
|---|---|
| Bytes: 1 × 10¹² | 1,000,000,000,000 |
| In GiB: 10¹² ÷ 1,024³ | 931.32 |
| What Windows shows | 931.3 GB |
Advertised vs shown in Windows
| Drive sold as | Windows shows about |
|---|---|
| 256 GB | 238 GB |
| 512 GB | 477 GB |
| 1 TB | 931 GB |
| 2 TB | 1.82 TB |
| 4 TB | 3.64 TB |
Your usable space will be a little lower still, after the operating system, recovery partition and formatting.
Frequently asked questions
Why does my 1 TB drive only show 931 GB?
Drive makers use decimal units, so 1 TB is 1,000,000,000,000 bytes. Windows divides by 1,024 at each step and labels the result GB, though it's really gibibytes (GiB). 10¹² ÷ 1,024³ = 931.3 GiB. No space is missing; it's two ways of counting.
Is 1 GB 1,000 MB or 1,024 MB?
Officially, 1 GB (gigabyte) is 1,000 MB, and 1 GiB (gibibyte) is 1,024 MiB. In practice Windows and some apps still write GB when they mean GiB, while macOS, drive packaging and internet speeds use true decimal units.
What is the difference between bits and bytes?
A byte is 8 bits. Storage and file sizes are measured in bytes (capital B, as in MB), while network speeds are measured in bits (lowercase b, as in Mbps). Divide Mbps by 8 for MB/s.
How many photos or songs fit in 1 GB?
Roughly 250 to 400 phone photos at 3 to 4 MB each, or about 250 songs as 4 MB MP3 files. A minute of 1080p phone video is around 100 to 200 MB.
Rates and figures last checked October 2026.
This calculator gives estimates for planning. Check current rates and product labels before you buy, list or file.