Byte prefixes
Prefixes for Bytes
A single byte is a very small unit of storage. Modern files, applications, and storage devices involve millions, billions, or even trillions of bytes. Rather than writing out these large numbers in full, computing uses standardised prefixes to name multiples of bytes in the same way that "kilometre" is used instead of "1,000 metres".
AQA defines these prefixes using powers of 1,000 (the decimal or SI system). Each step up the table multiplies by 1,000:
| Prefix | Symbol | Value in bytes | Approximate scale |
|---|---|---|---|
| Kilobyte | kB | 1,000 bytes | A short text message |
| Megabyte | MB | 1,000,000 bytes | A high-quality photograph |
| Gigabyte | GB | 1,000,000,000 bytes | A feature-length film |
| Terabyte | TB | 1,000,000,000,000 bytes | A large hard drive |
Each prefix is 1,000 times larger than the one below it. To convert downwards (e.g. GB to MB) multiply by 1,000. To convert upwards (e.g. MB to GB) divide by 1,000.
Deep Dive: Why Your Storage Device Looks Smaller Than Advertised
You may have noticed that a hard drive sold as "1 TB" appears smaller when you plug it into a computer. This is because manufacturers use the decimal definition (1 TB = 1,000,000,000,000 bytes) but operating systems such as Windows historically used the binary definition (1 TiB = 240 = 1,099,511,627,776 bytes). A drive with exactly 1,000,000,000,000 bytes is therefore reported as approximately 909 GiB by Windows, not 1,000 GB.
For AQA GCSE, you only need the decimal definitions (powers of 1,000) shown in the table above. The binary prefixes (kibibyte KiB, mebibyte MiB, gibibyte GiB, tebibyte TiB) are not required at GCSE but explain why the discrepancy exists in practice.
Key Takeaways
- Byte quantities are described using prefixes: kilo (kB), mega (MB), giga (GB), tera (TB).
- AQA defines each prefix as a power of 1,000: 1 kB = 1,000 B; 1 MB = 1,000 kB; 1 GB = 1,000 MB; 1 TB = 1,000 GB.
- To convert downwards multiply by 1,000; to convert upwards divide by 1,000.
- The apparent discrepancy between advertised storage capacity and reported capacity arises because manufacturers use decimal (1,000-based) prefixes while some operating systems use binary (1,024-based) prefixes - but only the decimal definitions are required for AQA GCSE.