The bit is the fundamental unit of data in computing (0 or 1). Multiple bits are grouped together to form larger units. The byte (8 bits) is the standard basic unit for measuring file sizes, memory capacity, and storage.
A nibble is a group of 4 bits (half a byte). A nibble can represent values from 0 to 15 (0000₂ to 1111₂), which corresponds to exactly one hexadecimal digit — this is why hex is a convenient shorthand for binary.
The International Electrotechnical Commission (IEC) defines binary prefixes based on powers of 2. These are the technically correct units for computer memory (RAM) and file sizes in binary systems:
| Unit | Symbol | Value in bytes | Power of 2 |
|---|---|---|---|
| Kibibyte | KiB | 1,024 bytes | 2¹⁰ |
| Mebibyte | MiB | 1,048,576 bytes | 2²⁰ |
| Gibibyte | GiB | 1,073,741,824 bytes | 2³⁰ |
| Tebibyte | TiB | 1,099,511,627,776 bytes | 2⁴⁰ |
Storage manufacturers often use SI (metric) prefixes based on powers of 10, which produce larger marketed capacity numbers. Network speeds and some storage devices also use these:
| Unit | Symbol | Value in bytes | Power of 10 |
|---|---|---|---|
| Kilobyte | kB | 1,000 bytes | 10³ |
| Megabyte | MB | 1,000,000 bytes | 10⁶ |
| Gigabyte | GB | 1,000,000,000 bytes | 10⁹ |
| Terabyte | TB | 1,000,000,000,000 bytes | 10¹² |
A hard drive marketed as "1 TB" (1,000,000,000,000 bytes) stores only about 0.91 TiB in binary terms. This is why a "1 TB" hard drive shows as ~931 GiB when formatted and used. The Edexcel 4CP0 specification requires you to know both systems.
| System | Prefix "kilo" means | Used for |
|---|---|---|
| IEC binary (KiB, MiB, GiB, TiB) | 1,024 (2¹⁰) | RAM, file sizes in binary systems |
| SI decimal (kB, MB, GB, TB) | 1,000 (10³) | Hard drive capacity, network speed marketing |