The base unit is the Bit. A bit is binary, and binary is the fundamental basis of computing. A bit has 2 possible states/values: 0 and 1 (zero and one). The 0 and 1 can be given different interpretations, for example:
| 0 | 1 |
|---|---|
| zero | one |
| off | on |
| false | true |
| no | yes |
| disabled | enabled |
| inactive | active |
The following units are groupings of bits, based on powers of 2 (binary). For example, a byte is 8 bits, and 8 is 2 to the power of 3.
| Power | Unit | Number of Bits |
|---|---|---|
| 2**0 | Bit | 1 bit |
| 2**1 | Half-Nibble | 2 bits |
| 2**2 | Nibble | 4 bits |
| 2**3 | Byte | 8 bits |
| 2**4 | Half-Word | 16 bits |
| 2**5 | Word | 32 bits |
| 2**6 | Double-Word | 64 bits |
| 2**7 | Quad-Word | 128 bits |
| 2**13 | Kilobyte | 8,192 bits |
| 2**23 | Megabyte | 8,388,608 bits |
| 2**33 | Gigabyte | 8,589,934,592 bits |
| 2**43 | Terabyte | 8,796,093,022,208 bits |
| 2**53 | Petabyte | 9,007,199,254,740,992 bits |
| 2**63 | Exabyte | 9,223,372,036,854,775,808 bits |
| 2**73 | Zettabyte | 9,444,732,965,739,290,427,392 bits |
| 2**83 | Yottabyte | 9,671,406,556,917,033,397,649,408 bits |
Following you can see the exact same units as above, but expressed as number of bytes instead of number of bits.
| Unit | Number of Bytes |
|---|---|
| Bit | 1/8 byte |
| Half-Nibble | 1/4 byte |
| Nibble | 1/2 byte |
| Byte | 1 byte |
| Half-Word | 2 bytes |
| Word | 4 bytes |
| Double-Word | 8 bytes |
| Quad-Word | 16 bytes |
| Kilobyte | 2**10 = 1024**1 = 1,024 bytes |
| Megabyte | 2**20 = 1024**2 = 1,048,576 bytes |
| Gigabyte | 2**30 = 1024**3 = 1,073,741,824 bytes |
| Terabyte | 2**40 = 1024**4 = 1,099,511,627,776 bytes |
| Petabyte | 2**50 = 1024**5 = 1,125,899,906,842,624 bytes |
| Exabyte | 2**60 = 1024**6 = 1,152,921,504,606,846,976 bytes |
| Zettabyte | 2**70 = 1024**7 = 1,180,591,620,717,411,303,424 bytes |
| Yottabyte | 2**80 = 1024**8 = 1,208,925,819,614,629,174,706,176 bytes |
In the early days of computing, a byte was not necessarily 8 bits. In modern computing, a byte is universally accepted to be 8 bits. In some older systems/documentation, a word was defined as 16 bits. In modern computing, a word is 32 bits.
Abbreviations may be used as in the examples following. It is recommended that the "B" for "byte" be in uppercase because some people use lowercase "b" to mean bit.
| Full | Abbrev 1 | Abbrev 2 |
|---|---|---|
| 16 kilobytes | 16 KB | 16K |
| 16 megabytes | 16 MB | 16M |
| 16 gigabytes | 16 GB | 16G |
| 16 terabytes | 16 TB | 16T |
It is recommended that "kilobits" and "megabits" (etc.) not be used. Use kilobytes and megabytes instead, because they are more practical and easier to understand. They are easier to understand because a letter of the alphabet uses a byte of memory, and a paragraph consisting of 300 characters uses 300 bytes of memory, etc.
Some people have tried to encourage or force a different/alternate definition of "kilobyte" (etc.) that uses a mix of power of 10 and power of 2 numbers, as follows:
| Unit | Number of Bits |
|---|---|
| Kilobyte (false) | (10**3) × (2**3) = 8,000 bits |
| Megabyte (false) | (10**6) × (2**3) = 8,000,000 bits |
| Gigabyte (false) | (10**9) × (2**3) = 8,000,000,000 bits |
It makes no sense to define units that are a mix of power of 10 and power of 2. Clearly the most sensible course of action is for data measurement units to be based on powers of 2 only, because computers are indisputably based on binary (2) not decimal (10).
In other words: A byte is indisputably a unit based on the binary / power of 2 system. A byte = 2**3 = 8 bits, NOT 10 bits. Therefore it makes sense that kilobytes and megabytes (etc.) should be based on the same system (powers of 2). There is no sense in mixing power of 10 and power of 2.
The marketing departments of some corporations have tried to force the use of these nonsensical mixed-power definitions because it makes the capacities of their devices seem bigger to naive customers.
For example, consider a disk manufactured with 156,330,000 sectors (blocks), where each sector is 2**12 = 4096 bits (512 bytes). That is a total of 74.5 gigabytes. However if the mixed-power definition of gigabyte is used, then it can be said to be 80 gigabytes, making the capacity of the disk seem bigger.
Thus the marketing departments prefer to use the mixed-power definitions. They are only interested in increasing profits, and not really interested in maintaining honesty. They are not bothered by the fact that they are deliberately attempting to mislead their customers.
The base unit for time is the Second, defined by Système International as:
"The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."
| 1 second | = 1 000 milliseconds |
| = 1 000 000 microseconds | |
| = 1 000 000 000 nanoseconds | |
| = 1 000 000 000 000 picoseconds |
Data speed is quantity over time. For example, "128 KB/sec" means 128 kilobytes per second, meaning if you start counting the number of bytes being transmitted, and you do this for a time period of 1 second, then you will have counted 128 kilobytes.