Time and Data CalculatorsBits and bytes: the difference and why it matters for speed and storage
A bit is a single 0 or 1. A byte is eight bits. Most confusion about speeds and file sizes comes from mixing them up.
The definitions
A bit is the smallest unit of information: 0 or 1. A byte is a group of 8 bits, enough for one letter in simple text, or a number from 0 to 255. Everything on a computer, from a photograph to a song, is a very long row of bits.
Why both are used
Network speeds are measured in bits per second, which comes from the history of telecommunications where lines carried one bit at a time. Storage and file sizes are measured in bytes, which come from the way computers handle data. The result is that a provider sells 100 megabits per second (Mbit/s) and your download window shows megabytes per second (MB/s).
Spelling the units
| Written | Meaning |
|---|---|
| b, bit | one bit |
| B, byte | one byte, 8 bits |
| kbit/s, Mbit/s, Gbit/s | thousands, millions, billions of bits per second |
| KB, MB, GB | thousands, millions, billions of bytes |
| Mbps | usually megabits per second |
| MBps or MB/s | megabytes per second |
A lower case b is a bit and an upper case B is a byte. Not every writer follows this, so look at the context: speeds from providers are in bits.
Converting
- Bits to bytes: divide by 8. 800 bits are 100 bytes.
- Mbit/s to MB/s: divide by 8. 100 Mbit/s is at most 12.5 MB/s.
- Bytes to bits: multiply by 8. A 5 MB photo is 40 megabits.
Examples
A 4.7 GB film on a 50 Mbit/s line: 4.7 GB is 37.6 gigabits, so about 752 seconds at full speed, 12.5 minutes. A 1 Gbit/s fibre line can deliver 125 MB/s at best, and an ordinary hard disk may be the slower part.
Why 8 bits?
The 8-bit byte became the standard early in the history of computing because it can hold a letter or a small number, and 8 is a power of two. Some old machines used other sizes.
Common mistakes
- Expecting 100 MB/s from a 100 Mbit/s connection.
- Thinking a megabit and a megabyte are close in size. A megabyte is eight times bigger.
- Mixing decimal (1000) and binary (1024) prefixes; see the page on KB and KiB.
How many bits in a kilobyte?
8,000 in the decimal system (1,000 bytes) and 8,192 in a kibibyte.
Why does my 200 Mbit/s plan show 25 MB/s?
200 divided by 8 is 25. That is the best case for your plan.
This is an independent guide. It is not connected with the IERS, BIPM, IANA, NIST or any company named here. Facts were read on 14 November 2026 from the sources named on each page; calculated values come from Python.