Why does your internet plan say "100 Mbps" but your download speed shows "12 MB/s"? Why does a file that's 1 GB take up more space than advertised? Digital data units confuse almost everyone. This guide breaks it all down.
The Basics: Bits and Bytes
Everything in computing comes down to two units: bits and bytes.
A bit is the smallest unit of digital information — a single 0 or 1. The word "bit" comes from "binary digit."
A byte is 8 bits. A byte can represent 256 different values (2⁸), which is enough to store a single character of text, like the letter "A."
The key rule to remember: 1 byte = 8 bits. This is why your 100 Mbps (megabits per second) internet connection only downloads at about 12.5 MB/s (megabytes per second) — you divide by 8.
The Scale: From Bytes to Petabytes
| Unit | Symbol | Size |
|---|---|---|
| Bit | b | 1/8 of a byte |
| Byte | B | 8 bits |
| Kilobyte | KB | 1,024 bytes |
| Megabyte | MB | 1,024 KB ≈ 1 million bytes |
| Gigabyte | GB | 1,024 MB ≈ 1 billion bytes |
| Terabyte | TB | 1,024 GB ≈ 1 trillion bytes |
| Petabyte | PB | 1,024 TB ≈ 1 quadrillion bytes |
Why 1,024 and Not 1,000?
Computers work in binary (base-2), so data storage naturally scales in powers of 2. The closest power of 2 to 1,000 is 2¹⁰ = 1,024. That's why 1 kilobyte is 1,024 bytes, not 1,000 bytes.
However, hard drive manufacturers typically use 1,000 (base-10) to make their drives sound larger. This is why a "1 TB" hard drive shows up as about 931 GB in Windows — your computer is measuring in true binary units.
Bits vs Bytes in Real Life
Internet speed is almost always measured in bits per second (Mbps, Gbps). This is an industry convention from the early days of networking.
File sizes and storage are almost always measured in bytes (KB, MB, GB). When you download a file or check your phone storage, you're seeing bytes.
The quick conversion: divide your Mbps speed by 8 to get your approximate MB/s download speed.
Real-World Size Reference
| Item | Approximate Size |
|---|---|
| One character of text | 1 byte |
| A short email | ~5 KB |
| A high-res photo | 3–8 MB |
| A song (MP3) | 3–10 MB |
| An HD movie | 4–8 GB |
| A smartphone (storage) | 64–512 GB |
| A laptop hard drive | 500 GB – 2 TB |
| All text ever written by humans | ~50 petabytes |
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