Bits, Bytes, KB, MB, GB, TB: Data Storage Units Explained
Your phone says 128 GB, Windows shows 119 GB, and your internet plan is sold in megabits while downloads are shown in megabytes. All of that confusion comes from a handful of units. This guide explains bits, bytes and the larger units, why “1 KB” can mean 1,000 or 1,024 bytes, and how to do the maths for storage and network speeds.
The bit: one yes-or-no
A bit (short for binary digit) is the smallest piece of digital data. It holds one of two values, 0 or 1, like a switch that is off or on. Inside a computer, those two states are represented by low and high voltages, magnetic directions, or charge in flash cells. Everything else, text, photos, video and programs, is built from long sequences of bits.
The byte: eight bits
A byte is a group of 8 bits, which can represent 28 = 256 different values (0 to 255). Bytes are the unit computers use to address memory and store data. A byte is not always “one character”: in ASCII and UTF-8, the English letters, digits and common punctuation take 1 byte each, so “cat” is 3 bytes, but accented letters usually take 2 bytes, most Indian-language and Chinese characters take 3, and an emoji usually takes 4. A file’s size is simply the number of bytes it occupies.
Abbreviations matter: a lower-case b means bit and a capital B means byte, so 100 Mb is one-eighth the size of 100 MB.
Bigger units: 1,000 or 1,024?
There are two systems, and mixing them up is the source of the “missing” storage.
| Decimal (SI) unit | Bytes | Binary (IEC) unit | Bytes |
|---|---|---|---|
| Kilobyte (KB) | 1,000 | Kibibyte (KiB) | 1,024 |
| Megabyte (MB) | 1,000,000 | Mebibyte (MiB) | 1,048,576 |
| Gigabyte (GB) | 1,000,000,000 | Gibibyte (GiB) | 1,073,741,824 |
| Terabyte (TB) | 1012 | Tebibyte (TiB) | 240 (about 1.0995 x 1012) |
| Petabyte (PB) | 1015 | Pebibyte (PiB) | 250 |
Computer memory is naturally built in powers of two, so early programmers used “kilobyte” for 1,024 bytes. Storage makers, however, use the decimal meaning, which makes their drives look larger and matches the SI prefixes. To end the ambiguity, the IEC introduced the binary names kibibyte, mebibyte, gibibyte and so on in 1998. In practice: hard drive, SSD and network capacities are sold in decimal units; RAM is measured in binary units; and Windows displays drive sizes in binary units but labels them GB. That mismatch is why a “1 TB” drive shows up as roughly 931 GB in Windows.
Worked example
A drive sold as 1 TB holds 1,000,000,000,000 bytes. Windows divides by 1,024 three times to show gibibytes: 1012 / 1,073,741,824 is about 931. The drive is not missing space; the unit is different. Part of the remaining space is then used by the file system and partition metadata, and on system drives by recovery partitions.
Bits vs bytes in networking
Network speeds are quoted in bits per second (Mbps, Gbps), while download managers and file sizes use bytes. Divide by 8 for a rough conversion:
- A 100 Mbps connection can move at most 12.5 MB of data per second (100 / 8).
- A 1 Gbps link is about 125 MB/s at best. Real speeds are a bit lower because of protocol overhead and other traffic.
- A 2 GB download at 50 Mbps needs 2 x 8,000 megabits / 50 = 320 seconds, about 5 minutes 20 seconds, ignoring overhead.
How many things fit? Rough sizes
| Item | Typical size |
|---|---|
| A plain text email | A few KB |
| A phone photo (JPEG or HEIC) | 2 to 5 MB |
| A 3-minute MP3 song (128 to 320 kbps) | 3 to 7 MB |
| One hour of 1080p video | 1 to 4 GB, depending on the compression settings |
| A large modern game | 50 to 150 GB or more |
These are estimates: compression settings change them a lot.
Check sizes yourself
- Windows:
Get-Volume | Format-Table DriveLetter, Size, SizeRemainingin PowerShell shows bytes. In PowerShell,1TB,1GBand1MBare built-in binary multipliers (1TB = 1,099,511,627,776), and(Get-Item file.iso).Length / 1MBgives a file’s size in mebibytes. - Linux:
df -huses binary units (powers of 1,024) anddf -Huses decimal (powers of 1,000), so you can compare against the label on the drive.lsblkshows the raw device sizes.
How much storage do I need?
- Browsing, email, documents: 128 GB is workable, though Windows and updates use a lot of it.
- Photos, music and some games: 256 to 512 GB.
- 4K video editing, big game libraries, VMs: 1 TB or more, ideally with a separate backup drive.
Whatever you choose, remember a drive is not full protection: keep a backup. Also try to keep 10 to 20 percent of an SSD free for the best performance and longevity.
Common questions
Why does a USB stick show less space than advertised?
Two reasons: decimal vs binary units, and the file system overhead. A “64 GB” card is 64,000,000,000 bytes, or about 59.6 GiB, and Windows shows a little less after formatting.
Is a megabit the same as a megabyte?
No. A megabyte is 8 megabits.
What comes after terabyte?
Petabyte, exabyte, zettabyte and yottabyte, each 1,000 times larger in the decimal system. Large data centres talk in petabytes; global internet traffic is measured in zettabytes.
Keep reading
- What Is a Computer? Hardware, Software and How They Work Together — The basics of how a computer works.
- How a Computer Works: A Beginner Guide From Power Button to Screen — A beginner walkthrough of the hardware.
- Windows File Systems Compared: NTFS, FAT32, exFAT and ReFS — How storage is organised into file systems.
