Data Storage Units Explained: Bits, Bytes, KB, MB, GB, TB (and Why Your 1 TB Drive Shows 931 GB)
Quick answer: 8 bits = 1 byte; then each step up is either ×1,000 (kilo, mega, giga, tera — the SI definition drive makers use) or ×1,024 (kibi, mebi, gibi, tebi — the binary sizes Windows still labels as KB/MB/GB). That mismatch is why a "1 TB" drive appears as 931 GB.
Bits and bytes
A bit is a single 0 or 1. A byte is 8 bits — enough for one character of plain text. Storage is quoted in bytes (B); network speeds are quoted in bits per second (b). So a "100 Mbps" connection moves 100 ÷ 8 = 12.5 MB per second — the single most common source of confusion in this topic.
The ladder of units
| Unit | SI (decimal) | Binary equivalent | Typical example |
|---|---|---|---|
| Kilobyte (KB) | 1,000 B | KiB = 1,024 B | A short email; a page of text |
| Megabyte (MB) | 1,000 KB | MiB = 1,048,576 B | A minute of MP3; a phone photo is 2–5 MB |
| Gigabyte (GB) | 1,000 MB | GiB = 1,073,741,824 B | An hour of HD streaming ≈ 3 GB |
| Terabyte (TB) | 1,000 GB | TiB = 1,099,511,627,776 B | ≈ 250,000 photos or 500 hours of HD video |
| Petabyte (PB) | 1,000 TB | PiB | Data-centre scale |
Why 1 TB shows as 931 GB
The manufacturer counts 1 TB = 1,000,000,000,000 bytes (SI). Windows divides that by 1,024 three times and labels the result "GB", although strictly it is GiB: 1,000,000,000,000 ÷ 1,073,741,824 = 931.3. Nothing is missing — the two sides use different rulers. macOS and most Linux tools now use SI, so the same drive shows 1 TB there.
How big things actually are
- Plain-text email: 5–20 KB · Word document: 50–500 KB · PDF: 0.5–5 MB
- Smartphone photo: 2–5 MB · RAW camera photo: 25–60 MB
- MP3 song: ~1 MB per minute · lossless FLAC: ~5 MB per minute
- Streaming video: SD ~0.7 GB/h, HD ~3 GB/h, 4K ~7 GB/h
- Modern game install: 50–150 GB · Operating system: 20–40 GB
Converting between units
Decimal: multiply or divide by 1,000 per step (2.5 GB = 2,500 MB). Binary: by 1,024 per step (2.5 GiB = 2,560 MiB). When a spec sheet just says "GB", it almost always means decimal for storage and binary for RAM (a "16 GB" memory module really is 16 GiB). Use the GB to MB converter or the complete data conversion guide for any pair.
Megabits and megabytes are not the same thing
The most expensive confusion in this whole area, because it decides whether your internet connection is doing what you paid for.
Internet speeds are sold in megabits per second (Mbps). Files are measured in megabytes (MB). There are 8 bits in a byte, so you divide by 8 to get from one to the other.
| Advertised speed | Realistic download rate | 1 GB file takes about |
|---|---|---|
| 10 Mbps | 1.25 MB/s | 13 minutes |
| 50 Mbps | 6.25 MB/s | 2.7 minutes |
| 100 Mbps | 12.5 MB/s | 80 seconds |
| 500 Mbps | 62.5 MB/s | 16 seconds |
| 1 Gbps | 125 MB/s | 8 seconds |
So a "100 Mbps" connection downloading at 12 MB/s is working correctly, not underperforming. The lowercase b means bits and the uppercase B means bytes — a one-character difference carrying a factor of eight.
How much space do things actually need?
| Item | Typical size |
|---|---|
| Plain text page | 2–5 KB |
| Photo from a phone | 2–5 MB |
| Photo, RAW format | 25–50 MB |
| Three-minute song, streaming quality | 3–5 MB |
| Hour of HD video streaming | 1–3 GB |
| Hour of 4K video streaming | 7–10 GB |
| Modern video game | 50–150 GB |
| Operating system install | 20–40 GB |
Two practical consequences. A 64 GB phone holds fewer 4K videos than people expect — under ten hours. And a "unlimited" mobile plan with a 50 GB fair-use cap is about 15 hours of HD streaming, not a month of it.
Planning storage you will not immediately outgrow
- Never fill a drive past about 85%. SSDs slow down when nearly full because they have less room to shuffle data, and Windows needs headroom for its page file and updates.
- Photos and video dominate. Documents are noise by comparison — 10,000 Word files are smaller than one 4K film.
- Backups multiply everything. A proper backup strategy needs more total space than the data, not the same amount.
- Buy for the size you will have in two years. Storage needs grow; camera resolutions and game sizes have risen steadily.
If a drive's advertised capacity does not match what your computer reports, that is the base-1000 versus base-1024 difference explained above — the drive is not faulty and you have not been short-changed.
When a file is too big for an upload limit, our guide to compressing a PDF covers what actually makes one large and how to shrink it without uploading it anywhere.
Frequently asked questions
Is a kilobyte 1,000 or 1,024 bytes?
Officially 1,000 (SI, IEC 80000-13). 1,024 bytes is a kibibyte (KiB). In everyday speech both are called "kilobyte", which is exactly the problem.
Why is my download slower than my internet plan?
Plans are sold in megabits per second, downloads are shown in megabytes per second. Divide by 8: 300 Mbps ≈ 37.5 MB/s at best.
How many photos fit on 128 GB?
At 4 MB per photo, about 30,000 (leaving some room for the system).
Sources
- IEC 80000-13:2008 — prefixes for binary multiples (kibi, mebi, gibi).
- NIST — Prefixes for binary multiples.
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