Kilobytes, Megabytes and Why Your 10 MB File Shows as 9.5 MB
Updated · by the My Gadgets team
Two different "megabytes" are in everyday use: 1,000,000 bytes, and 1,048,576 bytes (1,024 × 1,024). A file of 10,000,000 bytes is 10 MB by the first measure and about 9.54 by the second, which Windows displays as 9.53 MB. Nothing is missing; the same bytes are just being counted in different units. This explains why Windows and a Mac show different sizes for the same file, why a new 1 TB drive shows about 931 GB, and how to be sure a file fits an upload limit.
Why are there two kinds of megabyte?
Computers count in powers of two, and 2¹⁰ = 1,024 is close to 1,000, so early on "kilobyte" came to mean 1,024 bytes. Everywhere else in science, "kilo" means exactly 1,000. To end the confusion, standards bodies defined separate names for the binary units in 1998:
| Decimal (SI) | Bytes | Binary (IEC) | Bytes |
|---|---|---|---|
| 1 kilobyte (kB) | 1,000 | 1 kibibyte (KiB) | 1,024 |
| 1 megabyte (MB) | 1,000,000 | 1 mebibyte (MiB) | 1,048,576 |
| 1 gigabyte (GB) | 1,000,000,000 | 1 gibibyte (GiB) | 1,073,741,824 |
| 1 terabyte (TB) | 10¹² | 1 tebibyte (TiB) | 1,099,511,627,776 |
The gap grows with size: about 2.4 % at kilobytes, 4.9 % at megabytes, 7.4 % at gigabytes and 10 % at terabytes.
Who uses which?
At the time of writing:
| Where | Unit used | A 10,000,000-byte file shows as |
|---|---|---|
| Windows File Explorer | Binary, but labelled "KB", "MB", "GB" | 9.53 MB (or 9,766 KB) |
| macOS Finder (since 2009) | Decimal | 10 MB |
| iPhone and most Android phones | Decimal | 10 MB |
| Hard drive and memory card makers | Decimal | — |
| Linux | Varies by program; many show KiB/MiB | 9.5 MiB or 10 MB |
| Internet speeds (Mbps) | Decimal, in bits | — |
So the same photo can be "10 MB" on your phone and "9.53 MB" on your Windows PC.
Why does my 1 TB drive show 931 GB?
The maker counts a terabyte as 1,000,000,000,000 bytes (decimal). Windows divides that by 1,024³ and labels the result "GB": 10¹² ÷ 1,073,741,824 = 931.3. Nothing is missing.
A brand-new drive or phone also shows less free space than its size because of the file system and, on phones, the operating system and preinstalled apps. That is a separate, real loss; the 931 figure is just a unit difference.
Check any of these conversions with Unit Converter: under Data Size it has both decimal units (kB, MB, GB, TB) and binary ones (KiB, MiB, GiB, TiB), labelled with what Windows calls each.
Will my file fit under an upload limit?
This is where the confusion actually costs people. A form that says "max 2 MB" might mean 2,000,000 bytes or 2,097,152 bytes, and it rarely says which.
The safe rule: compare the size in bytes, and treat the limit as decimal (the smaller number).
- Find the exact size in bytes:
- Windows: right-click the file → Properties. The line Size shows the bytes in brackets.
- Mac: select it and press Cmd + I; Size shows the bytes.
- Anywhere: open it in File Inspector, which shows the size, the real file type and its first bytes.
- Compare with the limit × 1,000,000 for MB (or × 1,000 for kB).
- If it's close, aim 5–10 % under. Some systems also add a little overhead when the file is uploaded as part of a form.
| Limit says | Keep the file under (bytes) |
|---|---|
| 240 KB | 240,000 |
| 500 KB | 500,000 |
| 1 MB | 1,000,000 |
| 2 MB | 2,000,000 |
| 5 MB | 5,000,000 |
| 10 MB | 10,000,000 |
If a file is over, Compress Image and Compress PDF shrink it; why your PDF is so large explains what to try first.
Bits and bytes: reading internet speeds
Internet speeds are in bits per second (Mbps, with a lowercase b), and file sizes in bytes (MB, capital B). One byte is eight bits, so:
- 100 Mbps ÷ 8 = 12.5 MB per second at best.
- A 1 GB download on a 100 Mbps line takes about 80 seconds in ideal conditions, usually longer in practice.
Speeds are decimal: 100 Mbps is 100,000,000 bits per second. Unit Converter converts megabits to megabytes directly.
A note on "KB" vs "kB"
Strictly, the decimal kilobyte is kB (lowercase k, as in kilogram) and the binary one is KiB. In practice you'll see "KB" used for both. When precision matters, as in limits, specifications and contracts, say the number of bytes.
Quick answers
Is a file of 1,024 KB one megabyte? In Windows' binary units, yes (1 MiB). In decimal units it's 1.048576 MB.
Why does my phone say a photo is 3.1 MB but my PC says 2.95 MB? Same file, different units: the phone counts decimal megabytes, Windows binary ones.
Which unit do websites use for upload limits? It varies and is rarely stated. Assume decimal and stay a little under.
Does compressing change the bytes or just the units? Compression really changes the number of bytes. Unit differences only change how the same number is displayed.
What to do next
Check a file's exact size with File Inspector, convert between units with Unit Converter, and shrink anything over a limit with Compress Image or Compress PDF.