A KB (kilobyte) is a unit of file size equal to 1,000 bytes, and there are 1,000 KB in 1 MB (megabyte), so an MB is a thousand times bigger than a KB. Some software, including Windows, counts in powers of two instead, where 1 KB means 1,024 bytes and 1 MB means 1,024 of those, which is why the same file can show slightly different sizes on different devices.
KB vs MB: what is a KB and how many KB are in a MB?
Tools used in this guide
Compress PDF to SizeShrink a PDF to 100 KB, 200 KB, 500 KB or any size for online forms. No upload.Image CompressorCompress photos to a target size in KB (20, 50, 100 KB) without uploading.Increase Image Size in KBMake a photo fit a KB range like 20–50 KB: increase or reduce the file size. No upload.Unit ConverterLength, weight, volume, pressure, speed, energy, power, time, temperature, fuel economy, angles, data and land (cent, guntha, acre).
What is a KB?
A byte is the basic unit of file size, and one byte is 8 bits (IEC). "Kilo" means a thousand, as in kilometre, so a kilobyte is 1,000 bytes. The units go up in the same steps:
- 1 KB (kilobyte) = 1,000 bytes
- 1 MB (megabyte) = 1,000 KB
- 1 GB (gigabyte) = 1,000 MB
- 1 TB (terabyte) = 1,000 GB
Is KB or MB bigger?
MB is bigger. One MB is 1,000 KB, so 500 KB is half a megabyte, and a 2 MB photo is about ten times the size of a 200 KB one. From smallest to largest: byte, KB, MB, GB, TB.
How many KB are in a MB?
There are two answers, and both are in everyday use:
| Unit | Symbol | Bytes |
|---|---|---|
| kilobyte | kB or 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 |
So 1 MB = 1,000 KB in the decimal system, and 1 MiB = 1,024 KiB in the binary system.
Why are there two systems?
Computers work in binary, and 2 to the power of 10 is 1,024, very close to 1,000. Early engineers started calling 1,024 bytes a "kilobyte" for convenience. The IEC explains that the gap was only 2.4% at kilo scale, but it grows with every step, approaching 10% at tera scale (IEC).
To end the confusion, the International Electrotechnical Commission approved separate binary prefixes in December 1998: kibi (Ki) for 1,024, mebi (Mi) for 1,024², gibi (Gi) for 1,024³ and so on (NIST). They are now part of the IEC 80000-13 standard. Under the International System of Units, kilo always means 1,000, and NIST notes that the binary prefixes are not SI prefixes. In practice, few apps show KiB or MiB, so you will mostly see "KB" and "MB" used both ways.
Which one does your device use?
- Windows counts in 1,024s but labels the result KB, MB and GB. A Microsoft engineer explained on Microsoft's developer blog why File Explorer says KB rather than KiB (Microsoft). So a file of exactly 200,000 bytes shows as 195 KB in Windows.
- Mac, iPhone and iPad use decimal units today. Apple lists iOS 10 and earlier and OS X Leopard and earlier among the systems that reported sizes in binary (Apple). The same 200,000-byte file shows as 200 KB on a current Mac.
- Android's built-in size formatter has used 1,000-byte kilobytes since Android 8, and 1,024 before that (Android Developers). Individual apps can still choose either.
- Storage makers sell drives, memory cards and phones in decimal units (Apple). That's why a 1 TB drive shows as about 931 GB in Windows: 1,000,000,000,000 bytes divided by 1,073,741,824 is 931.3.
None of this changes from country to country. The units are international standards; what differs is how each app or website is programmed.
What does "under 200 KB" mean on a form?
Honestly, it varies. Most upload forms, whether for exams, visas, job applications or government services, don't say whether 200 KB means 200,000 bytes or 204,800 bytes, and each site is built differently. You can make a file that passes either way:
- For a maximum ("under 200 KB"), keep the file at or below 200 × 1,000 = 200,000 bytes. Windows shows that as 195 KB.
- For a minimum ("at least 20 KB"), make it at least 20 × 1,024 = 20,480 bytes.
To see the exact number of bytes on Windows, right-click the file and choose Properties; the size in bytes is shown in brackets.
Our Compress PDF to size, Image compressor and Increase image size tools count 1 KB as 1,024 bytes, the same as Windows. For a strict 200,000-byte limit, set the target to 195 KB. If a photo is below a minimum, Increase image size can raise it into a range such as 20–50 KB without changing how it looks.
Conversion table
| Size | Decimal (bytes) | Binary (bytes) |
|---|---|---|
| 20 KB | 20,000 | 20,480 |
| 50 KB | 50,000 | 51,200 |
| 100 KB | 100,000 | 102,400 |
| 200 KB | 200,000 | 204,800 |
| 500 KB | 500,000 | 512,000 |
| 1 MB | 1,000,000 | 1,048,576 |
| 2 MB | 2,000,000 | 2,097,152 |
| 5 MB | 5,000,000 | 5,242,880 |
| 10 MB | 10,000,000 | 10,485,760 |
| 1 GB | 1,000,000,000 | 1,073,741,824 |
Quick rules:
- KB to MB: divide by 1,000 (or 1,024). 750 KB is 0.75 MB.
- MB to KB: multiply by 1,000 (or 1,024). 2.5 MB is 2,500 KB, or 2,560 KiB.
- MB to GB: divide by 1,000 (or 1,024).
For other values, the unit converter converts between bytes, KB, MB, GB and TB. It uses the 1,024 system, like Windows.
KB, Kb and kB: bytes versus bits
A capital B means bytes and a lowercase b means bits. Data transfer, such as an internet connection, is usually measured in bits per second (IEC), so a speed of 100 Mbps (megabits per second) moves at most 12.5 MB (megabytes) per second, because there are 8 bits in a byte. Real downloads are a little slower because of network overheads.
The SI symbol for kilo is a lowercase k, as in kbit for kilobit (NIST), which is why you will also see "kB". "KB" with a capital K is more common in apps and on forms, and means the same thing.
Common mistakes
- Aiming exactly at the limit. A file that Windows shows as 200 KB is at least 204,800 bytes, which fails a form that means 200,000. Leave a small margin.
- Mixing up bits and bytes. A 50 Mbps plan does not download 50 MB each second.
- Expecting the full capacity of a new drive. Formatting and the decimal-versus-binary difference both make it look smaller in Windows.