Data Size Converter (Bytes, KB, MB, GB, TB)
Enter a byte count to convert it into all common units — KB, MB, GB, TB, and more — at once. Shows both the decimal (SI, base-1000) and binary (IEC, base-1024) systems side by side, and explains why file size displays don't always match across apps.
Decimal (SI)
Binary (IEC)
Why "file size" displays don't match across apps
Units like KB, MB, and GB actually come in two systems: "decimal (SI)," which advances by 1000, and "binary (IEC)," which advances by 1024. The same byte count produces different displayed numbers depending on which system is used, which is exactly why "the file size shown doesn't match" confusion happens between different OSes and apps.
| Unit | Bytes in decimal (SI) | Bytes in binary (IEC) |
|---|---|---|
| KB / KiB | 1,000 B | 1,024 B |
| MB / MiB | 1,000,000 B | 1,048,576 B |
| GB / GiB | 1,000,000,000 B | 1,073,741,824 B |
| TB / TiB | 1,000,000,000,000 B | 1,099,511,627,776 B |
Tips
- macOS Finder uses decimal (SI) notation, while Windows Explorer keeps labels like GB and MB but actually calculates in binary (IEC) under the hood — so the same file can show a different size number depending on the OS.
- Hard drive and SSD manufacturers commonly use decimal (SI) notation for advertised capacity, which is why a drive can appear to have "less capacity than advertised" once your OS reports it — that's not actually missing storage, just a difference in how the units are calculated.
- Network speeds (bps) are generally calculated in decimal (1 Mbps = 1,000,000 bps), so naively estimating download time as "file size (often IEC) ÷ line speed (SI)" introduces some error worth keeping in mind.
- You can select a unit and enter a value in the input field, so this also works directly for converting between units — e.g. "how many MiB is 5 GB?" or "how many MB is 500 MiB?"
Frequently Asked Questions
Side Note — why "KiB" was born in 1998
The convention of "1 KB = 1024 bytes" took hold in computing because 2 to the 10th power (1024) was a natural, round number for machines built on binary. But under the International System of Units (SI), the prefix "kilo" strictly means a factor of 1000, so this convention was at odds with the actual definition of the SI prefix all along.
This contradiction became especially visible around hard drive capacity labeling. Drive manufacturers strictly followed SI, advertising "1 GB" as one billion bytes, while operating systems calculated 1 GB as roughly 1.074 billion bytes (1024 cubed) — leading to a steady stream of consumer complaints that a purchased drive showed "less capacity than advertised" once viewed in the OS. In reality, no capacity was actually missing; it was purely a difference in calculation method.
To resolve this confusion, the International Electrotechnical Commission (IEC) defined new prefixes dedicated to binary (base-1024) counting in 1998 — Kibi (Ki), Mebi (Mi), Gibi (Gi), and so on. In theory, this cleanly separated the roles: "KB" strictly follows SI's base-1000, while "KiB" denotes binary's base-1024. In practice, though, the habit of using "KB" to mean base-1024 remains deeply entrenched among everyday users to this day.