Two competing definitions of 'kilobyte' have coexisted for decades
Buy a '1TB' hard drive and plug it into your computer, and the operating system will typically report something closer to 931GB of usable space. This isn't a manufacturing shortfall or a scam — it's two legitimate but incompatible definitions of the same unit colliding. Storage manufacturers use decimal SI units, where 1 kilobyte means exactly 1000 bytes, matching how every other metric prefix works (a kilometer is 1000 meters, no exceptions). Operating systems and file explorers have traditionally used binary units instead, where 1 'KB' actually means 1024 bytes, because 1024 is a power of two (2^10) and computers fundamentally operate in binary.
This tool follows the binary, 1024-based interpretation — the same convention most file explorers use when they label something 'KB', 'MB', or 'GB' — which is why its numbers will match what you see in Windows Explorer or macOS Finder rather than what a storage manufacturer's marketing materials advertise.
Binary units and their byte counts
| Unit | Bytes | Power of 1024 |
|---|---|---|
| KB | 1,024 | 1024¹ |
| MB | 1,048,576 | 1024² |
| GB | 1,073,741,824 | 1024³ |
| TB | 1,099,511,627,776 | 1024⁴ |
| PB | 1,125,899,906,842,624 | 1024⁵ |
Converting a file size
Type the numeric value of your file size.
Pick the unit: B, KB, MB, GB, TB, or PB.
Read the equivalent value in every other unit, shown together in one table.
Why the KB/KiB naming split exists, and why it's mostly ignored in practice
Technically, there's an official fix for this ambiguity: KiB (kibibyte), MiB (mebibyte), and so on are defined specifically as the 1024-based binary units, leaving KB, MB, and GB to mean their strict decimal (1000-based) SI values. This distinction is precise and unambiguous on paper — but in everyday use, almost nobody actually uses 'KiB' outside of specifications and some Linux tooling. Most file explorers, most software, and most everyday conversation still say 'KB' while meaning the 1024-based value, which is exactly the convention this tool follows to match what you'll actually see reported elsewhere on your system.
Converting between the two systems if you need to
Precision: exact math, limited display
The underlying conversion math is exact — no rounding happens during calculation itself, only when the result is formatted for display, which shows up to 6 significant digits. This matters for anyone chaining conversions or doing further math with the output: the displayed number is a faithful rounded representation of an exact underlying value, not an accumulation of rounding error from earlier steps in the conversion.
Common mistakes
Assuming a drive's reported capacity in an OS should exactly match its advertised marketing capacity — the gap is the binary-vs-decimal unit difference, not missing storage.
Using KB and KiB interchangeably in technical writing when precision matters — they mean different byte counts, even though everyday usage often blurs them.
Manually converting between binary and decimal units with a rough estimate instead of the exact 1.024 multiplier, compounding small errors across large values.
Assuming this tool's results will match a manufacturer's decimal-based spec sheet directly — they represent the same byte count differently, not different amounts of data.
Real use cases
Figuring out how many photos or videos of a known file size will fit on a drive with a specific reported capacity.
Converting a file size mentioned in a document or spec sheet into the unit your particular tool or system expects.
Understanding why a purchased drive's usable space differs from its advertised capacity.
Quickly checking the byte-level size of a value expressed in an unfamiliar unit like PB or TB.
Frequently asked questions
Q: Why 1024 not 1000?
A: Operating systems traditionally report file sizes in binary units, where 1 KB equals 1024 bytes. Decimal SI units, where 1 KB equals 1000 bytes, are also valid but typically used for storage marketing; multiply by 1.024 to convert between the two.
Q: What's the difference between KB and KiB?
A: Technically, KB now officially means 1000 bytes (decimal) while KiB means 1024 bytes (binary). This tool uses the 1024-based binary interpretation, matching what most file explorers display as 'KB'.
Q: How precise is the result?
A: Values are displayed with up to 6 significant digits. The underlying math is exact, with no rounding applied until the final display step.
Q: Can I copy a single conversion?
A: Yes — each row has a Copy button that copies the value along with its unit.
Q: What's the largest size supported?
A: Up to petabytes (PB equals 1024⁵ bytes, roughly 1.13 × 10¹⁵ bytes). Beyond that, results are shown in scientific notation.
Q: If I have exactly 1,000,000 bytes, why doesn't that equal exactly 1MB in this tool?
A: Because this tool uses the binary definition, where 1MB equals 1,048,576 bytes (1024²), not 1,000,000. 1,000,000 bytes converts to slightly less than 1MB under the binary convention, even though it would equal exactly 1MB under the decimal SI convention.
Convert a file size now
Try the File Size Converter. Need a different kind of unit conversion? Check the Unit Converter. Working with time zones or aspect ratios instead? See the Timezone Converter or Aspect Ratio Calculator.
