KiB vs KB: Why Mac and Windows Show Different File Sizes
File sizes are written in two units, KB and KiB:
- KB = kilobyte = 1,000 bytes (decimal, base 10)
- KiB = kibibyte = 1,024 bytes (binary, base 2)
That is why Finder, which uses decimal, and File Explorer, which labels binary sizes “KB”, show the same file at different sizes.
They’re not the same unit, and the difference grows as sizes get larger. DuoBolt uses KiB, MiB, and GiB throughout the app, so every size it shows is counted in steps of 1,024 and labeled that way.
What Is a Kibibyte?
Section titled “What Is a Kibibyte?”A kibibyte (KiB) is a unit of digital information equal to 1,024 bytes. It’s part of the binary prefix system introduced by the International Electrotechnical Commission (IEC) in 1998 to resolve the long-standing ambiguity between decimal and binary interpretations of “kilobyte.”
The full set of IEC binary units:
| Prefix | Symbol | Power of 2 | Bytes |
|---|---|---|---|
| kibi | Ki | 2¹⁰ | 1,024 |
| mebi | Mi | 2²⁰ | 1,048,576 |
| gibi | Gi | 2³⁰ | 1,073,741,824 |
| tebi | Ti | 2⁴⁰ | 1,099,511,627,776 |
The “bi” in each prefix stands for “binary” — a reminder that these are base-2 measurements, unlike the metric kilo/mega/giga which use base 10.
Quick Comparison Table
Section titled “Quick Comparison Table”| Unit | Bytes | System | Where you’ll see it |
|---|---|---|---|
| 1 KB | 1,000 | SI (decimal) | Hard drives, USB sticks, network speeds, macOS Finder |
| 1 KiB | 1,024 | IEC (binary) | RAM, Windows file properties, Linux tools, DuoBolt |
| 1 MB | 1,000,000 | SI (decimal) | Storage manufacturer labels, internet speed tests |
| 1 MiB | 1,048,576 | IEC (binary) | Memory allocation, system utilities, DuoBolt |
| 1 GB | 1,000,000,000 | SI (decimal) | Drive packaging (“1 TB drive”) |
| 1 GiB | 1,073,741,824 | IEC (binary) | Actual capacity reports, DuoBolt |
Example: A file that DuoBolt reports as 100 KiB is exactly 102,400 bytes. If it were 100 KB, it would be 100,000 bytes. The difference is 2.4% — enough to matter when you’re comparing across tools.
Why the Confusion Exists
Section titled “Why the Confusion Exists”For decades, “kilobyte” meant 1,024 bytes in computing. Memory, file systems, and operating systems all used base-2 because computers work in powers of two. But storage manufacturers used 1,000 bytes — a “1 MB” hard drive had 1,000,000 bytes, not 1,048,576.
In 1998, the IEC introduced kibibyte (KiB), mebibyte (MiB), gibibyte (GiB), and tebibyte (TiB) to end the ambiguity.
Today, adoption is mixed:
- Windows labels sizes “KB” but actually reports KiB values in File Explorer
- macOS uses decimal units (KB = 1,000 bytes) in Finder since macOS 10.6
- Linux tools such as
ls -lhanddu -huse KiB but print “K” - Storage boxes always use decimal (a “2 TB” SSD is ~1.82 TiB of usable space)
Why DuoBolt Uses Binary Units
Section titled “Why DuoBolt Uses Binary Units”The i in KiB, MiB, GiB, and TiB is what tells you a size was counted in steps of 1,024, and DuoBolt prints it on every size it shows.
- On Windows it does the same arithmetic as File Explorer. The difference is the label: File Explorer writes “KB” where DuoBolt writes “KiB”.
- On a Mac it will not match Finder, which counts in steps of 1,000. DuoBolt’s 100 KiB is Finder’s 102.4 KB, and the gap grows with the unit, as the table below shows.
A size on screen is rounded to one decimal, so two files that both show as “1.0 MiB” can differ by up to about 100 KiB. That rounding never decides anything: duplicates are matched by their exact size in bytes and then by a full BLAKE3 hash of their contents.
How the Gap Grows
Section titled “How the Gap Grows”The larger the unit, the bigger the discrepancy between decimal and binary:
| Unit | Decimal bytes | Binary bytes | Difference |
|---|---|---|---|
| Kilo | 1,000 | 1,024 | 2.4% |
| Mega | 1,000,000 | 1,048,576 | 4.9% |
| Giga | 1,000,000,000 | 1,073,741,824 | 7.4% |
| Tera | 1,000,000,000,000 | 1,099,511,627,776 | 10% |
This is why a “1 TB” drive shows up as roughly 931 GiB in DuoBolt — it hasn’t lost space, the manufacturer and the OS are just using different units.
A unit mismatch is not the only way a size can mean less than it looks. On macOS two identical files can share the same blocks on disk, so deleting one of them frees nothing at all — no matter which unit the number is printed in. DuoBolt reads that from the filesystem and reports it as a separate figure: see why identical files can free no space.
Practical Tips
Section titled “Practical Tips”- Reading DuoBolt’s output: Ki = ×1,024, Mi = ×1,048,576, Gi = ×1,073,741,824
- Converting between them: multiply or divide by 1,024 at each step, not 1,000
- Comparing with Finder (macOS): Finder uses decimal. DuoBolt’s 100 KiB ≈ Finder’s 102.4 KB
- Comparing with File Explorer (Windows): Windows says “KB” but counts in KiB, the same arithmetic DuoBolt uses
- Photo libraries: a photo is rarely stored once — a RAW, its JPEG, an export and a re-save are four files of four different sizes and one picture. The photo finder groups them by what they look like rather than by what they weigh.
When in doubt, consult the byte count — bytes are unambiguous.