File Size Converter

SI decimal vs IEC binary — finally explained. All 9 units, live.

= 1,000,000,000 bytes
Decimal SI — powers of 1,000
Binary IEC — powers of 1,024
Why two systems?

1 GB (SI) = 1,000,000,000 bytes. 1 GiB (IEC) = 1,073,741,824 bytes. Hard drive manufacturers use SI (powers of 1,000). Operating systems like Windows use IEC (powers of 1,024).

Example: a 1 TB drive (SI) shows as 931 GiB in Windows. No data is missing — it's purely a display difference. Standard: IEC 60027-2.

How the File Size Converter Works

Enter any value, select its unit, and all 9 units update simultaneously. The tool never rounds the intermediate bytes value — it converts to exact bytes first, then divides by the target unit's factor. This means you see the full precision of each conversion.

  • SI units (KB, MB, GB, TB) — use powers of 1,000. 1 KB = 1,000 bytes. Used by storage manufacturers and network specs.
  • IEC units (KiB, MiB, GiB, TiB) — use powers of 1,024. 1 KiB = 1,024 bytes. Used by operating systems internally. Defined in IEC 60027-2.

The IEC Standard and Why It Matters

The IEC 60027-2 standard (published 1998, updated 2000) introduced binary prefixes — kibi, mebi, gibi, tebi — precisely to end the ambiguity between powers-of-1000 and powers-of-1024 unit names. Before this standard, "kilobyte" was ambiguous: it sometimes meant 1,000 bytes (SI) and sometimes 1,024 bytes (computing). The confusion is especially visible at larger scales: a "1 TB" hard drive advertised by a manufacturer holds 1,000,000,000,000 bytes (SI). Windows reports this as roughly 931 GiB (IEC) because the OS divides by 1,024³, not 1,000³. No data is missing — it is purely a labeling difference.

In practice: network speeds (Mbps, Gbps) always use SI (powers of 1,000). RAM is traditionally sold and reported in IEC binary multiples. SSD and HDD manufacturers use SI. macOS switched from IEC to SI reporting in macOS 10.6 (Snow Leopard, 2009), so it now agrees with the label on the box. Linux and Windows still report in IEC.

All 9 units, one grid, copy on click

Most file size tools show you one result at a time. This one shows all 9 simultaneously — 5 SI and 4 IEC — in two clearly labelled sections. Click any unit card to copy the value and unit to your clipboard. The active input unit is highlighted so you always know which direction you started from.

What isn't here: a file size estimator for images (which depends on format, bit depth, and compression), network transfer time calculations, or storage cost calculators. Those require additional inputs and belong in separate tools.

Frequently Asked Questions

Why does my 1 TB hard drive show as 931 GB in Windows?
The manufacturer labels it as 1 TB using SI (1,000,000,000,000 bytes). Windows reports in GiB (IEC), dividing by 1,024³ = 1,073,741,824 bytes, which gives 931.32 GiB. No data is missing — it is purely a labeling difference between SI and IEC conventions.
What is the difference between GB and GiB?
GB (gigabyte, SI) = 1,000,000,000 bytes. GiB (gibibyte, IEC) = 1,073,741,824 bytes. The difference is about 7.4%. The IEC binary prefixes (KiB, MiB, GiB, TiB) were introduced by IEC standard 60027-2 to eliminate this ambiguity.
Are network speeds in SI or IEC?
Network speeds (Mbps, Gbps) always use SI — powers of 1,000. A 100 Mbps connection transfers 100,000,000 bits per second, or 12,500,000 bytes per second (12.5 MB/s SI). This is why downloading a 1 GB file on a 100 Mbps line takes about 80 seconds, not 71 seconds.
Which operating systems use IEC vs SI?
Windows and Linux report file sizes in IEC binary units (GiB) but label them as 'GB' — which is technically incorrect. macOS switched to correct SI labeling in Snow Leopard (2009), so a 1 TB drive shows as ~1 TB in macOS but ~931 GB in Windows.
What standard defines binary prefixes?
IEC 60027-2 (International Electrotechnical Commission), first published in 1998 and updated in 2000. It defines kibi (2¹⁰), mebi (2²⁰), gibi (2³⁰), and tebi (2⁴⁰) prefixes to unambiguously represent powers of 1,024.