File Size Converter

Decimal (SI) vs. Binary (IEC) Units — What's the Difference?

There are two competing standards for file size units, which causes widespread confusion — especially when buying storage devices:

Decimal / SI Units (used by hard drive manufacturers)

In the SI system, prefixes like "kilo" mean 1,000 (as they do everywhere else in science). So:

• 1 KB = 1,000 bytes
• 1 MB = 1,000,000 bytes
• 1 GB = 1,000,000,000 bytes

Hard drive manufacturers use decimal units. A drive advertised as "1 TB" contains 1,000,000,000,000 bytes.

Binary / IEC Units (used by operating systems)

Because computers work in binary (base 2), memory and storage architectures often favor powers of 2, where 2¹⁰ = 1,024 ≈ 1,000. Windows and older software used KB/MB/GB to mean binary multiples:

• 1 KiB = 1,024 bytes
• 1 MiB = 1,048,576 bytes
• 1 GiB = 1,073,741,824 bytes

The IEC introduced the "kibibyte/mebibyte/gibibyte" (KiB/MiB/GiB) naming to eliminate confusion. Modern operating systems increasingly use these IEC designations.

The "Missing Space" Problem

When Windows reports a 1 TB hard drive as having only 931 GB of space, this is not a scam — it is a unit mismatch. The drive contains 1,000,000,000,000 bytes. Windows (using binary units) divides by 1,073,741,824 per true gigabyte, yielding about 931. Use this converter to check any such discrepancy.

For compressing files to reduce their size, try the PDF compressor, image compressor, or convert images to the highly efficient AVIF format.

Common File Size Reference

• Plain text page: ~2 KB
• Average email: ~75 KB
• MP3 song (3 min): ~3–5 MB
• 720p video (1 hour): ~1–2 GB
• 4K Blu-ray movie: ~80–100 GB
• Typical smartphone (256 GB): ~238 GiB

Frequently Asked Questions

Why does my 256 GB phone only show 238 GB available?

The phone's storage uses binary units internally. 256 × 1,000,000,000 bytes ÷ 1,073,741,824 bytes/GiB ≈ 238.4 GiB. Once you subtract space reserved for the operating system and pre-installed software, you get the remaining usable space shown.

What is a bit vs. a byte?

A bit is the smallest unit of digital data — a single 0 or 1. A byte is 8 bits. Bytes are used for file sizes; bits are used for network speeds (e.g., a 100 Mbps internet connection transfers 100 megabits — or just 12.5 megabytes — per second).

How many MB is 1 GB?

In decimal (SI): 1 GB = 1,000 MB. In binary (IEC): 1 GiB = 1,024 MiB. The difference is small for everyday use, but matters when you're comparing storage specifications precisely.