💾 💾 Data Size Converter: Bits, Bytes, KB, MB, GB, TB Explained

Digital storage is measured in bits and bytes, but the prefixes (kilo, mega, giga, tera) create persistent confusion because they can mean different things depending on context. Operating systems, hard drive manufacturers, and network equipment all use slightly different conventions. Understanding these differences explains why a "1 TB" hard drive shows as 931 GB in Windows.

Bits and Bytes: The Foundation

• Bit (b): The smallest unit of digital information — a 0 or 1
• Byte (B): 8 bits. The standard unit for file sizes and storage capacity
• Nibble: 4 bits (half a byte) — used in some low-level computing contexts

Important: bits (lowercase b) are used for network speeds (Mbps = megabits per second). Bytes (uppercase B) are used for file sizes and storage (MB = megabytes). A 100 Mbps internet connection transfers 100 megabits = 12.5 megabytes per second.

The Two Prefix Systems

This is the root of all data size confusion. Two systems exist:

Decimal (SI) prefixes — used by storage manufacturers

• 1 kilobyte (KB) = 1,000 bytes (10³)
• 1 megabyte (MB) = 1,000,000 bytes (10⁶)
• 1 gigabyte (GB) = 1,000,000,000 bytes (10⁹)
• 1 terabyte (TB) = 1,000,000,000,000 bytes (10¹²)

Binary prefixes — used by operating systems

• 1 kibibyte (KiB) = 1,024 bytes (2¹⁰)
• 1 mebibyte (MiB) = 1,048,576 bytes (2²⁰)
• 1 gibibyte (GiB) = 1,073,741,824 bytes (2³⁰)
• 1 tebibyte (TiB) = 1,099,511,627,776 bytes (2⁴⁰)

Conversion Table

Why a 1 TB Drive Shows as 931 GB in Windows

Hard drive manufacturers advertise capacity using decimal prefixes (1 TB = 1,000,000,000,000 bytes). Windows historically displays storage in binary GiB but labels them "GB." So Windows reads 1,000,000,000,000 bytes ÷ 1,073,741,824 = 931.32 GiB and displays it as "931 GB." No storage is missing — it's a labeling difference between decimal and binary interpretations of the same prefix.

macOS switched to using decimal GB in macOS 10.6 (Snow Leopard, 2009), so a 1 TB drive shows as approximately 1 TB on a Mac — matching the manufacturer's label.

Network Speed vs File Size

Internet speed is measured in megabits per second (Mbps), while file sizes are in megabytes (MB). To convert download speed to file-transfer rate:

MB/s = Mbps ÷ 8

• 100 Mbps connection → 12.5 MB/s actual transfer speed
• 1 Gbps connection → 125 MB/s actual transfer speed
• A 4 GB file on a 100 Mbps connection: 4,000 MB ÷ 12.5 MB/s ≈ 320 seconds ≈ 5.3 minutes

Real-World Size Reference

• 1 byte: One ASCII character
• 1 KB: A short text email
• 1 MB: A high-quality JPEG photo; about 1 minute of MP3 audio
• 1 GB: About 250 high-quality photos; 1 hour of HD video (compressed)
• 1 TB: About 250,000 photos; 500 hours of HD video
• 1 PB (petabyte): 1,000 TB — the scale of large data centers