Why Your 1TB Drive Shows 931GB: Storage Units & Binary vs Decimal Explained

The Smallest Unit: Bit

The smallest unit of information in computing is the bit , representing a single 0 or 1.

A bit alone is too small to represent useful information, so we need larger units.

Basic Unit: Byte

1 Byte = 8 bits

This is the fundamental "packaging unit" in computers. One byte can represent 256 different values (00000000 to 11111111), enough for a single English character.

Important distinction:

bit (lowercase b): the smallest unit

Byte (uppercase B): = 8 bits

This difference matters. Network bandwidth "100 Mbps" means 100 megabits per second, while download speed "10 MB/s" means 10 megabytes per second — an 8× difference. 100 Mbps theoretical download speed is actually 12.5 MB/s.

The Storage Unit Ladder

Above bytes are larger units. Key point: computers use 1024, manufacturers use 1000 .

Computer Calculation (Binary)

Inside computers, adjacent units have a 1024× relationship (2¹⁰ = 1024):

B (Byte) = 1 byte = 8 bits

KB (Kilobyte) = 1024 B ≈ 1 thousand bytes

MB (Megabyte) = 1024 KB ≈ 1 million bytes

GB (Gigabyte) = 1024 MB ≈ 1 billion bytes

TB (Terabyte) = 1024 GB ≈ 1 trillion bytes

PB (Petabyte) = 1024 TB ≈ 1 quadrillion bytes

Manufacturer Calculation (Decimal)

Drive and USB vendors use base-1000 for simplicity:

KB = 1000 B

MB = 1000 KB

GB = 1000 MB

TB = 1000 GB

Why Drives "Shrink"

This is the classic "bought drive smaller than advertised" issue.

Example: a nominal 1 TB drive:

Vendor: 1 TB = 1000 × 1000 × 1000 × 1000 = 1,000,000,000,000 bytes

OS: 1 TB = 1024 × 1024 × 1024 × 1024 = 1,099,511,627,776 bytes

Same byte count divided by 1024 appears smaller. A nominal 1 TB drive shows about 931 GB in the OS. This isn't a defect or deception — just different standards.

Common nominal vs. displayed capacities:

128 GB nominal → ~119 GB displayed

256 GB nominal → ~238 GB displayed

512 GB nominal → ~476 GB displayed

1 TB nominal → ~931 GB displayed

2 TB nominal → ~1862 GB displayed

How Big Are These Units Really?

Concrete examples:

1 KB holds:

~500 Chinese characters of plain text

A tiny, blurry icon

1 MB holds:

A short novel's text

One ordinary compressed phone photo

One low-quality MP3 song

1 GB holds:

~500 million Chinese characters

~250 MP3 songs

~500 phone photos

One 720p video episode

1 TB holds:

~500,000 photos

~250,000 songs

~500 HD movies

Several years of an average user's files

1 PB holds:

One day of data for a large internet company

~500,000 HD movies

Global daily internet data volume scale

File Size Quick Reference

Typical file sizes:

Plain text : Few KB – tens of KB

Word document : Hundreds of KB – few MB

PPT : Few MB – tens of MB

Phone photo : 2–5 MB

DSLR RAW photo : 20–50 MB

MP3 song : 3–10 MB

Lossless music : 20–50 MB

720p video (1 hour) : 1–2 GB

1080p video (1 hour) : 2–4 GB

4K video (1 hour) : 10–20 GB

Practical Storage Advice

Phone Storage

64 GB : Barely enough, frequent cleanup needed

128 GB : Sufficient for most users

256 GB : Recommended for heavy photo/app users

512 GB+ : For video recording, large file storage

Computer Drive

256 GB SSD : OS only, light users

512 GB SSD : Recommended for daily use

1 TB SSD : Gamers, content creators

2 TB+ : Video editing, large asset libraries

Final Summary

Storage units are simple: bit is smallest, Byte is basic, then KB, MB, GB, TB — each roughly 1000× larger.

Remember key numbers: 1 Byte = 8 bits, 1 KB ≈ 1000 B, 1 MB ≈ 1 million B, 1 GB ≈ 1 billion B. With this mental model you can instantly gauge any file size.