File Size Converter

Convert file sizes between Bytes, KB, MB, GB, and TB instantly. I built this after my student asked "Why does my 64GB USB only show 59GB?" This tool shows both binary (what your computer uses) and decimal (what manufacturers use) conversions.

Convert File Sizes

Quick Reference

Binary (1024-based):

  • 1 KB = 1,024 Bytes
  • 1 MB = 1,024 KB = 1,048,576 Bytes
  • 1 GB = 1,024 MB = 1,073,741,824 Bytes
  • 1 TB = 1,024 GB = 1,099,511,627,776 Bytes

Decimal (1000-based):

  • 1 KB = 1,000 Bytes
  • 1 MB = 1,000 KB = 1,000,000 Bytes
  • 1 GB = 1,000 MB = 1,000,000,000 Bytes
  • 1 TB = 1,000 GB = 1,000,000,000,000 Bytes

Why Your "1TB" Hard Drive Shows Only 931GB (I Tested This)

Last week, a student emailed me: "I bought a 1TB Samsung SSD, but Windows shows only 931GB. Did I get scammed?" Nope. Here's what's actually happening.

There are two different systems for measuring file sizes: binary (used by computers) and decimal (used by storage manufacturers). Neither is wrong—they're just different math systems.

Binary (1024-based): What Your Computer Uses

Computers work in binary (base-2), so they calculate storage using powers of 2. In this system:

  • 1 KB = 1,024 Bytes (not 1,000)
  • 1 MB = 1,024 KB = 1,048,576 Bytes
  • 1 GB = 1,024 MB = 1,073,741,824 Bytes
  • 1 TB = 1,024 GB = 1,099,511,627,776 Bytes

Real Test: I plugged in a "1TB" external drive. Windows showed 931GB. I did the math: 1,000,000,000,000 bytes (what the manufacturer calls "1TB") ÷ 1,073,741,824 (what Windows calls "1GB") = 931GB. Mystery solved.

Decimal (1000-based): What Manufacturers Use

Storage companies use the International System of Units (SI), which is based on powers of 10:

  • 1 KB = 1,000 Bytes
  • 1 MB = 1,000 KB = 1,000,000 Bytes
  • 1 GB = 1,000 MB = 1,000,000,000 Bytes
  • 1 TB = 1,000 GB = 1,000,000,000,000 Bytes

Why Manufacturers Do This: It's not a scam. The SI system is the international standard for measurements (like meters and kilograms). Hard drive companies follow this standard. Your computer just uses different math.

Real-World File Sizes (From My Computer Right Now)

  • My Thesis (50 pages, plain text): 87 KB
  • Same Thesis (Word doc with images): 2.3 MB
  • High-res photo from my phone: 4.8 MB
  • 3-minute MP3 song: 4.2 MB
  • 1-hour Zoom recording (1080p): 1.8 GB
  • Call of Duty: Modern Warfare (game install): 231 GB

When to Use Binary vs Decimal

Use Binary (1024-based) when:

  • Checking storage space in Windows, macOS, or Linux
  • Calculating RAM sizes (8GB RAM = 8,192 MB)
  • Working with computer memory or cache

Use Decimal (1000-based) when:

  • Reading hard drive or SSD specifications
  • Comparing storage products (all manufacturers use decimal)
  • Calculating internet speeds (100 Mbps = megabits per second, decimal)

FAQ: Questions My Students Always Ask

Q: Is this false advertising?
A: No. Manufacturers clearly state capacity in decimal gigabytes. It's just a different measurement system, like Celsius vs Fahrenheit.

Q: Can I make Windows show decimal sizes?
A: Not easily. Windows is hardcoded to use binary. macOS switched to decimal in 2009, which is why a "1TB" drive shows as "1TB" on Mac but "931GB" on Windows.

Q: Which system is "correct"?
A: Both. Binary makes sense for computers (base-2 math). Decimal makes sense for humans (base-10 math). Use whichever matches your context.

Pro Tip: When buying storage, assume you'll get about 93% of the advertised capacity in Windows. A "500GB" drive will show ~465GB. A "2TB" drive will show ~1.86TB.