USB flash drive storage capacity explained

A question we’re often asked is “why does my USB flash drive show less storage space when plugged into a computer?”. You might even think you’ve been sold less storage capacity than was advertised!

Let’s say you’ve bought a USB flash drive advertising 32GB of storage space. However, when you go to plug it into your computer it shows the available space to be 29.2GB. What’s happened to the extra 2.8 GB of space you were expecting?

The short answer is: nothing!

A tiny fraction of advertised space is indeed unavailable due to being used by the memory stick’s hardware. But the main reason comes down to the different ways that memory device manufacturers and computer operating systems are programmed to calculate the amount of disk space to display. More specifically, the amount of bytes in a kilobyte (KB), kilobytes in a megabyte (MB), and megabytes in a gigabyte (GB).

The USB flash drive manufacturer determines that:

• 1000 bytes = 1 KB
• 1000 KB = 1 MB
• 1000 MB = 1 GB

Therefore they will say 32 GB of storage equals 32 * 1000 * 1000 * 1000 = 32,000,000,000 bytes.

If we look at the major operating systems (Windows, Mac OS, Linux), they say:

• 1024 bytes = 1 KB
• 1024 KB = 1 MB
• 1024 MB = 1 GB

So when you plug in your 32 GB USB flash drive, your operating system will display 32,000,000,000 / (1024 * 1024 * 1024) which equals roughly 29.8 GB!

What are the correct definitions of a kilobyte, megabyte and gigabyte?

Ultimately it’s the data storage manufacturers that are correct. The prefixes ‘kilo’, ‘mega’ and ‘giga’-byte all mean raising to powers of 1000 (whereas the corresponding names for powers of 1024 are actually ‘kibi’, ‘mebi’ and ‘gigi’. You may sometimes see the correct usage of these in the abbreviations ‘KiB’, ‘MiB’ and ‘GiB’ respectively).

So the next time you plug a storage device into your USB port and see a different storage capacity than you were expecting, remember this quirk of measuring disk space. In most cases it will be the explanation!