So, you just bought a “1 TB” SSD. You plug it into your computer, open up your file explorer, and only 931 GB shows up.

First thought?
“Where did the rest of my storage go?” 🤯

If so, you’re not alone, and there’s nothing wrong with your drive. It all comes down to two different ways of measuring digital storage: SI units and IEC units. In this article, we’ll explain what those are, how they differ, why both systems exist, and what it means for you in real life.

What Are SI and IEC Units?

When we talk about data—like kilobytes (KB), megabytes (MB), or gigabytes (GB), we’re actually talking about how many bytes something takes up. A byte is the basic unit of digital information. Everything on your computer—files, apps, videos, and photos, is stored in bytes.

But here’s the complex part: there are two different systems used to define what these larger units mean.

The SI (Decimal) System

SI stands for Système International, or the International System of Units. It’s the same system we use to measure things like kilograms, kilometers, and liters.

SI uses powers of 10, which makes the math easy to understand.

Unit	Stands for	Bytes
1 kB	kilobyte	1,000 bytes
1 MB	megabyte	1,000,000 bytes
1 GB	gigabyte	1,000,000,000 bytes
1 TB	terabyte	1,000,000,000,000 bytes

Most storage device manufacturers (like hard drive and SSD makers) use SI units.

The IEC (Binary) System

Computers don’t think in 10s. They work in binary, using powers of 2. That means memory and storage inside your computer is often based on powers of 2.

To avoid confusion, the International Electrotechnical Commission (IEC) introduced a new system in 1998 that uses different names for binary measurements. These names end in “-bibyte” to show they’re based on binary, not decimal.

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

These binary units are used more accurately by computers, operating systems, and programmers.

Why Is This Confusing?

Here’s the problem:

• When a storage company says “1 TB,” they mean 1,000,000,000,000 bytes (SI).
• When your computer reads that number, it often divides by 1,073,741,824 bytes (1 GiB) instead of 1,000,000,000, giving you 931 GiB.
• Your operating system then calls it “931 GB” even though it’s really 931 GiB, not 931 billion bytes. (Microsoft Windows)

This makes it look like you’ve lost space—but you haven’t. It’s just a difference in how things are counted.

Who Uses Which System?

Device or System	Uses This System	Notes
Hard drives & SSDs	SI (decimal)	1 TB = 1,000,000,000,000 bytes
USB drives & SD cards	SI (decimal)	Same as storage drives
RAM (memory)	Binary (but labeled with SI terms)	8 GB RAM = 8 GiB
Windows	Binary (IEC), but calls it “GB”	1 GB = 1 GiB (2³⁰ bytes)
macOS and branches (iOS, tvOS)	Decimal (SI)	1 GB = 1,000,000,000 bytes
Linux	Mixed	Different commands use different systems

So, depending on what you’re looking at, 1 GB might mean different things.

Real-Life Example: 1 TB Drive

You buy a 1 TB SSD. The manufacturer means:
1 TB = 1,000,000,000,000 bytes (SI)

Your computer reads this and divides by:
1 GiB = 1,073,741,824 bytes (IEC)

So,
1,000,000,000,000 ÷ 1,073,741,824 ≈ 931 GiB

Your system shows: 931 GB (but actually, GiB)

Nothing is broken. It’s just two different measurement systems being used.

What About RAM?

RAM (Random Access Memory) is a special case.

• RAM chips are built in binary, so 8 GB of RAM = 8 × 2³⁰ = 8,589,934,592 bytes
• But it’s usually labeled as “8 GB” even though it should be called “8 GiB”

How to Avoid Confusion

Here are some easy tips to stay on top of this:
It’s normal for your 1 TB drive to show 931 GB. Nothing is wrong.

Check what system is being used.

OS (especially MS Windows) file managers might show sizes in IEC (but call them GB)
Marketing labels are always SI (especially on storage devices)

Use IEC units in code or technical settings.
If you’re calculating memory or disk usage in a program, use the exact values.

Label clearly.
If you’re writing documentation or software, you should show both formats like this: “512 GB (476 GiB) total storage”

Final Thoughts

The difference between SI and IEC units has been around for decades. It started with good intentions, but today it causes confusion for many users. Still, once you understand the difference, it becomes much easier to work with storage, memory, and file sizes without surprises.

So just remember:

• SI = base 10, used in marketing
• IEC = base 2, used by computers
• Always check the context

And next time someone asks, “Why is my drive smaller than advertised?”, you will know exactly what to say.