GB vs. GiB: why your new drive shows less space than the box says

You buy a 1 TB drive, plug it in, and your computer reports about 931 GB. It's natural to wonder whether you've been short-changed or the drive is faulty. You haven't, and it isn't. This is the GB vs. GiB problem: the box and your computer are simply measuring in two different systems that happen to use nearly identical names. Once you know the difference, the numbers line up perfectly.

GB vs. GiB: what is the difference?

A gigabyte (GB) is a decimal unit: 1 GB = 1,000,000,000 bytes. A gibibyte (GiB) is a binary unit: 1 GiB = 1,073,741,824 bytes, which is 2 to the power of 30. A GiB is about 7.4% bigger than a GB, and the gap widens as the units get larger.

Why does my 1 TB drive only show 931 GB?

Drive makers label capacity in decimal units, so "1 TB" means one trillion bytes. Windows, among other systems, measures in binary units but labels the result "GB". Divide one trillion bytes by 1,073,741,824 and you get about 931.3. So the "931 GB" on screen is really 931 GiB. It's the same amount of storage, described in a different unit.

A little more space is taken by the file system, and on phones and laptops by the operating system and preinstalled apps, which is why the usable space you actually get is lower still. You can convert any value with the data storage converter.

Why are there two systems?

Computers work in binary, so memory naturally comes in powers of two, like 1,024. In the early days, engineers borrowed the familiar metric prefixes because 1,024 is so close to 1,000. The difference hardly mattered at the kilobyte scale. As storage grew into gigabytes and terabytes, the gap became impossible to ignore, so in 1998 the International Electrotechnical Commission introduced the binary names kibi, mebi, gibi and tebi. Adoption has been patchy, which is why the confusion lives on.

Does Windows or macOS use GB or GiB?

Windows calculates in binary units but labels them KB, MB and GB, which is where most of the confusion comes from. macOS has reported storage in decimal units since 2009, so a 1 TB drive shows as roughly 1 TB on a Mac. Many Linux tools let you choose and often use the explicit GiB labels. Memory (RAM) is almost always sold in binary sizes, so 16 GB of RAM is effectively 16 GiB.

How to convert GB to GiB

GiB ≈ GB × 0.9313
GB ≈ GiB × 1.0737

So 500 GB is about 465.7 GiB, and 8 GiB is about 8.59 GB.

Why is my download slower than my internet speed?

This is a separate mix-up that often comes up at the same time. Internet speeds are sold in megabits per second (Mbps), while file sizes are shown in megabytes (MB). There are 8 bits in a byte, so a 100 Mbps connection moves at most about 12.5 MB per second. A 5 GB file is 40,000 megabits, so on that connection it takes at least 400 seconds, roughly 7 minutes, and usually a bit longer because of network overhead and server limits.

Do memory cards and USB drives work the same way?

Yes. Memory cards, USB sticks and SSDs are all sold using decimal capacities, so they show the same apparent shortfall on systems that display binary units. A 64 GB card typically appears as about 59.6 GiB before you've saved anything on it.

What about cloud storage and data plans?

Providers use both conventions, and they don't always say which. If the exact amount matters, for example when you're close to a data cap, check the provider's fine print. For everyday decisions, it's enough to know that binary figures look about 7% smaller at the gigabyte level and about 9% smaller at the terabyte level.

How can you see the exact size of a drive in bytes?

If you want to confirm there's nothing missing, look at the raw byte count. On Windows, right-click the drive in File Explorer and choose Properties; the capacity is shown in bytes as well as GB. On a Mac, the Get Info window shows the byte count too. Divide that number by 1,000,000,000 and you'll get the decimal figure printed on the box; divide it by 1,073,741,824 and you'll get the binary figure your system displays.

Mix-ups to watch for

• Assuming a drive is faulty because it shows less than advertised.
• Confusing bits and bytes when estimating download times.
• Using 1,000 where a system counts in 1,024, or the reverse, in technical work.
• Forgetting that the operating system and file system take space too.

For sizing a new phone or laptop, see how much storage you need. For converting other units with the same care, our guide to converting units explains the general method.