Why Hard Drives Show Less Capacity Than Advertised

Updated 8 November 2026 · Standard formula, shown below

Hard drives usually show less capacity than advertised because manufacturers count in decimal units, while many operating systems display capacity using larger binary units—or label binary units imprecisely. Formatting and system files can reduce the space available for your own files further, but the decimal-versus-binary difference is usually the main reason.

Decimal and Binary Units Explain the Difference

Storage makers use the International System of Units (SI), where each step is a multiple of 1,000. A drive advertised as 1 terabyte (TB) therefore contains 1,000,000,000,000 bytes. This is a valid decimal measurement, not a claim that the drive contains a trillion units of some larger, differently defined byte.

Computers work with binary powers of 1,024, which historically led software to use familiar names such as kilobyte, megabyte, gigabyte, and terabyte for binary quantities. The standards distinguish those binary units with different names:

1 kilobyte (kB) = 1,000 bytes; 1 kibibyte (KiB) = 1,024 bytes.
1 megabyte (MB) = 1,000,000 bytes; 1 mebibyte (MiB) = 1,048,576 bytes.
1 gigabyte (GB) = 1,000,000,000 bytes; 1 gibibyte (GiB) = 1,073,741,824 bytes.
1 terabyte (TB) = 1,000,000,000,000 bytes; 1 tebibyte (TiB) = 1,099,511,627,776 bytes.

When software divides a byte count by 1,024 at each step but calls the result “GB” or “TB,” its displayed number will be lower than the decimal figure on the package. The number of bytes has not vanished; the two figures use different-sized units.

A 1 TB Drive, Worked Through

Using the advertised decimal capacity, 1 TB is 1,000,000,000,000 bytes. Divide by 1,024 four times to express that same byte count in tebibytes:

1,000,000,000,000 ÷ 1,099,511,627,776 ≈ 0.91 TiB.

So a drive sold as 1 TB may appear as roughly 0.91 when a system reports binary capacity but uses the label “TB.” Some operating systems instead display decimal units more clearly, while others show the byte count or offer a separate capacity view. The exact rounded number depends on the interface and its conventions.

The same effect appears at other sizes. A decimal 500 GB drive works out to about 465 GiB; a 2 TB drive works out to about 1.82 TiB. Those lower-looking figures are ordinary conversions, not evidence that the manufacturer supplied a smaller drive.

Formatting and Reserved Space

Unit conversion explains the large, predictable difference, but it is not the whole story. A new drive also needs a partition and a file system before most computers can store and organize ordinary files on it. Formatting writes structures that track files and available space. These structures use some capacity, though their overhead is generally much smaller than the decimal-to-binary difference.

Other space may be reserved or used by the device or computer:

A drive may include recovery, diagnostic, or system partitions that do not appear as ordinary file storage.
Some file systems reserve a portion for bookkeeping or system use; the amount depends on the file system and configuration.
On a computer’s main drive, the operating system, updates, recovery tools, and preinstalled software occupy space after setup.
Drive-management tools may report a partition’s size, the whole device’s capacity, or available space. These are different measurements.

It helps to distinguish capacity from free space. Capacity is the size of the drive or partition; free space is what remains after formatting and saved data. A computer that reports substantially less than the expected converted capacity deserves a closer check, especially if the drive is not new or contains partitions you cannot see in a file browser.

How to Check What Your Computer Is Reporting

First, compare the byte count or the units shown in the storage settings with the advertised decimal capacity. If the displayed value is close to the expected binary conversion, the difference is probably just a unit convention. Then check whether you are looking at the full physical device, a single partition, or available space on a volume.

For an external drive, a file browser may show only a formatted volume, while a disk-management utility can show the physical device and all its partitions. On an internal system drive, the operating system’s storage view may focus on usable space and include files already installed. Do not delete an unfamiliar partition merely because its size looks like “missing” capacity; it may contain recovery or startup files.

If the numbers are far below the expected conversion, check that the drive is detected at its full size and that no partition is unallocated or hidden. If the capacity changes unexpectedly, the drive disconnects, or files become unreadable, back up any accessible data and investigate the connection or device health. A reporting difference alone does not prove that a drive is faulty.

Why Storage Devices Use Different Conventions

Decimal prefixes are standard for manufacturer capacity labels and make storage sizes easy to scale consistently: 1 TB is exactly 1,000 GB. Binary quantities fit the way digital information is addressed and grouped, where powers of two are common. Both systems are useful, but their names became blurred in everyday software and marketing, which is why two accurate displays can seem contradictory.

Operating systems and applications have not always used the same convention, and the same interface may use different units in different places. A file’s properties might use one convention, while a disk utility uses another. That can make two screens disagree even when both are describing the same underlying bytes. Look for labels such as GB versus GiB, or compare exact byte counts when precision matters.

Frequently Asked Questions

Is a hard drive advertised as 1 TB actually smaller?
Not necessarily. The advertised figure means 1,000,000,000,000 bytes. Expressed in binary tebibytes, that is about 0.91 TiB, before allowing for formatting or other reserved space.

Why does my computer call a 1 TB drive 931 GB?
Many systems divide the byte count using powers of 1,024 but display the result with the label “GB.” A decimal 1 TB is about 931 GiB, which some interfaces call 931 GB even though the unit label is technically imprecise.

Does formatting remove a lot of storage?
Formatting uses some space for file-system structures, but it usually does not explain the full difference between the package capacity and a binary-style display. The unit conversion is typically the biggest factor.

Why does my new drive show less than the expected 0.91 TiB per TB?
Check whether the display is showing free space rather than total capacity, or only one partition rather than the physical drive. Recovery partitions, formatting overhead, or an unusual partition layout can also affect the visible amount.

Do SSDs show less capacity for the same reason?
Yes. Solid-state drives and hard disk drives use the same decimal capacity convention, and computers may display either decimal or binary units for both.

Can I make the drive’s capacity match the advertised number?
You cannot change the number of bytes by changing the display setting. Some systems show decimal units or exact bytes, which can make the figure look closer to the label; that changes how capacity is expressed, not the drive itself.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.