Why your 1 TB SSD shows 931 GB in Windows

You install a 1 TB SSD, open Windows and see roughly 931 GB. It looks as though a sizeable chunk of the drive has vanished before you have copied anything onto it.

A 1 TB drive showing about 931 GB in Windows is usually normal. The manufacturer’s terabyte and the binary units used in familiar Windows capacity displays count the same bytes differently. Intel’s 2022 explanation of SSD capacity gives this exact 1 TB-to-931.32 example.

That explains the familiar number. It does not explain every smaller reading: the capacity of C:, the size of the whole disk and the space still free for your files are different things.

This guide explains the distinction and the checks you can make in Windows 10 and Windows 11, without changing the drive’s layout.

Where the 931 GB figure comes from

Storage manufacturers normally use decimal units. A gigabyte, or GB, is one billion bytes; a terabyte, or TB, is one trillion bytes. For a concrete example, Samsung’s June 2021 datasheet for the 970 EVO Plus explicitly describes its 1 TB model as 1,000 GB.

The binary unit with a similar name is a gibibyte, written GiB. NIST’s unit definitions distinguish the two:

• 1 GB = 1,000,000,000 bytes
• 1 GiB = 1,073,741,824 bytes

Because a GiB is larger, you need fewer of them to describe the same quantity of data. Divide the advertised byte count by the size of one GiB:

1,000,000,000,000 ÷ 1,073,741,824 ≈ 931.32 GiB

The confusing part is that familiar Windows displays label this binary calculation “GB”. Here, we use GiB for the arithmetic so the units stay clear.

The conversion itself removes no bytes. Subtracting 931 from 1,000 and calling the difference “missing gigabytes” mixes two different units.

Common drive capacities, converted

These are calculated equivalents of the advertised decimal capacities, rounded to two decimal places. They are a comparison baseline, not a promise of the exact formatted capacity or free-space reading on every drive.

Notice that 500 GB and 512 GB are different advertised capacities. Comparing both with the same expected Windows reading would create a discrepancy that the drives never had.

A model’s exact reported byte count, its partition layout and file-system bookkeeping can also affect the final readings. The conversion table is the starting point for checking them.

Disk capacity, volume capacity and free space

The drive letter you click in File Explorer does not necessarily represent the entire physical SSD.

Disk capacity is the size Windows reports for the selected disk. On a straightforward PC with an ordinary internal SSD, this is the whole device as exposed to Windows.

Volume capacity is the size of a particular file-storage area, such as C:. A disk can contain several partitions, and its volumes can appear under different drive letters. Microsoft’s explanation of disks and partitions describes how one physical disk can appear as multiple drives.

Free space is the room currently available within that volume. Windows distinguishes this from its total size; the two are separate properties in Microsoft’s logical-disk documentation .

A Windows installation can also have separate system and recovery partitions that do not appear as ordinary folders under C:. Microsoft’s partition-layout guidance explains those roles. Their space is a separate consideration from converting TB to GiB.

A worked example: why 300 GB free can be perfectly consistent

Suppose a drive sold as 1 TB reports about 931.3 GiB overall. One data volume has a capacity of 900 GiB, with 600 GiB used and 300 GiB free.

This is an illustrative example with simplified figures, not a measurement from a particular SSD.

The four readings tell a coherent story:

1. 1 TB on the label describes the advertised capacity in decimal units.
2. About 931.3 GiB for the disk expresses that scale of capacity in binary units.
3. 900 GiB for the volume tells you how much belongs to that particular storage area.
4. 300 GiB free tells you what remains available within it.

The roughly 31.3 GiB outside that volume calls for a look at the disk layout. Other partitions, unallocated space and layout details may account for it. You cannot identify its purpose from the subtraction alone.

If you remove 100 GiB of files and that space is actually released, the volume would have 400 GiB free. Its capacity would still be 900 GiB, and the disk’s reported capacity would remain about 931.3 GiB.

That is why a cleanup can increase free space while leaving the drive’s total-capacity figure unchanged.

Check the drive in Windows

Start by recording which number concerns you. There is no need to format a drive to investigate a capacity reading.

1. Read the volume’s Properties

In File Explorer, open This PC, right-click the relevant drive and choose Properties. On the General tab, note Capacity, Used space and Free space.

Look at the byte counts as well as the rounded GB values. Bytes provide a common unit when two displays seem to disagree.

If the concern is “300 GB free of 900 GB”, you are looking at the free space within a volume. You have not yet established the size of the entire disk.

2. Look at the whole disk and its partitions

Press Windows + R, enter diskmgmt.msc and press Enter to open Disk Management .

In the lower pane, find the row containing the drive letter you checked. Read the disk’s total size beside its Disk 0, Disk 1 or other disk label, then inspect the blocks across that same row.

Look for other volumes, system or recovery partitions, and areas labelled Unallocated. A smaller C: volume can be one part of a correctly sized disk.

At this stage, just inspect the layout. If an unexpected prompt asks you to initialise or format a drive that already contains files, cancel it. Deleting an unfamiliar partition to make the numbers look closer can remove data or files needed to start or recover Windows.

3. Use a byte count if the rounded figures remain confusing

For an optional numerical check, open Windows PowerShell as an administrator on your own PC and run:

Get-Disk | Select-Object Number, FriendlyName, Size

The command displays information; it does not format or resize a disk. Microsoft’s Get-Disk reference documents the disk query, and its MSFT_Disk reference defines Size as the total disk size in bytes.

Match FriendlyName and Number to the device you are checking. A size around 1,000,000,000,000 bytes is consistent with a 1 TB device, even when another view shows about 931 GB. Compare an exact result with the specification for that model.

This optional command is intended for ordinary basic disks; Get-Disk does not list dynamic disks. On systems using virtual disks or storage pools, the device presented to Windows may not correspond to one physical SSD.

What your result means

Use the whole-disk reading first, then the volume and free-space readings:

• The disk is around 931 GiB and was sold as 1 TB. The unit conversion explains the headline difference. That reading alone is not evidence that capacity has disappeared.
• The disk is around 931 GiB, but C: is much smaller. Inspect the other partitions and unallocated space. The remaining capacity may be outside C:. Any later resizing needs a plan based on the actual layout and a backup.
• The volume has the expected capacity, but little free space. Investigate what is stored within it. Removing files addresses used space; it cannot change how GB and GiB relate.
• The whole disk reports roughly 465 GiB despite a 1 TB label. That is closer to the conversion for 500 GB. Confirm you selected the correct device and check its exact model against the manufacturer’s specification. A substantial whole-disk mismatch needs further investigation.

A plausible capacity reading does not establish that every part of a drive works or that the product is genuine. If files are corrupting, the drive disconnects or errors recur, those symptoms deserve their own diagnosis. Repeated formatting is not a useful capacity test.

Is the difference SSD over-provisioning?

SSDs can reserve flash for their controller to use internally, a mechanism called over-provisioning. Kingston’s March 2019 explanation describes factory-reserved capacity that is unavailable to the user and invisible to the host operating system.

That distinguishes the flash fitted to a device from the capacity it exposes for storage. It is a separate issue from expressing the advertised capacity in GB or GiB.

For the specific 1 TB-to-about-931 GiB comparison, the unit conversion already explains the numbers. Do not subtract an assumed reserve percentage a second time, or assume every SSD model reserves the same amount.

Compare the whole-disk capacity with the model’s specification in consistent units. If that matches but C: looks smaller or has little free space, inspect the partition layout and the space used within that volume. Follow the number that concerns you before changing anything.

Sources and further reading

• NIST: Prefixes for binary multiples , including the GB/GiB definitions and the IEC standards published in 1999 and 2000.
• Intel: SSD Advertised Size vs. Operating Systems Reported Size , last reviewed 8 November 2022.
• Samsung: 970 EVO Plus datasheet, revision 3.0 , June 2021, pages 3–5.
• Microsoft: Disk Devices and Partitions , 7 January 2021; Hard drives and partitions , 16 March 2022.
• Microsoft: Win32_LogicalDisk , 7 May 2022; MSFT_Disk , 9 January 2023.
• Microsoft: The Easy Way to Manage Disk Drives , 25 November 2012; Get-Disk reference for Windows Server 2016 .
• Kingston: Understanding SSD Over-provisioning , March 2019.