Why Does a 1 TB Drive Show Only 931 GB? Decimal vs Binary Storage Explained

A 1 TB drive shows about 931 GB because the maker and your computer count in different units. The box uses decimal terabytes, where 1 TB is 1,000,000,000,000 bytes, while Windows divides that byte count by 1,024 three times and labels the result "GB". The drive is not short: 1,000,000,000,000 bytes is exactly 931.32 binary gibibytes, and nothing is missing.

That covers most of the gap. Formatting, hidden partitions and preinstalled software take away a bit more, and those are separate issues with different fixes. Below you'll find exactly how big the gap is at each common drive size, why memory is counted differently from storage, and how to check what your own drive holds.

Two meanings of "gigabyte"

The prefixes kilo, mega, giga and tera belong to the International System of Units (SI), maintained by the BIPM. In the SI they are powers of ten: kilo is 1,000, mega is 1,000,000, giga is 10⁹ and tera is 10¹². A kilometer is 1,000 meters, and by the same logic a kilobyte is 1,000 bytes.

Computers, though, address memory in powers of two. Early engineers noticed that 2¹⁰ = 1,024 is close to 1,000 and began calling 1,024 bytes a "kilobyte". The shortcut stuck and scaled up: 2²⁰ = 1,048,576 bytes became a "megabyte" and 2³⁰ = 1,073,741,824 bytes a "gigabyte". For decades the same word meant two different quantities depending on who was speaking.

To end the confusion, the International Electrotechnical Commission (IEC) introduced separate binary prefixes in 1998. They are now part of the IEC 80000-13 standard, and NIST recommends them too:

• KiB (kibibyte) = 1,024 bytes
• MiB (mebibyte) = 1,024 KiB = 1,048,576 bytes
• GiB (gibibyte) = 1,024 MiB = 1,073,741,824 bytes
• TiB (tebibyte) = 1,024 GiB = 1,099,511,627,776 bytes

Under these standards the answer to "is a gigabyte 1,000 or 1,024 megabytes?" is clear: a GB is 1,000 MB, and a GiB is 1,024 MiB. The trouble is that not all software uses the right labels. You can see how the two ladders line up on the data storage converter, which lists decimal and binary units side by side.

How drive makers count and how Windows reports

Hard drive and SSD makers use SI prefixes, which matches the standards. A 1 TB drive has at least 1,000,000,000,000 bytes of user-addressable space. In practice it usually has slightly more. The industry sector-count formula published by IDEMA gives a 1 TB drive 1,953,525,168 sectors of 512 bytes, which is 1,000,204,886,016 bytes, or about 204.9 million bytes over the round number.

Windows File Explorer takes that byte count and divides it by 1,073,741,824 to get a figure it calls "GB". It is really showing gibibytes with the decimal label. So:

• 1,000,000,000,000 bytes ÷ 1,073,741,824 = 931.32, which Explorer shows as 931 GB
• Once the figure passes 1,024 GiB, Explorer switches to "TB", which is really TiB. A 2 TB drive appears as about 1.82 TB and a 4 TB drive as about 3.64 TB

Every byte the maker promised is on the drive. Only the unit changed, and the label hides the change. If you want to move between decimal sizes yourself, the GB to TB converter shows both the TB and TiB figures for any capacity.

macOS, iOS and Linux

Apple switched macOS to decimal units with Mac OS X 10.6 Snow Leopard in 2009. Since then a 1 TB drive shows as 1 TB in Finder, and a 500 MB file shows as 500 MB. iPhones and iPads also report storage in decimal units, so a 256 GB phone lists 256 GB of total capacity before the system share is taken out.

Linux varies by tool. Many desktop file managers use decimal units, while command-line tools let you choose: df -h uses powers of 1,024 and df -H uses powers of 1,000. Tools that print "GiB" or "TiB" are telling you exactly which convention they use.

How much space looks missing at each drive size

The table below takes the advertised capacity as an exact decimal byte count and converts it the way Windows does. Because the ratio between GB and GiB is fixed, the gap is always 6.87% while Windows reports in "GB". Once it switches to "TB", the displayed gap jumps to 9.05%, because the TiB is 1,024 times bigger again.

Explorer truncates rather than rounds in some views, so you may see 1.81 TB or 3.63 TB instead. Either way the "shortfall" is a unit difference, not lost storage. For file-sized numbers, the MB to GB converter shows the same effect one step down the ladder, where the gap is 4.63%.

Other things that really do take space

Unit confusion explains the jump from 1,000 to 931. If your new drive shows noticeably less than the binary figure in the table, something else is using capacity. These are real reductions, and they are worth telling apart.

File-system overhead

Formatting a drive writes structures the file system needs to track files: the master file table on NTFS, allocation bitmaps, journals and similar metadata. On a fresh, empty drive this usually costs well under 1% of the capacity, so a formatted 1 TB drive typically shows a volume size a little below 931 GB. File systems also store data in fixed-size clusters, often 4 KiB, so a 1-byte file still occupies a full cluster on disk. That is why "size on disk" in Windows Properties can be larger than "size".

Hidden and recovery partitions

A system drive in a Windows PC carries several small partitions you don't see in File Explorer: an EFI system partition used to boot, a tiny reserved partition, and a Windows Recovery Environment partition. Together these take from a few hundred megabytes to about a gigabyte or two. Many laptop makers add their own factory-recovery partition as well, which can take many gigabytes. Disk Management lists all of them.

The operating system and preinstalled apps

On a laptop or phone, the operating system, updates, page file, hibernation file and bundled apps are already on the drive when you first switch it on. A phone sold with 128 GB may show a large "System" share in its storage settings. That space is in use, not missing, and some of it can be reclaimed by removing apps you don't need.

SSD spare area

Flash memory chips are built in binary sizes. Many SSDs contain more raw flash than the capacity on the label and keep the difference hidden as spare area for wear leveling and replacing worn cells. A drive with 256 GiB of flash sold as 256 GB, for example, has a hidden reserve of about 6.9%. You never see this space, and it does not count against the advertised capacity.

Why RAM is always binary

Memory is the one place where the old binary meaning never went away. RAM chips are addressed with binary address lines, so every module comes in an exact power-of-two size. A stick sold as 16 GB holds 16 × 1,073,741,824 = 17,179,869,184 bytes, which is 16 GiB. The JEDEC memory standards still use "GB" for this binary quantity.

That produces a neat reversal. When Windows reports 16 GB of installed memory, it is right, because it uses binary units and the RAM really is 16 GiB. When it reports 931 GB for a 1 TB drive, the number is also correct in binary terms; only the label is off. Measured in decimal, that same 16 GB of RAM is 17.18 GB. If you want to see why powers of two come so naturally to computers, try writing 1,024 on the decimal to binary converter: it is a 1 followed by ten zeros, 10000000000 in binary.

Network speeds, by contrast, are always decimal. A 1 Gbps link carries 1,000,000,000 bits per second, which is 125 MB/s in decimal bytes. The Gbps to Mbps converter handles those conversions and the bits-to-bytes step.

How to check your own drive

The most reliable figure is the exact byte count, because bytes mean the same thing everywhere. Here is where to find it.

Windows

1. Open File Explorer, right-click the drive and choose Properties. The General tab shows Used space, Free space and Capacity, each in bytes and in binary "GB".
2. For the whole physical disk, including hidden partitions, right-click the Start button and open Disk Management.
3. In PowerShell, Get-Disk shows each disk's total size, and Get-Volume lists every volume with its size and free space.

macOS

1. Open Disk Utility and select the drive; capacity is shown in decimal units.
2. In Terminal, diskutil info disk0 prints the disk size in both decimal units and exact bytes.

Linux

1. lsblk -b lists every disk and partition in bytes.
2. df -H shows mounted file systems in decimal units, and df -h in binary units.

Once you have the byte count, divide by 1,000,000,000 for decimal GB or by 1,073,741,824 for GiB. If the decimal figure matches the label, the drive is exactly as advertised. If it is far below, with no partitions to explain it, the drive may be faulty or counterfeit. Fake USB sticks and memory cards that report a false capacity do exist, and a full write-and-verify test is the way to expose them.

Quick rules to remember

• Multiply an advertised GB or TB size by 0.9313 to predict what Windows will show in "GB".
• Multiply an advertised TB size by 0.9095 to predict what Windows will show in "TB".
• Expect roughly 1% less again after formatting, plus whatever the recovery and system partitions take.
• RAM sizes are binary and match what the operating system reports.
• Network speeds are decimal, and you divide bits by 8 to get bytes.

The two numbers describe the same bytes counted two ways. Once you know which convention a screen uses, the "missing" space turns into simple arithmetic. For any other storage conversion, the data converters cover both decimal and binary units.