Why does my hard drive show less space?

A 1 TB drive showing 931 GB in Windows is not missing space. Drive makers count in powers of 1,000, Windows counts in powers of 1,024 but labels the result GB, and formatting and hidden partitions account for the rest of the gap. This page works through the arithmetic and the signs that shrinking space is a real fault.

Written by Essential Data RecoveryPublished 2026-09-13Owner-reviewedRevision 1

A closed portable hard drive resting beside a small empty ceramic bowl on pale oak wood in warm morning light

Plug a new drive in and the number on screen is smaller than the one on the box: a 1 TB drive shows about 931 GB in Windows. Nothing is missing. Drive makers and Windows are counting the same bytes in two different units, and the drive holds every byte it was sold with. Smaller deductions come from formatting and, on some drives, hidden system partitions. This page works through each one, then covers the case where shrinking free space really is a warning.^[1]

The short answer

The drive is fine and the arithmetic checks out. Seagate, which prints the big number on the box, puts it plainly: decimal and binary translate to the same amount of storage capacity. The company compares the two figures to measuring the same distance in different units; it is the same distance, reported differently because of the measurement. Windows shows fewer gigabytes because its gigabyte is bigger, not because the drive is smaller.^[1]

Two ways of counting a terabyte

Drive makers count in decimal, powers of 1,000. Seagate defines one megabyte as 1,000,000 bytes, one gigabyte as 1,000,000,000 bytes, and one terabyte as 1,000,000,000,000 bytes, and that is the number printed on the label. Apple counts the same way: the fine print on its packaging reads 1GB = 1 billion bytes and 1TB = 1 trillion bytes, actual formatted capacity less.^[1][2]

Windows counts in binary, powers of 1,024, but keeps the same GB and TB labels. In Seagate's list of binary-counting software, which includes FDISK, the system BIOS, Windows, and older versions of macOS, one gigabyte is 1,073,741,824 bytes and one terabyte is 1,099,511,627,776 bytes. The standards bodies gave these bigger units their own names, GiB and TiB, precisely so they would not be confused with the decimal ones. Wikipedia's binary prefix article notes that some operating systems, Windows among them, still display drive capacities using the MB and GB labels for the binary numbers.^[3][1]

Advertised size against what each system reports, from Seagate's capacity table and worked example. A blank cell means the page quoted here gives no figure for that row.

Size on the box	Windows reports	macOS reports
500 GB	465 GB
1 TB	931 GB	1 TB (1,000 GB)
2 TB	1.81 TB
5 TB	4.54 TB
16 TB	14.55 TB

Seagate's worked example shows where the Windows column comes from. A 500 GB drive is about 500,000,000,000 bytes, and dividing that by the binary gigabyte of 1,073,741,824 gives 465 GB. The 1 TB row is the same arithmetic, which is where the familiar 931 GB figure comes from. Every row is the same bytes measured twice, not a drive that arrived short.^[1]

The Mac column explains the other common surprise: the same drive showing two different numbers on two computers. Apple states that makers measure storage capacity in the decimal system and that older software, naming iOS 10 and earlier and OS X Leopard and earlier, reported it in binary instead. Current macOS reports decimal, which is why Seagate's table lists the Mac output for a 1 TB drive as 1 TB (1,000 GB) while Windows on the same drive shows 931 GB. Neither number is wrong; they are different units.^[1][2]

What formatting takes

A smaller and entirely genuine deduction happens when the drive is formatted. Apple's explanation of formatted capacity says the storage device's formatting uses some of this storage space. A file system has to write its own tables and indexes onto the drive before a single one of your files lands, so usable space sits a little below raw capacity. This effect is much smaller than the decimal-versus-binary gap, and it is the reason packaging says actual formatted capacity less. Apple does not put a percentage on it and neither will we.^[2]

Hidden partitions

Drives that have been set up on a Windows PC can also carry partitions Windows never shows you. Microsoft's partitioning documentation requires an EFI system partition of at least 200 MB on standard drives (300 MB on 4K native ones), a 16 MB Microsoft Reserved partition that cannot store user data, and a recovery partition holding the Windows recovery image. Utility partitions can be flagged so they do not appear in File Explorer, and Microsoft confirms that these partitions exist without being assigned a drive letter. The space they take is real and counted; it is just invisible from the File Explorer window.^[4]

Two everyday versions of this catch people out. A drive initialized and formatted on the other operating system, or a drive that ships with factory partitions for bundled backup software, can show less usable space than expected or refuse to open properly on first use. Reformatting the drive for the system you use reclaims that space, but reformatting erases everything on the drive, so it is only the answer when nothing on it matters yet.^[4]

When missing space is a real problem

Everything above describes a one-time gap between the number on the box and the number on screen. It is worth understanding once and then ignoring. The situations that deserve attention look different: free space that keeps shrinking over days with nothing being added, files that fail to open or copy, new clicking or beeping noises, or a drive that drops out and reconnects on its own. Those point at the drive rather than the arithmetic, and the right response is to copy anything important off the drive first, then look at its health.^[5]

Failing drives often warn first. Backblaze's study of SMART statistics describes a small nonzero count of reallocated sectors (SMART 5) as meaning the drive has started remapping, as few as two sectors in its example, and found that 76.7 percent of the failed drives in its fleet had at least one of five warning attributes above zero, against 4.2 percent of drives that kept running. The same study is blunt about the limits: 23.3 percent of failed drives gave no warning at all, and a small value means little on its own. Treat remapped sectors, failed reads, or new noises as the cue to copy your data off now rather than to keep filling the drive.^[5]