Why does my 1 TB drive only show 931 GB?

Your 1 TB drive isn't missing any space. Drive makers count a terabyte as 1,000,000,000,000 bytes (10¹²). Windows divides the byte count by 1,073,741,824 (2³⁰) and calls the result “GB”. The math is 10¹² ÷ 2³⁰ = 931.32, which Windows shows as 931 GB. Both numbers describe exactly the same bytes. The 931 figure is really in gibibytes (GiB).

Here's the exact math, a table for common drive and SSD sizes, which devices count which way, and how to check whether any space really is missing.

The math, step by step

Advertised: 1 TB = 1,000,000,000,000 bytes (10^12) Windows' “1 GB”: 1 GiB = 1,073,741,824 bytes (2^30 = 1024 × 1024 × 1024) Shown in Windows: 1,000,000,000,000 ÷ 1,073,741,824 = 931.3225… → “931 GB”

The “missing” 68.68 GB is an illusion of units, like measuring the same road in miles and then in kilometers. Try any value in the GB to GiB converter, or go back with the GiB to GB converter.

Two meanings of “gigabyte”

In everyday science and trade, kilo means 1,000, mega 1,000,000 and so on. Early computing borrowed the same prefixes for powers of two, because memory addresses come in binary and 2¹⁰ = 1,024 is close to 1,000. To end the ambiguity, the International Electrotechnical Commission (IEC) adopted separate binary prefixes in December 1998: kibi, mebi, gibi and tebi. NIST, the US standards agency, documents the same prefixes.

The difference grows at every step, because each prefix multiplies the previous gap by another 1.024:

The gap column shows how much smaller the decimal unit is: 1 − 1000ⁿ ÷ 1024ⁿ. For gigabytes that's 1 − 0.931323 = 6.87%, which is exactly how far 931.32 falls short of 1,000.

What common drives show in Windows

For bigger drives File Explorer switches to “TB” (really TiB). It typically shows three significant digits, cut off rather than rounded, which is why 1.819 appears as 1.81.

Worked example: a 2 TB external drive

2 TB = 2,000,000,000,000 bytes 1 TiB = 2^40 = 1,099,511,627,776 bytes 2,000,000,000,000 ÷ 1,099,511,627,776 = 1.8190… → shown as “1.81 TB”

At first glance this looks worse than the 1 TB case: the number drops by about 9% instead of about 7%. That's because Windows now compares tera with tebi, a 9.05% gap, instead of giga with gibi at 6.87%.

Worked example: a 512 GB laptop SSD

512 GB = 512,000,000,000 bytes 512,000,000,000 ÷ 1,073,741,824 = 476.837… → shown as “476 GB”

On a laptop's main drive, the C: drive often shows a little less than that, because some space goes to other partitions. See below.

Who counts which way

• Drive makers use decimal. Seagate, for example, says the storage industry standard is 1 GB = 1,000,000,000 bytes.
• Windows divides by powers of 1,024 but uses the labels KB, MB, GB and TB.
• macOS and iOS use decimal, according to Apple. OS X Leopard and earlier, and iOS 10 and earlier, used binary. A 1 TB drive on a current Mac shows about 1 TB, minus what formatting uses.
• Linux command line: in GNU df, df -h uses powers of 1,024 and df -H (or --si) uses powers of 1,000. The same drive can show two different sizes on the same computer.
• RAM is built in binary. The JEDEC memory standards use “GB” to mean 2³⁰ bytes, so a 16 GB memory kit is 17,179,869,184 bytes.
• Internet speeds are in decimal bits per second. A 100 Mbps connection moves at most 12.5 megabytes per second. The megabits to megabytes converter does this math.

Planning storage: don't mix the two units

The units matter most when you estimate how much will fit. Windows reports file sizes in binary units too. A video that File Explorer lists as 3.5 GB is really 3.5 GiB, which is 3.5 × 1.073741824 ≈ 3.758 decimal GB. If you compare that with the decimal size on the drive's box, your estimate will be too high.

Worked example: how many 3.5 GB videos fit on a 1 TB drive?

Right (both binary): 931.32 GiB ÷ 3.5 GiB = 266.09 → 266 files Wrong (mixed units): 1,000 ÷ 3.5 = 285.71 → 285 files Difference: 19 files, or 66.5 GiB (about 71.4 GB) you don't have

The fix is to pick one system and convert everything into it before dividing. This matters when the file sizes come from one place, such as a Mac or a download page (usually decimal), and the free space from another, such as Windows (binary). Leave some headroom too, because the file system and any software updates need free space of their own.

The same decimal labeling is the storage-industry norm, so it applies to USB flash drives and memory cards as well as hard drives and SSDs. A card sold as 64 GB is 64,000,000,000 bytes, or 59.6 GiB. Windows will show at most about 59.6 GB for it, and slightly less once it's formatted.

Drives are often slightly bigger than advertised

Hard drives built to the IDEMA LBA1-03 standard get their sector count from a formula: 97,696,368 + 1,953,504 × (advertised GB − 50) sectors of 512 bytes. For a 1 TB drive:

97,696,368 + 1,953,504 × (1,000 − 50) = 1,953,525,168 sectors 1,953,525,168 × 512 = 1,000,204,886,016 bytes ÷ 1,073,741,824 = 931.51 GiB → still shown as “931 GB”

The drive holds about 205 million bytes more than 1 TB, but that doesn't change the headline number.

When space really is missing

Unit conversion explains the 931 figure. A few other things can reduce the space you can use, and these are real:

• Formatting. The file system uses some capacity for its own records, so an empty drive already shows some space used.
• Extra partitions. System drives often hold EFI or recovery partitions that don't appear as separate letters in File Explorer.
• The operating system itself. Apple notes that formatting and the operating system use part of a device's capacity.

How to check: in Windows, right-click the drive and choose Properties. The Capacity line shows the exact byte count next to the rounded figure. Divide that by 1,000,000,000 to get decimal GB. If the result is close to the size on the box, you have all the bytes you paid for. On Linux, lsblk -b lists every disk and partition in bytes.

Convert it yourself

GiB = GB × 1,000,000,000 ÷ 1,073,741,824 = GB × 0.931323 GB = GiB × 1.073741824 TiB = TB × 0.909495

Our converters show every step: GB to GiB, TB to GB, MB to MiB, and the full data storage converter list. If you'd like to see why powers of two come naturally to computers, try converting 1,024 in the decimal to binary converter. It's 10000000000, a 1 followed by ten zeros.