Why Does a 1TB Drive Only Show 931GB?

By KT Apps · Updated October 16, 2026

The short answer

Drive makers define 1 TB as 1,000,000,000,000 bytes, using the decimal meaning of tera. Windows divides that number by 1,024 three times to get gigabytes, and the result is about 931. The label still says GB, but strictly the figure is in gibibytes (GiB).

So 1 TB and 931 GB describe the same capacity. Only the size of the unit is different.

The maths, step by step

1. Start from the byte count on the box: 1 TB = 1,000,000,000,000 bytes.
2. Divide by 1,024 to get KiB: about 976,562,500.
3. Divide by 1,024 again to get MiB: about 953,674.3.
4. Divide by 1,024 once more to get GiB: about 931.3.
5. Divide by 1,024 a fourth time to get TiB: about 0.91.

Each division by 1,024 instead of 1,000 shaves off about 2.4%, and the losses compound: three steps give roughly 7%, four steps roughly 9%. That is why the gap looks bigger on larger drives.

What common sizes show in Windows

Capacity as labelled, followed by the approximate size reported in 1,024-based units, before any space is used:

• 64 GB: about 59.6 GB
• 128 GB: about 119.2 GB
• 256 GB: about 238.4 GB
• 500 GB: about 465.7 GB
• 1 TB: about 931.3 GB
• 2 TB: about 1,862.6 GB, or 1.82 TB
• 4 TB: about 3,725.3 GB, or 3.64 TB

macOS, iPhone and iPad report sizes in decimal units, so on those devices a 1 TB drive shows close to 1 TB. The remaining difference there comes from the reasons below.

Where the rest of the space goes

Even after the unit difference, the free space is usually lower than the table suggests. Common reasons are:

• File system overhead. Formatting a drive reserves space for the structures that keep track of files.
• Hidden partitions. Computers often ship with recovery or system partitions that do not appear as a normal drive.
• The operating system and preinstalled apps, which on a phone or laptop take up a noticeable share before you add anything.
• Caches and temporary files that grow over time.

None of this is a fault. If a drive reports far less than the table, for example 32 GB on a card sold as 256 GB, that is different: it can mean the drive was formatted with the wrong partition size or, with unknown sellers, that the card is counterfeit. Checking the full byte count, as described below, helps tell these cases apart.

How to check a drive yourself

1. On Windows, right-click the drive in File Explorer and choose Properties. The Capacity line shows both the number of bytes and the GB figure.
2. On a Mac, select the drive in Finder and choose File > Get Info, or open Disk Utility, which shows the capacity in decimal units and in bytes.
3. Take the byte count and convert it: divide by 1,000,000,000 for decimal GB, or by 1,073,741,824 (1,024 × 1,024 × 1,024) for binary GB.
4. Compare the decimal figure with the label. It should be very close; small differences come from how manufacturers round capacities.

Calculator, spreadsheet or converter?

A phone calculator works if you are careful with the zeros, but a trillion is easy to mistype. A spreadsheet formula such as =A1/1024^3 is reliable once set up and is good for comparing several drives. A storage unit converter saves you counting digits and can show the result in several units at once, which is the quickest option when you are comparing drives in a shop or online.

How to do the conversion in Bitcon

Bitcon uses 1,024-based steps, the same as Windows, so the trick is to enter the drive's capacity in bytes:

1. Open the Convert tab and type the capacity in bytes into Enter Value: 1000000000000 for a 1 TB drive, or 256000000000 for a 256 GB card.
2. Set the From Unit to byte and the To Unit to GB. The Conversion Result shows about 931.32 for the 1 TB drive.
3. Switch the To Unit to TB to see about 0.91, or open Compare Units and select GB and TB to see both at once.
4. Use the precision button to choose how many decimal places to show.
5. If you compare drives often, save the byte → GB pair on the Templates tab so it is one tap away next time.

Bitcon converts amounts of data, not drive health or free space, so use your operating system's tools to see how much of the drive is actually in use. For the background on decimal and binary units, see how many MB are in a GB.

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