Convert TB to GB

A terabyte is a trillion bytes. A drive sold as 1 TB is exactly that — the space that seems to go missing is a unit disagreement, not a defect.

A gigabyte is a billion bytes in the decimal sense used by drive manufacturers, phone plans and video sizes.

Going from terabytes to gigabytes moves the decimal point 3 places to the right and changes nothing else. There is no factor to remember and no rounding to decide: the digits stay in the same order and only their position changes.

1,234 TB is 1234000 GB — same digits, moved along. That is worth knowing because it is the one kind of conversion you can check at a glance: if the digits of the answer are not the digits you started with, something other than the conversion has happened to them.

The manufacturer sold you 1,000,000,000,000 bytes and put 1 TB on the box, and both of those statements are exact. Tera is the SI prefix for 10¹², the same prefix that means the same thing on a terawatt and a terahertz, and storage capacity has been quoted decimally since long before anyone was troubled by the alternative. A 2 TB drive is 2,000 GB and a 500 GB drive is exactly half a terabyte.

The number on your screen is smaller because it was calculated with a different divisor, not because it was measured on a different drive. That distinction is the whole content of this page: no capacity has gone missing, nothing is reserved for the manufacturer, and no amount of reformatting will recover the difference — because there is no difference to recover, only two units printed with the same three letters.

Take the byte count and divide by 1,024 three times. 1,000,000,000,000 ÷ 1,024 = 976,562,500. Divide again: 953,674.32. Once more: 931.32. That last figure is a count of gibibytes — units of 1,073,741,824 bytes each — and Windows prints it with the label GB, which is where the impression of missing space comes from.

Doing it in one step gives the same answer: 1,000,000,000,000 ÷ 1,073,741,824 = 931.32. The shortfall against 1,000 is 6.9 per cent of the decimal figure, and it is fixed. A 2 TB drive shows 1,862.6; a 500 GB drive shows 465.7; a 256 GB phone shows 238.4. Every one of those is the label multiplied by 0.9313, and once you have seen the pattern the numbers stop being surprising.

The difference is not a constant number of bytes; it is a percentage that compounds with each 1,024-against-1,000 step taken. A kibibyte is 2.4 per cent larger than a kilobyte, a mebibyte 4.9 per cent larger than a megabyte, a gibibyte 7.4 per cent larger than a gigabyte, and a tebibyte 10 per cent larger than a terabyte. Each step multiplies the previous ratio by 1.024 again.

That is why the effect is barely noticeable on a memory card and unmistakable on a drive. Nobody investigates a 32 GB card showing 29.8, but a 12 TB array reporting 10.9 looks like more than a terabyte has vanished — and it is the same 1.024 applied one more time. Expect the reported figure to be about 93 per cent of the label at gigabyte scale and about 91 per cent at terabyte scale.

Connect the same drive to a Mac and it reports 1 TB. macOS moved to decimal reporting in Mac OS X 10.6, so it divides by 1,000 and its numbers agree with the label and with the invoice. Windows kept dividing by 1,024 while continuing to print KB, MB, GB and TB, which is the single decision responsible for most of the confusion around this pair.

Linux tools sit on both sides and usually say which they mean. Most report in 1,024-based units by default and accept a flag for powers of ten, and better-behaved ones print KiB, MiB and GiB so the reader can tell. When two tools disagree about one drive by roughly seven or nine per cent, the divisor is the explanation, and the fastest way to settle it is to ask for the raw byte count, which every one of them will give.

Separately from the relabelling, a formatted volume genuinely has less usable space than the drive has capacity, and it is worth telling the two apart. The filesystem writes allocation structures, an inode or record table and a journal, which on a terabyte volume runs to a few gigabytes. Some filesystems reserve a percentage for privileged use by default. Manufacturers often ship a hidden recovery partition on a system drive, typically tens of gigabytes.

Solid-state drives lose a further slice invisibly: the controller keeps spare blocks for wear levelling, deducted from the raw flash before the label is set. Adding those up, a 1 TB drive that shows 931 GB of capacity and about 900 GB of free space when new is behaving normally. A 1 TB drive showing 850 GB has something else on it, and that is a question worth asking rather than a units question.

Expressed in the gigabyte figures things are actually quoted in: a terabyte is 1,000 GB, so it holds around 330 hours of HD video at the roughly 3 GB an hour streaming services publish, or about 140 hours at 4K. Modern games are frequently 100 GB or more, which puts fewer than ten of them on a 1 TB drive — the reason console storage fills so quickly despite sounding generous.

Photographs and documents barely register by comparison. At 4 MB per phone photograph, a terabyte holds a quarter of a million of them; a raw file from a serious camera is 25 to 50 MB, so the same drive holds twenty to forty thousand. If a terabyte is filling up, the answer is almost always video or games, and the arithmetic above is usually enough to identify which without looking.

The habit worth forming is to convert once, deliberately, and know which system produced every number you are comparing. Multiply the terabyte label by 1,000 for the decimal gigabyte figure, or by 931.3 for what a binary-counting system will display. A drive within a per cent or two of either of those is exactly as advertised.

Where a figure sits well outside both, the units have stopped being the explanation. A capacity substantially below 93 per cent of the label means something is occupying space — a recovery partition, a system volume, snapshots or previous versions held by the filesystem. Those are all findable, and separating them from the display arithmetic is what turns a vague sense of missing space into a specific thing to look at.