One terabyte is 1,000 gigabytes in decimal terms, so a gigabyte is 0.001 TB. This is the step where the decimal-versus-binary disagreement becomes visible to ordinary buyers, because a drive sold as 1 TB shows up in Windows as 931 GB.
About the units
The terabyte is 10¹² bytes under the SI convention that storage manufacturers use, and 1,099,511,627,776 bytes under the binary convention that Windows uses — the latter being 2⁴⁰, correctly called a tebibyte. Neither party is lying: the manufacturer sells a decimal terabyte and the operating system counts a binary one, and the labels collide. The gap is ten per cent at this prefix and grows to twelve at petabyte scale. Several class-action lawsuits in the United States in the 2000s ended with manufacturers adding a footnote rather than changing the number.
The exact factor
The factor is exactly 1,000 decimal. The reason a 1 TB drive reads as 931 GB is not lost capacity but a change of unit midway: 1,000,000,000,000 bytes ÷ 1,073,741,824 bytes per binary GB = 931.32. Nothing is missing. Formatting does consume a little real space for the file system, typically well under one per cent, but that is a separate and much smaller effect than the unit mismatch.
Where you meet this conversion
Drive and SSD purchasing, NAS and RAID capacity planning, and cloud storage tiers. Backup planning is where the conversion earns its keep: a 4 TB drive holds 4,000 GB of decimal data, but a backup tool reporting in binary units will say 3,725 GiB, and a plan built on the wrong figure runs out of space near the end of the job. Video production works at this scale routinely, with an hour of high-bitrate footage running to hundreds of gigabytes.