Convert units

Nothing has been taken. The drive holds exactly what it says. The disagreement is over what a gigabyte means.

Drive manufacturers use powers of ten, so a gigabyte is 1,000,000,000 bytes. Most operating systems use powers of two, where a gigabyte works out as 1,073,741,824 bytes. Divide one by the other and you lose about 7.4%, which is where 1TB becomes 931GB.

Both are correct. They're just different definitions, and the gap widens as the numbers get bigger: about 2.4% at kilobytes, 4.9% at megabytes, 7.4% at gigabytes, and roughly 10% at terabytes.

The names that were supposed to fix it

There's a standard for this. Powers of two are meant to be called kibibytes, mebibytes and gibibytes, written KiB, MiB and GiB, leaving kilobyte and megabyte to mean the powers of ten they mean everywhere else in science.

Nobody adopted it consistently. Linux and macOS mostly report in powers of ten now. Windows still reports powers of two while calling them GB. So the same drive plugged into two machines shows two different numbers, both labelled the same way.

Both meanings are listed here for that reason. Picking one and hiding the other would just move the confusion.

Where else this bites

Network speeds are bits, not bytes. A 100 Mbps connection carries 100 megabits a second, which is about 12.5 megabytes. That factor of eight is why a fast connection seems to download slower than expected. The lowercase b is bits and the uppercase B is bytes, which is a lot of weight for one letter to carry.

RAM is genuinely powers of two. Memory is addressed in binary, so a 16GB stick really is 16 gibibytes. Storage isn't built that way, which is why the two are counted differently even inside the same machine.

Video and photo sizes are usually quoted in powers of ten by cameras and powers of two by the computer you copy them onto.

The other conversions worth a word

Temperature is not a ratio. Twice 10°C is not 20°C in any meaningful sense, because zero on that scale is an arbitrary point rather than an absence of heat. Converting a temperature works; scaling one doesn't.

Fluid ounces differ by country. A US fluid ounce is about 29.57ml and an imperial one is about 28.41ml. That's a 4% difference, which matters in a recipe and matters more in a cocktail. US and imperial pints and gallons differ by considerably more: an imperial pint is 568ml against the US 473ml.

Tons come in three sizes. A metric tonne is 1000kg, a US short ton is about 907kg, and an imperial long ton is about 1016kg. Shipping quotes that don't say which one are a recurring source of argument.

Miles and nautical miles are different by about 15%. A nautical mile is one minute of latitude, which is why it exists and why aviation and shipping use it.

Rounding

Results are rounded for reading rather than truncated, so a converted value may not round-trip to exactly the number you started with. That's normal and it's the right trade for a display: showing 0.9999999999 instead of 1 helps nobody.

If you need the exact figure for something that has to reconcile, like an invoice, do the arithmetic at full precision and round once at the end. Round at every step and the errors accumulate.

Nothing is sent anywhere

The conversion happens as you type, in your browser. There's no request and nothing recorded, which is worth knowing given how often the number being converted is a salary, a dose or a measurement from work.