File size converter

Why there are two answers

Everywhere else in measurement, kilo means a thousand. A kilometre is 1,000 metres and nobody argues about it. Computing borrowed the prefix and then used it for 1,024, because 1,024 is 2¹⁰ and it is the number that falls out of how memory is addressed, close enough to a thousand that the name seemed harmless.

It was harmless while files were small. At kilobytes the two readings are 2.4% apart. By gigabytes they are 7.4% apart, and by terabytes 10%, which is large enough to look like somebody is lying.

In 1998 the IEC named the binary ones separately, kibibyte, mebibyte, gibibyte, written KiB, MiB, GiB, so that kB could go back to meaning a thousand. The names are correct, unambiguous, and still not what most software prints.

Who uses which

• Storage manufacturers use decimal. A “500 GB” drive holds 500,000,000,000 bytes.
• Windows uses binary values with decimal labels, so the same drive reads as 465 GB.
• macOS switched to decimal in 2009, so it agrees with the box.
• RAM is always binary, from everyone. 16 GB of memory really is 16 GiB.
• Network speeds are decimal, and in bits rather than bytes.

The 500 GB drive, worked through

The drive holds 500,000,000,000 bytes. Divide by 1,024 three times, for kibi, mebi and gibi, and you get 465.66. Windows prints that number with a “GB” label.

So nothing is missing and nobody has overstated anything. The same bytes are being counted in one system and labelled in the other. A little more disappears to the file system when the drive is formatted, which is real, but it is a fraction of a percent rather than the seven you can see.

MB and Mb are not the same thing

The case of the second letter is a factor of eight. A capital B is bytes; a lowercase b is bits, and there are eight bits in a byte.

Internet connections are sold in megabits per second. A 100 Mbps line moves at most about 12.5 MB a second, so a 1 GB download takes at least 80 seconds and not 10. That single letter is the most common reason a connection feels eight times slower than advertised.

Rough sizes worth carrying around

A phone photo is 2–5 MB. A compressed song is 3–10 MB. An hour of ordinary video is around 1 GB; an hour of 4K is 15 GB or more. A plain text novel is under 1 MB, text is very nearly free, and it is images and video that fill a disk.

These vary hugely with quality settings, so they are worth using to sanity-check a figure rather than to plan storage with.