Convert GB to MB

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

A megabyte is a million bytes. Storage manufacturers have always used this decimal meaning, which is why their capacities look larger than what a computer reports.

Going from gigabytes to megabytes 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 GB is 1234000 MB — 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.

This page divides by a thousand because that is what the prefix means. Kilo, mega and giga are SI prefixes for 10³, 10⁶ and 10⁹, and they carry those values on every other quantity they are attached to — a kilometre, a megawatt, a gigahertz. Applied to bytes they give 1,000, 1,000,000 and 1,000,000,000, which is the meaning used by drive manufacturers, by every network standard, by mobile data plans and by the file sizes services quote in their own documentation.

The rival system is real and this page is not it. Powers of 1,024 give the kibibyte, mebibyte and gibibyte — KiB, MiB and GiB — and those are what most desktop operating systems count in, whatever letters they print. The two systems answer different questions and both are correct inside their own conventions; the mistake is reading a number from one and a limit from the other without noticing which is which.

A limit stated in megabytes is nearly always a decimal one, because it was written by a person choosing a round number rather than derived from any hardware boundary. A mail service that caps a message at 25 MB, a form that refuses an upload over 100 MB, a plan that allows a 500 MB file: all of those are 25, 100 and 500 million bytes as the service counts them. So the comparison is direct. 1.4 GB is 1,400 MB and is fourteen times over a 100 MB ceiling.

The awkward case is the file that sits near the line. A download listed as 0.1 GB is 100 MB exactly against a 100 MB limit, and whether it passes depends on rounding neither side shows you. If the figure you have been given is 0.1 GB, the real size is anywhere between 50 and 150 MB, which is the difference between an upload that works and one that fails after four minutes of waiting.

The single most expensive confusion around this pair has nothing to do with 1,000 against 1,024. The upper-case B is a byte and the lower-case b is a bit, and there are eight bits in a byte, so MB and Mb differ by a factor of eight — far larger than the decimal-against-binary question the rest of this page is about. Connection speeds are quoted in bits per second by long-standing convention; files are measured in bytes.

The arithmetic is a division by eight. A 100 Mbit/s line delivers at most 12.5 MB/s, a 500 Mbit/s line at most 62.5 MB/s, and a gigabit line at most 125 MB/s. Those are ceilings rather than expectations: framing, protocol headers and retransmission take a few per cent before anything else does. A 2 GB download over a 100 Mbit/s connection is 2,000 MB at 12.5 MB/s, which is 160 seconds in the best case and closer to three minutes in a real one.

Copy a 1 GB file onto a Windows machine and the properties dialog will report roughly 953 MB. Nothing has been removed. The file is 1,000,000,000 bytes; Windows divides by 1,048,576 to get a mebibyte count and then writes the letters MB next to it. 1,000,000,000 ÷ 1,048,576 = 953.67, and the shortfall is 4.6 per cent of the decimal figure — the same fact as the mebibyte being 4.9 per cent larger than the megabyte, counted from the other end.

macOS has reported decimal sizes since Snow Leopard, so the same file on a Mac shows 1 GB and 1,000 MB. Two computers, one file, two different numbers, both printed with the same letters. This is worth knowing before you argue with an upload form: if the limit is decimal and your file manager is binary, the figure on screen understates your file by 4.6 per cent at megabyte scale and 6.9 at gigabyte scale.

Photographs from a modern phone run about 4 MB each, so a gigabyte is roughly 250 of them and a 64 GB phone that is entirely full of pictures is holding somewhere near sixteen thousand. Music at a good bitrate is around 5 MB a track, which puts about 200 songs in a gigabyte. A long PDF report with images is a few megabytes; a plain text document is a few tens of kilobytes and effectively free at this scale.

Video is where the number moves. Streaming services publish figures of roughly 1 GB per hour for standard definition, about 3 GB per hour for HD and up to 7 GB per hour at 4K. In megabytes that is 1,000, 3,000 and 7,000 per hour — which is why a video file is the thing that meets an upload limit and a spreadsheet never is.

Where a file genuinely will not fit, the usual answer is to split it, and the arithmetic is clean in decimal. A 1.4 GB archive at 100 MB per volume is fourteen parts; at 250 MB per volume it is six, five full and one holding the remainder. Choosing round decimal volume sizes keeps the count obvious, and it means the part sizes match the way the receiving limit was written.

Compression is worth trying first and is worth measuring rather than assuming. Video, photographs and audio are already compressed and will shrink by a per cent or two at most; text, logs, spreadsheets and uncompressed database dumps often fall to a tenth of their size. That is a much larger lever than any unit question on this page: a 1.4 GB SQL dump can arrive comfortably under a 200 MB limit without being split at all.

A size shown as 1.4 GB has been rounded to two significant figures and could be anything from 1.35 to 1.45 GB — a span of 100 MB, which is larger than many of the limits people are checking against. If the decision is close, get the byte count rather than the gigabyte figure: it is an integer, it has no rounding in it, and every tool that reports a size can be persuaded to show it.

The same caution applies to totals. Adding twenty items each shown as "0.4 GB" gives 8 GB with an uncertainty of a full gigabyte, because each of those twenty figures carries up to 50 MB of rounding and the errors do not reliably cancel. Sum the byte counts and divide once at the end; the answer will be right to the byte instead of right to within a rounding step you cannot see.