Convert MB to GB

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.

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

Going from megabytes to gigabytes moves the decimal point 3 places to the left 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 MB is 1.234 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.

Every allowance in this shape — a mailbox limit, a data bundle, a storage tier — is a round decimal number chosen by whoever is selling it. 15 GB means 15,000,000,000 bytes, 5 GB means 5,000,000,000, and the metering behind it counts bytes and divides by a power of ten. That is the same convention networks and drive manufacturers use, and it is the SI meaning of the prefix: giga is 10⁹ wherever it appears.

So the division is by a thousand. 2,400 MB is 2.4 GB, 800 MB is 0.8 GB, and 15,000 MB is the whole of a 15 GB allowance. No part of the provider side of this involves 1,024. Where 1,024 arrives is on the device that displays the total back to you, and that is a separate step with its own arithmetic.

Divide 15,000,000,000 bytes by 1,073,741,824 and the answer is 13.97. That divisor is a gibibyte — 1,024³ — and it is what most desktop and many mobile file managers count in before printing the letters GB. The result is a display that is 6.9 per cent below the number on the invoice, every time, for every allowance, with not a byte unaccounted for.

The gap is fixed and it is worth memorising in both directions. A decimal figure read in binary units loses 6.9 per cent at gigabyte scale and 4.6 per cent at megabyte scale; going the other way, a gibibyte is 7.4 per cent larger than a gigabyte and a mebibyte 4.9 per cent larger than a megabyte. Those are the same two facts stated from opposite ends, and mixing the two percentages up is the commonest way this arithmetic goes wrong.

Quota screens round hard. A list of two hundred items each shown as "3.4 MB" is a list of two hundred numbers that are somewhere between 3.35 and 3.45, and the total of 680 MB carries up to 10 MB of uncertainty that no amount of care in the division will remove. Against a 1 GB allowance that is one per cent; against a 700 MB one it is the difference between fitting and not.

The fix is to work from byte counts wherever they are available and to divide exactly once. Mail clients, storage dashboards and command-line tools will all produce a byte total if asked, and a byte count is an integer with no rounding in it at all. Convert at the end, to as many digits as you need, and the answer is exact rather than approximately exact.

A mailbox does not store your attachments as files. Mail is a text protocol, so binary attachments are encoded into printable characters, and that encoding turns every three bytes into four — an increase of about 33 per cent. A 6 MB photograph occupies roughly 8 MB of quota, and a folder of a hundred such photographs is 800 MB rather than 600.

This is the same mechanism behind sending limits that seem to reject files smaller than they should. A service capping messages at 25 MB is capping the encoded message, so the largest attachment that reliably gets through is closer to 18 MB on disk, before headers and any second recipient copy. Working out how much of a gigabyte a mail archive occupies means adding a third to the raw sizes first.

Streaming is the whole of most data plans. Published figures put standard-definition video at roughly 1 GB an hour, HD at about 3 GB and 4K at up to 7, so a 5 GB monthly bundle is five hours of ordinary video or under two of HD. Music streaming is far cheaper — a high-quality stream is on the order of 100 MB an hour, so the same 5 GB carries fifty hours of it.

Everything else on a phone is noise by comparison. A web page averages a few megabytes, a messaging conversation with photographs is tens of megabytes over a month, and email without attachments is measured in kilobytes. When a bundle disappears faster than expected, the answer is almost always video quality left on automatic over a mobile connection rather than a thousand small things adding up.

Two industries took the same prefix in opposite directions and both stuck. Memory is addressed by binary offsets, so a module is sized in binary units: a stick sold as 16 GB holds 17,179,869,184 bytes, which is 16 GiB, and there is no decimal 16 GB module anywhere. The capacity is usually a power of two and no longer always — Crucial and Kingston ship 24 GB and 48 GB DDR5 modules built on 24 Gbit dies — but the unit behind the label is binary either way. Storage is sold by capacity rather than by address range, so a drive sold as 1 TB holds exactly 1,000,000,000,000 bytes.

That matters whenever both numbers appear in one place. A machine specified with 16 GB of memory and 512 GB of storage is carrying 17.2 billion bytes of the first and 512 billion of the second, and a container limit written as "4 GB of memory, 4 GB of disk" is asking for two different quantities that differ by 7.4 per cent. Reading the two numbers with the same divisor is what produces a memory limit set a little too low.

Order of magnitude first. A thousand megabytes is a gigabyte, so a total in the hundreds of MB cannot be more than a fraction of a gigabyte and a total in the tens of thousands is tens of gigabytes. Most errors in this arithmetic are factors of a thousand rather than of 1.07, and a quick check against the scale catches them before a decimal-against-binary argument becomes relevant.

Then check the direction of any disagreement. If your figure is larger than the device says, by around seven per cent at gigabyte scale, the device is counting in gibibytes and you are right. If the disagreement is much larger than that, or runs the other way, the units are not the explanation and something else is being counted — deleted items still held in a trash folder, previous versions, or a shared allowance being consumed by something other than the account you are looking at.