How many megabytes in a gigabyte? 1,000 or 1,024 bytes?

This is one of the few conversions where the honest answer is “it depends” — and the disagreement isn't sloppy rounding, it's two genuinely different, both-correct definitions colliding under the same abbreviation. Decimal: 1,000 MB per GB. Binary: 1,024 MiB per GiB. Storage vendors, most modern operating systems, and the IEC 80000-13 standard all use the decimal figure for anything labeled plain “MB” or “GB”.

Decimal: 1,000 MB = 1 GB

Under the SI/IEC decimal convention, a kilobyte is 1,000 bytes, a megabyte is 1,000,000 bytes, and a gigabyte is 1,000,000,000 bytes. That makes the megabyte-to-gigabyte relationship a clean:

1 GB = 1,000 MB

This is what you'll see on a drive's packaging, in most cloud-storage pricing pages, and in macOS's Finder or most Linux file managers. It's also the definition Convertitive's megabyte-to-gigabyte converter uses by default.

Binary: 1,024 MiB = 1 GiB

Computer memory is naturally organized in powers of two, so older software — and RAM specifications to this day — used “kilobyte,” “megabyte,” and “gigabyte” to mean 1,024, 1,048,576, and 1,073,741,824 bytes respectively. To remove the ambiguity, the IEC formally standardized separate binary-prefix names in 1998: kibibyte (KiB), mebibyte (MiB), and gibibyte (GiB).

Why the two definitions coexist

Windows is the main holdout still labeling binary values with decimal unit names in its UI (showing “GB” when it means GiB), which is the direct cause of the famous “why does my 1 TB drive show 931 GB” complaint: the drive is exactly 1,000,000,000,000 bytes (decimal, as advertised), but Windows divides by 1,073,741,824 and still labels the result “GB.” Neither number is wrong — they're measuring the same bytes with two different rulers.

Which one should you use?

For everyday file sizes, downloads, and storage-plan comparisons, use the decimal definition (1,000 MB = 1 GB) — it matches how storage is sold and how most modern software reports it. For RAM capacity or anything explicitly labeled with an “i” (KiB, MiB, GiB), use the binary definition instead. When in doubt, check NIST's guidance on SI vs. binary prefixes for the full reference table.

A quick way to check which one you're looking at

If two “GB” figures for what should be the same thing differ by a few percent, decimal-vs-binary is almost always the cause — not a data-loss bug or a miscalculation. The two conventions never differ by more than about 7.4% at the gigabyte scale (and by more at larger scales, since the gap compounds), so if you see a bigger mismatch than that, look for a different explanation, such as filesystem overhead or compression, rather than assuming it's a unit issue.