Convert GiB to GB

A gibibyte is 1,073,741,824 bytes — about 7% more than a gigabyte. Windows measures in gibibytes and labels them GB, which is the whole of the missing-space mystery.

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

The factor is 1.073742, and almost nobody carries that around. Rounded to 1.07 it is off by 0.35 % — which stays invisible on small numbers and turns into a whole unit somewhere around 1,000 GiB.

That is the number worth knowing before you round: not the error itself, but where it stops being ignorable. Below that point the shorter factor is the sensible one; above it, use the field above, which never rounds until it prints.

One GiB is 1,024 of the unit below it; one GB is 1,000. On this page that is the difference between 1.0737 GB and 1 GB — 7.4 % — and the gap grows at every step up the scale, which is why it is a rounding error on a photograph and a visible chunk of a hard disk.

This is the whole of the missing-storage mystery, and on this page it is worth 7.4 %. A drive sold in GB holds exactly what the label says; Windows divides by 1,024 instead of 1,000, keeps the decimal name, and reports 1 GB where the box said 1.0737. macOS has counted these in the decimal units since 10.6, which is why the same drive can look two sizes on two machines — nothing is missing and nobody is rounding, the same bytes have two names.

Multiply by 1.073741824 and the answer is exact, because a gibibyte is exactly 1,073,741,824 bytes and a gigabyte is exactly a billion. Nothing in this conversion is rounded or approximate: both units are defined counts of bytes, and the factor is the ratio of two integers. That makes it safe to use at any scale, which matters when the number is going into a purchase order rather than a conversation.

The figures that recur: 8 GiB is 8.59 GB, 16 GiB is 17.18, 100 GiB is 107.37, 465.66 GiB is 500 and 931.32 GiB is 1,000. That last pair is the one to recognise on sight — a console reporting 931 GiB is describing a one-terabyte drive, and quoting 931 GB to a supplier would have you buying a size that is not sold.

The International Electrotechnical Commission published the binary prefixes in 1998, as an amendment to IEC 60027-2, and they were carried into IEC 80000-13 in 2008. The names are contractions of the decimal prefixes with the word binary: kibi from kilo binary, mebi from mega binary, gibi from giga binary, tebi from tera binary. Each is written as two letters ending in i, so KiB, MiB, GiB and TiB, and the capital K in KiB is deliberate — the SI symbol for kilo is a lowercase k.

The intent was precise and modest. Kilobyte, megabyte and gigabyte would go back to meaning powers of a thousand, as they do everywhere else in science and engineering, and the new words would carry the powers of 1,024 that computing had been quietly using instead. IEEE adopted them in 2002, the standards bodies were unanimous, and the ambiguity was, on paper, resolved.

Adoption split along a line that still holds. Standards documents, most Linux tooling, several programming language libraries and a good deal of technical writing use the IEC prefixes as intended. Consumer operating systems did not: Windows still writes GB in File Explorer while dividing by 1,024, and a generation of users has learned the binary meaning as the normal one because that is the label their computer shows them.

The result is that the standard succeeded at making the units unambiguous and failed at making them universal, which is the worst of both outcomes for a reader trying to interpret a number. A figure written GiB is reliable. A figure written GB may be either, and the only way to know is to ask what produced it. That uncertainty is why this conversion is still being looked up decades after it was supposed to have become unnecessary.

Block storage is the common source. AWS sizes its elastic block volumes in gibibytes, so a volume created as 100 will report 100 GiB and hold 107.37 GB of bytes. Hypervisor consoles usually follow the same convention, as do most Linux volume managers when asked for exact sizes rather than human-readable ones. A figure with a decimal fraction attached — 465.66, 931.32 — is nearly always a decimal capacity that has been divided by 1,024 on its way to the display.

Memory is the other source and it is unambiguous in a different way. RAM is genuinely binary: a module holds a power of two bytes because it is addressed that way, so a stick sold as 8 GB holds 8 GiB, which is 8,589,934,592 bytes. Converting a memory figure into decimal gigabytes is arithmetically valid and practically pointless, which is a useful test of whether a number needs this page at all.

The failure mode is under-ordering. A backup target measured at 900 GiB is 966 GB, and a one-terabyte drive covers it with 34 GB to spare rather than the 100 the numbers appeared to promise. A dataset measured at 1,900 GiB is 2.04 TB and does not fit on a 2 TB drive at all. Converting before quoting is the difference between a comfortable margin and a rebuild.

The same care applies in reverse when reading someone else’s requirement. If a vendor specifies 500 GB of free space and your monitoring reports free space in GiB, the threshold to watch is 465.66, not 500. Setting the alert at the unconverted number means it fires 34 GB early, every time, and the team learns to ignore it.

A gibibyte figure multiplied by 1,073,741,824 gives a byte count, and byte counts carry no convention at all — they are the one representation that two systems can agree on without agreeing on anything else. When a dashboard, an invoice and a drive label disagree, converting all three to bytes usually shows that two of them were the same number the whole time.

This is also the quickest way to catch a doubled or halved conversion. If the byte count that comes out is around seven per cent away from where it should be, the factor was applied in the wrong direction; if it is fifteen per cent away, it was applied twice. Both mistakes are common and both are invisible until the number is expressed in bytes.

A ratio is unit-free, which makes a whole category of these calculations unnecessary. A volume that is eighty-five per cent full is eighty-five per cent full whether the figures behind it are gibibytes, gigabytes or bytes, because the same factor appears in the numerator and the denominator and cancels. Alert thresholds, utilisation graphs and fill rates can all be expressed as proportions and stay correct across every convention.

The place this stops being true is where a threshold is absolute rather than proportional. An alert that fires below 50 GB free has to know which gigabyte it means; one that fires below five per cent free does not. Where a policy can be written as a proportion it should be, and the conversion on this page becomes something needed only at the point of purchase.

Write the unit you mean every time a number leaves your hands. A ticket that says 400 GiB free is actionable; one that says 400 GB free is a question. The cost of the extra letter is nothing, and it removes the step where somebody downstream has to guess which tool produced the figure and what that tool divides by.

Where a system forces the ambiguous label — and Windows will — carry the byte count alongside it. A capacity line reading 931 GB (1,000,204,886,016 bytes) cannot be misread by anybody, in either convention, and it survives being pasted into a spreadsheet by a person who has never heard of a gibibyte.