Convert MB to TB

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 terabyte is a trillion bytes. A drive sold as 1 TB is exactly that — the space that seems to go missing is a unit disagreement, not a defect.

Going from megabytes to terabytes moves the decimal point 6 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 0.001234 TB — 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.

The conversion is six places of decimal point, which puts the two units far enough apart that they belong to different activities. Megabytes are what individual files are, one at a time: a raw photograph, a video clip, an album in a lossless format. Terabytes are what storage is sold in. The conversion is the moment a pile of files becomes a purchasing decision, and it usually happens once.

That distance is also why the inversion is more useful than the division. A terabyte holds two hundred thousand five-megabyte files, or forty thousand at twenty-five megabytes, or two hundred and fifty at four gigabytes. Those counts are easier to check against reality than a decimal is — somebody knows roughly how many photographs they have taken, and nobody has an intuition for 1.85 TB.

Real libraries are lopsided, and the total is nearly always dominated by one category. Video overwhelms photographs, raw files overwhelm exported ones, and lossless audio overwhelms compressed. Summing everything and converting once gives a correct number and hides that structure, so it is worth knowing which category is carrying the total before deciding what to buy — halving one of them is often easier than buying twice the drive.

What the total leaves out is the second thing to check. Photo libraries keep generated previews and sidecar files that can add a substantial fraction; video projects keep render caches and proxies that are not part of the footage but do occupy the disk; a music library may hold both an original and a converted copy. A total taken from a catalogue application counts what the catalogue knows about, and a total taken from the filesystem counts everything, including the parts that could be regenerated.

Two separate reductions sit between the number on the box and the space available for files, and they are constantly confused with each other. The first is not a reduction at all: a drive sold as 1 TB holds a trillion bytes, and an operating system that divides by 1,024 three times reports 931 and writes GB next to it. Every byte is present and the label changed, not the capacity.

The second is real. A filesystem spends capacity on its own structures — allocation tables, journals, directory records — and some reserve space for repair or for the system’s own use. The amount is small on a large modern drive, generally well under one per cent, but it is genuinely unavailable for files. Solid-state drives also hold back a portion for wear levelling, which is why the advertised capacity and the formatted capacity differ by a bit more on those than on a spinning disk.

The megabyte total is exact for the instant it was measured, and that instant is already past. A library that is being added to changes between counting and buying; a catalogue rebuild finds files the last count missed; a phone syncs another few gigabytes. Two decimal places on a terabyte figure claims accuracy to ten gigabytes, which is roughly one afternoon of shooting video.

One decimal is the sensible form, and the number it produces should be treated as the lower bound rather than the answer. The decision the figure feeds into is a choice between drive sizes that come in a small number of steps — one, two, four, eight terabytes — so the arithmetic only has to be good enough to pick a step, and precision beyond that is spent on a question nobody is asking.

Take the terabyte figure, add whatever the library is expected to grow by over the life of the drive, and then leave headroom. Filesystems slow down and fragment as they approach full, moving files around becomes difficult when there is nowhere to move them to, and a drive with no free space cannot be reorganised without a second drive. Buying for about twice the current total is the rule of thumb that survives contact with most libraries.

A backup drive needs more again, and for a different reason. Versioned backups keep previous states of files, so the space required is the current total plus whatever has changed over the retention period — for a photo library that changes slowly this is modest, and for a video project that re-renders constantly it can exceed the source. Sizing a backup drive from the library total alone is the commonest way to run out within months.

A total assembled from a Windows file listing is in mebibytes wearing the MB label, and is about 4.9% below the decimal figure the drive will be sold in. On a Mac the total is already decimal and needs no adjustment. On Linux it depends on the flag: the human-readable output of most tools divides by 1,024, and the SI option switches them to powers of ten.

Five per cent of a two-terabyte library is a hundred gigabytes, which is not enough to change which drive to buy and is exactly enough to make two totals of the same library disagree in a way that looks like an error. Where the numbers have to reconcile — against a cloud bill, against a quota, against another person’s count — take the byte totals rather than the rounded megabyte columns, because that is the one figure both systems agree on.

The current total is the input that is easiest to measure and the least important to the decision. What matters is the rate: a camera that produces forty gigabytes on a good weekend fills a terabyte in a couple of dozen outings, while a library that has been stable for five years will still be roughly its own size when the drive is replaced. Converting the total gives a starting point, and the slope gives the answer.

It is worth measuring that slope rather than guessing it. The difference between two totals a few months apart, divided by the months, is usually a more reliable predictor than any intuition, and it exposes the step changes — a new camera with larger files, a switch to raw, a move to higher-resolution video — that make a library grow by a factor rather than a percentage.