Convert TB to MB

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.

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 terabytes to megabytes moves the decimal point 6 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 TB is 1234000000 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.

A terabyte is not a quantity anybody has an intuition for, and it does not become one by being written as 1,000,000 MB. What makes it legible is dividing it by the size of the thing that will fill it: forty thousand raw frames at 25 MB, four thousand scans at 250 MB, twenty-two hours of footage at a bitrate somebody chose. The conversion is a step on the way to a count, and the count is the answer.

This is also why the per-file figure deserves measuring rather than looking up. Raw file sizes vary with the sensor, the compression setting and, for most cameras, with the content of the frame itself — a detailed forest costs more than a plain sky. Taking the megabyte figure from a full card of your own material gives a number that predicts the next card, which a published average does not.

Both units are decimal and both are powers of a thousand: a megabyte is a million bytes, a terabyte a trillion, and the gap between them is two full steps of the ladder. That is what makes the conversion a clean six places with no remainder, and it is a property of the decimal system rather than a coincidence. In the binary units the same two rungs give 1,048,576, which is a number nobody divides by in their head.

The clean ratio has a practical use beyond arithmetic. Because the answer is a round million, the count of files per terabyte is a million divided by the file size in megabytes, and that division can be done approximately at a glance: 40 MB gives twenty-five thousand, 80 MB gives twelve and a half thousand, 200 MB gives five thousand. Halving the file size doubles the count, which is the trade the setting on the camera is actually offering.

Stills and video fill capacity in completely different ways and are worth converting separately. A stills shoot is a count of frames at a fairly stable size, so a card of a known capacity holds a predictable number and the only variable is how fast the shutter is used. Video is a rate: the file grows at the codec’s bitrate regardless of what is in front of the lens, which makes the arithmetic simpler and the totals much larger.

For video the conversion to use is bitrate to megabytes per second — divide the megabit figure by eight — and then multiply out. A 100 Mbit/s codec is 12.5 MB a second, 750 MB a minute, 45 GB an hour, so a terabyte is about twenty-two hours. A 400 Mbit/s intermediate codec is four times that and gives about five and a half hours. Two files both labelled 4K can therefore differ in size by a factor of eight, and the resolution told you none of it.

Drives and cards are sold in decimal units: a megabyte is a million bytes and a terabyte a trillion, and the capacity on the label is honest in those terms. Much of the software that reports file sizes divides by 1,024 instead, making its megabyte 4.9% larger, so a file the computer calls 24 MB is 25.2 in the units the card’s capacity was quoted in. Over a full card that gap is several hundred frames.

It also explains the shortfall a card appears to have before a single file is written. Some of it is the naming difference and some is real — the filesystem occupies space for its own structures, and a card formatted in the camera reserves a little more. The remaining-shots counter in the viewfinder resolves all of this for you, which is why it is more trustworthy than any calculation and why it changes as you shoot.

Converting a terabyte to a million megabytes gives a theoretical count that assumes the last file lands exactly on the boundary, and nothing works that way. Filesystems slow down as free space runs out, a nearly full drive cannot be reorganised because there is nowhere to move anything to, and a card that fills in the middle of a take costs far more than the frames it saved. Working to about eighty per cent of the converted figure is the version of the number to plan against.

The reserve matters more on solid-state media than on spinning disks, because wear levelling needs free blocks to move data into and a drive kept at capacity has fewer of them to work with. That shows up as write speed falling away rather than as an error, which makes it easy to blame on the card or the cable. The count of files a terabyte holds is a ceiling, not a target.

The count of files is more useful again when divided by a rate. If a typical day produces eight hundred frames at 30 MB, that is 24 GB, and a terabyte is about forty days of shooting. If a typical shoot is three hours of footage at 45 GB an hour, the same terabyte is seven shoots. Those are numbers that answer the real question, which is not how much fits but how often the drive has to be dealt with.

Measuring the rate from history beats estimating it. The difference between two capacity readings a month apart, divided by the month, accounts for everything an estimate forgets — the days nothing was shot, the exports, the duplicates kept for safety. It also exposes the step changes, because a new camera or a switch to a higher bitrate does not increase the rate by a percentage but multiplies it.

Capacity has a second dimension that a conversion does not show: how long it takes to move. At 100 MB a second — a realistic sustained figure for a spinning drive over USB 3 — a terabyte takes about two hours and forty-five minutes to copy. At 35 MB a second it takes eight hours. A backup, a migration or a card offload is an operation measured in hours, and the number of megabytes is what sets it.

That turns the same conversion into a scheduling tool. Divide the million megabytes by the transfer rate in megabytes per second and the answer is in seconds, which is worth doing before committing to a plan that assumes a copy finishes over lunch. It is also the argument for keeping working files and archives on separate volumes: the cost of moving a terabyte is paid every time, and the conversion is the only warning you get.