File Size Converter (GB to MB)

How this is calculated

There are two systems, they are both correct, and almost every complaint about storage sizes comes from comparing one against the other. DECIMAL - the SI convention. 1 kB = 1,000 bytes, 1 MB = 1,000², 1 GB = 1,000³, 1 TB = 1,000⁴. This is what drive manufacturers use, and it is the ordinary meaning of the prefixes kilo, mega, giga and tera everywhere else in measurement. BINARY - powers of two. 1 KiB = 1,024 bytes, 1 MiB = 1,024², 1 GiB = 1,024³, 1 TiB = 1,024⁴. Computers address memory in powers of two, so these are the natural units inside a machine. ⚠️ WINDOWS DISPLAYS BINARY SIZES BUT LABELS THEM WITH DECIMAL NAMES. It computes in units of 1,024 and writes 'GB' where the standard name is 'GiB'. That single labelling decision is the source of essentially every 'my drive is smaller than advertised' complaint. A 1 TB drive holds 1,000,000,000,000 bytes exactly as advertised. Divided by 1,024³ that is 931.32, and Windows prints '931 GB'. Nothing is missing. The same bytes have been divided by a different number and given the same name. The divergence COMPOUNDS with each prefix, which is why nobody notices it on small files and everybody notices it on drives. It can be stated two ways and they are different numbers, so it is worth being careful. A binary unit is BIGGER than its decimal namesake by 2.4 percent at kilo, 4.9 at mega, 7.4 at giga and 10.0 at tera. Equivalently, the same quantity of bytes READS SMALLER when divided into binary units - by 2.3, 4.6, 6.9 and 9.1 percent respectively. Both describe one fact. The second is the one you see on a drive, which is why 1 TB shows as 931 GB: 6.9 percent smaller at the giga step, and 9.1 at tera. A 4 TB drive shows about 3,725 'GB' where the decimal figure is 4,000; a 4 KB file loses almost nothing. macOS and most Linux file managers use decimal units and label them decimally, so the same drive reads as 1 TB there and 931 GB on Windows. Neither machine is wrong and neither drive changed. RAM IS THE EXCEPTION and it goes the other way. Memory is genuinely manufactured and sold in powers of two, so 16 GB of RAM really is 16 GiB - it is one of the few places the marketing figure is the binary one. NETWORK SPEEDS ARE A DIFFERENT TRAP ENTIRELY: those are BITS, not bytes. A 100 Mbps connection carries at most about 12.5 MB per second, because there are 8 bits in a byte. Lowercase b is bits, uppercase B is bytes, and the difference is a factor of eight.

Common questions

Why does my 1 TB drive show as 931 GB?

Because the drive holds 1,000,000,000,000 bytes exactly as advertised, and Windows divides that by 1,024³ rather than 1,000³. The result is 931.32, which it prints as '931 GB'. Nothing is missing and nothing was taken - the same bytes have been divided by a different number and given the same name. The standard name for what Windows is showing is GiB, and if it printed that, this question would not exist.

So who is wrong, the manufacturer or Windows?

Neither, exactly - but only one of them is using the label incorrectly. Drive manufacturers use kilo, mega, giga and tera in their ordinary SI sense of thousands, which is how those prefixes work everywhere else in measurement. Windows computes in powers of two, which is genuinely the natural unit inside a computer, and then labels the result with the decimal names. The arithmetic on both sides is right; it is the shared label that causes the argument. macOS and most Linux file managers use decimal units and label them decimally, so the same drive reads 1 TB there.

Why does the gap get bigger with larger units?

Because it compounds with each prefix. The ratio between 1,024 and 1,000 is 2.4 percent, and each step up multiplies it again. A binary unit is bigger than its decimal namesake by 2.4 percent at kilo, 4.9 at mega, 7.4 at giga and 10.0 at tera - which means the same bytes read 2.3, 4.6, 6.9 and 9.1 percent SMALLER when divided into binary units. That is exactly why nobody notices this on a text file and everybody notices it on a hard drive. A 4 TB drive is 4,000 GB decimal and shows as about 3,725 in Windows - an apparent shortfall of roughly 275 'GB', which looks like a scandal and is a division.

What are KiB, MiB and GiB?

The standard names for the binary units - kibibyte, mebibyte, gibibyte - introduced precisely to end this confusion by giving the powers of two their own labels. They are used consistently in some software and ignored in a great deal more, including Windows. When you see them you can trust them completely; when you see plain 'GB' you have to know which program wrote it. That is the entire practical problem in one sentence.

Is RAM the same?

No - RAM is the exception, and it goes the other way. Memory is genuinely manufactured and sold in powers of two, so 16 GB of RAM really is 16 GiB. It is one of the few places where the marketing figure is the binary one, which is why RAM never produces the 'smaller than advertised' complaint that storage does. If you are checking whether a machine has what it was sold with, memory and storage need to be checked differently.

What about internet speeds?

Different trap entirely, and a bigger one. Network speeds are quoted in BITS per second, not bytes, and there are eight bits in a byte. A 100 Mbps connection therefore carries at most about 12.5 megabytes per second - so a 1 GB download takes at least 80 seconds even at full speed. Lowercase b means bits and uppercase B means bytes, a distinction that is easy to miss in print and is a factor of eight. That is why this converter shows a megabits output as well.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the File Size Converter (GB to MB) at https://www.bfcbrilliance.com/tools/file-size-converter.

What I entered:
- Size: ___
- Unit: ___

What it calculated:
- In GiB — what Windows shows as "GB": ___
- In GB — decimal, what it is sold as: ___
- Bytes: ___
- In MB (decimal): ___
- In MiB (binary): ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

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