You buy a 1 TB hard drive, plug it in, and your computer reports something closer to 931 GB. You download a file listed as 500 MB and your network meter shows a number that does not seem to match. Behind these everyday puzzles sits one of the most persistent sources of confusion in computing: the units we use to measure data. Kilobytes, megabytes, and gigabytes sound simple, but they hide a quiet disagreement about whether the prefixes mean 1000 or 1024. This guide untangles all of it.
Bits and Bytes: The Foundation
At the lowest level, a computer stores everything as bits. A bit is a single binary digit, either 0 or 1, the smallest possible unit of information. On its own a bit cannot say much, so bits are grouped together.
The fundamental grouping is the byte, which is 8 bits. With 8 bits you can represent 256 different values (2 to the power of 8), which historically was enough to hold one character of text. Almost every storage measurement you see, from a tiny text file to a massive video, is counted in bytes or multiples of bytes. When you see a capital B (as in MB or GB), it refers to bytes. A lowercase b (as in Mb or Gb) refers to bits, and that single letter changes the number by a factor of eight.
The Decimal Prefixes: Kilo, Mega, Giga
The familiar prefixes kilo, mega, giga, and tera come from the metric system (SI), where they have always meant powers of 1000. In this decimal world, a kilobyte is 1000 bytes, a megabyte is 1,000,000 bytes (1000 kilobytes), a gigabyte is 1,000,000,000 bytes, and a terabyte is 1,000,000,000,000 bytes.
Each step up multiplies by 1000. This is clean and intuitive because it matches the way we count in everyday life, and it is exactly how a scientist would interpret kilo or mega anywhere else. Storage manufacturers and networking equipment overwhelmingly use these decimal definitions.
The Binary Prefixes: Kibi, Mebi, Gibi
Computers, however, think in powers of two. The closest round binary number to 1000 is 1024 (2 to the power of 10). Because memory chips are built in binary, it was natural for early programmers to use 1024 as the step between units rather than 1000.
To end the ambiguity, the IEC introduced a separate set of binary prefixes in 1998. A kibibyte (KiB) is exactly 1024 bytes. A mebibyte (MiB) is 1024 kibibytes, or 1,048,576 bytes. A gibibyte (GiB) is 1024 mebibytes, or 1,073,741,824 bytes, and a tebibyte (TiB) continues the pattern. These -bi- names look odd at first, but they are precise: KiB, MiB, GiB, and TiB always mean powers of 1024, with no room for misinterpretation.
Why the Confusion Exists
The mess comes from history. Long before the IEC prefixes existed, programmers casually wrote KB to mean 1024 bytes because it was close enough to a thousand and matched the binary nature of memory. MB came to mean 1024 KB, and GB meant 1024 MB. For decades the same symbols, KB and MB and GB, were used for both the 1000-based and 1024-based meanings depending on who was speaking.
This worked tolerably well when files were small, because the gap between 1000 and 1024 is only 2.4 percent. But the discrepancy compounds at every level. By the time you reach gigabytes the difference is about 7 percent, and at terabytes it is nearly 10 percent. That growing gap is exactly why the units now visibly disagree on large drives.
Why Your Hard Drive Shows Less Space
This brings us back to the missing space on a new drive. Storage manufacturers advertise capacity using the decimal definition, where 1 TB means 1,000,000,000,000 bytes. That is the bigger, friendlier-looking number for marketing.
Many operating systems, however, report file sizes and disk capacity using the binary definition while still labelling it GB. So the operating system takes those 1,000,000,000,000 bytes and divides by 1,073,741,824 (a true gibibyte) to get about 931, then prints 931 GB. Nothing was lost; the same bytes are simply being divided by a larger unit. The drive is honest, the operating system is honest, and they merely use different definitions of the word gigabyte. Modern versions of macOS report decimal GB to match the label, while Windows historically reports the binary value, which is why the same drive can show two different numbers.
Where Each Convention Is Used
Knowing which convention applies in a given context removes most surprises. Hard drives, SSDs, USB sticks, and most storage marketing use decimal units, so a 256 GB phone means 256 billion bytes. Network and data-transfer speeds are also decimal, and they are usually measured in bits per second rather than bytes.
On the other side, RAM is always sold and measured in binary, so an 8 GB stick of memory really is 8 gibibytes. Operating-system file managers often count in 1024 even when they print GB, and many software tools that report memory usage do the same. As a rule of thumb: physical storage and networking lean decimal, while memory and the inner workings of the operating system lean binary.
Bits per Second vs Bytes
Network speed adds one more twist. Internet plans and connection speeds are advertised in bits per second: megabits (Mbps) or gigabits (Gbps), not megabytes. File sizes, meanwhile, are measured in bytes.
Because there are 8 bits in a byte, a 100 Mbps connection can transfer at most about 12.5 megabytes per second under ideal conditions (100 divided by 8). This is why a download that should take eight seconds on paper still feels slow: you are mentally comparing a bit-based speed against a byte-based file size. Whenever a download seems eight times slower than expected, the bits-versus-bytes mismatch is usually the culprit.
A Worked Example: GB to MB Both Ways
Suppose you want to convert 2 GB into megabytes. Using the decimal convention, you multiply by 1000: 2 GB equals 2000 MB, because each gigabyte is 1000 megabytes. This is the answer a storage manufacturer would give.
Using the binary convention, you multiply by 1024 instead: 2 GiB equals 2048 MiB, because each gibibyte is 1024 mebibytes. The two results, 2000 and 2048, differ by 48 units, a 2.4 percent gap at this scale that widens as the numbers grow. Neither answer is wrong; they simply answer two slightly different questions. The key is to know which definition the context expects before you trust the figure.
Convert Storage Units Instantly with ToolboxHub
Rather than juggling powers of 1000 and 1024 in your head, the free ToolboxHub Data Storage Converter handles both conventions for you. Enter a value in any unit, from bits and bytes up through terabytes, and the tool shows the equivalent in every other unit, with clear support for both the decimal (1000) and binary (1024) interpretations so you can see exactly where the numbers diverge. It runs entirely in your browser with no sign-up, making it easy to settle the next debate over how big a file really is.
Key Takeaways
A bit is a single 0 or 1, and 8 bits make 1 byte, the basic unit of stored data. The decimal SI prefixes (kilo, mega, giga, tera) step by 1000, while the binary IEC prefixes (kibi, mebi, gibi, tebi, written KiB, MiB, GiB, TiB) step by 1024. The historical habit of writing KB, MB, and GB for the 1024-based values is the root of nearly all the confusion.
Your hard drive looks smaller because the manufacturer counts in decimal and the operating system often divides by 1024. Storage and networking lean decimal, memory and the operating system lean binary, and network speeds are in bits per second rather than bytes. When the exact figure matters, let the free ToolboxHub Data Storage Converter show both conventions side by side.