Byte Size Conversion: KB, KiB, and the Missing Gigabytes

After reading this you will know why a "500 GB" drive shows 465 in your file manager, how to convert between decimal and binary units by hand, and which unit each part of your system actually uses.

The one number that never matches

You buy a drive labeled 500 GB. You plug it in, and your operating system reports something like 465 GB. Nothing is broken, and nothing was stolen. The two numbers describe the same physical bytes using two different definitions of the letters "GB".

The drive maker counts in powers of 1000. To them, 500 GB means 500 \times 1000^3 = 500{,}000{,}000{,}000 bytes. Your OS counts in powers of 1024 but prints the same three-letter label. It takes that identical byte count and divides by 1024^3:

Same bytes, two rulers. The Byte Size Converter shows you both rulers side by side and always prints the exact byte count in the middle, so you can see where the gap comes from.

Two families of units

The confusion has a fix that most software ignores. Since 1998 the IEC standard 60027-2 (later folded into ISO/IEC 80000-13) defines a separate set of binary prefixes. The decimal prefixes keep their SI meaning of powers of 1000. The binary prefixes get a small "i" and mean powers of 1024.

Decimal (SI) units

KB, MB, GB, TB. Each step multiplies by 1000. Used by storage vendors, network speeds, and anything measured in SI.

Binary (IEC) units

KiB, MiB, GiB, TiB, pronounced "kibibyte", "mebibyte", "gibibyte". Each step multiplies by 1024. Used for RAM and file sizes at the OS level.

The trap is that Windows and many older tools compute in powers of 1024 but print the SI labels KB, MB, GB. So the number is binary, the label is decimal, and you cannot tell which meaning applies without knowing the tool.

The formula and the intuition

Every conversion routes through one honest number: the total byte count. To go from a unit to bytes, multiply by that unit's size. To go from bytes to a unit, divide.

Here value is the number you typed, B is the base (1000 for decimal, 1024 for binary), and n is the step count (1 for kilo, 2 for mega, 3 for giga, 4 for tera). To convert between two units, get to bytes first, then divide out the target unit.

The drift between the two families grows with each step because you are comparing 1024^n against 1000^n. One step is 1024/1000 = 1.024, a 2.4% gap. Three steps compound it:

So a "gigabyte" of binary is about 7.37% larger than a decimal gigabyte, and a "terabyte" of binary is about 10% larger. That compounding is why the missing space looks worse on bigger drives.

Reading the converter output

The tool always prints three things: your input, the exact integer byte count, and the equivalent in every other unit. Trust the byte count. It is the only value with no ambiguity. The unit rows are just that byte count divided by different constants.

Notice that 1,024 bytes is exactly 1 KiB but 1.024 KB. Whenever a value looks slightly above a round number in one column, it is landing exactly on a round number in the other.

When decimal, when binary

You do not get to choose the unit family freely. Each domain has a fixed convention, and mixing them produces wrong answers.

• RAM is always binary. A 16 GB memory stick is 16 \times 1024^3 = 17{,}179{,}869{,}184 bytes. Memory addressing is inherently powers of two, so there is no decimal version.
• Storage is sold decimal. Drives, SSDs, and SD cards use powers of 1000 on the box.
• Network speed is decimal bits. A 100 Mbps link is 100 \times 1000^2 = 100{,}000{,}000 bits per second, which is 12.5 megabytes per second (divide by 8, then note bytes vs bits).
• File sizes at the OS level are usually binary, though the label depends on the OS as noted above.

Common mistakes

Three errors cause almost every byte-size bug.

1. Assuming the label tells you the base. A MB from Windows is really a mebibyte. A MB from a drive vendor is a real megabyte. Same label, different math.
2. Using 1024 in code that reports "MB". If your program divides by 1024*1024 and prints "MB", it is lying by 4.86% at that step. Either divide by 1000*1000, or print "MiB".
3. Confusing bits and bytes in transfer estimates. Dividing a file size in megabytes by a link speed in megabits without the factor of 8 makes downloads look 8 times faster than reality.

Related tools

Byte counting shows up whenever you measure encoded data. If you are sizing a payload before transport, the Base64 / URL / HTML Encoder-Decoder helps you see the roughly 33% size increase Base64 adds. To measure the byte length of text specifically, including multibyte UTF-8 characters, use the String Length Inspector. When you hash a file or verify a signature, the Hash Generator and HMAC Calculator work on the same underlying bytes this converter counts, and the JWT Decoder shows how those bytes get Base64url-packed into a token.

Related tools

Base64 / URL / HTML Encoder-Decoder

Encode and decode Base64, hex, URL percent-encoding and HTML entities — with auto-detect, bulk lines and file drop.

Hash Generator

MD5, SHA-1/256/384/512, CRC32, file checksums, bcrypt and digest compare.

JWT Decoder

Decode JWT header and payload, check expiry — locally.

HMAC Calculator

HMAC-SHA256/512/1/MD5 with text or hex inputs, hex + Base64 output, and timing-safe verify.

String Length Inspector

Characters, UTF-8 bytes, UTF-16 units and graphemes of any text.

CSS Gradient, Shadow & Radius Generator

Visual generators for gradients, box-shadows and border radii.