File Size Converter

Why a 1 TB Drive Shows About 931 GB

A drive sold as 1 TB holds 1,000,000,000,000 bytes. Some computers report it as about 931 "GB", which looks like missing space but isn't. Storage makers count in decimal units, where 1 GB is 1,000,000,000 bytes. Some software divides the same bytes by 1,073,741,824 to show gibibytes, then still labels the result "GB": 1,000,000,000,000 ÷ 1,073,741,824 is about 931.32. No storage has disappeared, because the same bytes are being counted in larger units.

Advertised capacity versus the size shown by software that divides by 1,024
Advertised (decimal)	Bytes	Shown by binary arithmetic
128 GB	128,000,000,000	119.21 GiB
256 GB	256,000,000,000	238.42 GiB
512 GB	512,000,000,000	476.84 GiB
1 TB	1,000,000,000,000	931.32 GiB
2 TB	2,000,000,000,000	1,862.65 GiB

Which figure you see depends on the software. Microsoft Windows is the best-known example of a system that calculates in binary units but labels the result "GB". macOS has used decimal units since Snow Leopard in 2009, and Linux tools vary, so the same drive can show different figures on different systems. Formatting, system files and pre-installed apps then reduce the free space further.

The confusion goes back to early computers, which worked in blocks of 1,024 bytes (2¹⁰). Because 1,024 is close to 1,000, that block became known as a kilobyte, and the same shortcut was applied to megabytes and gigabytes. The IEC introduced the separate binary names KiB, MiB and GiB in the late 1990s to remove the ambiguity, but many systems kept the older labels.

Decimal and Binary Units Side by Side

Each decimal unit has a binary counterpart that is slightly larger, and the gap grows at every step because the 2.4% difference compounds. The table shows the bytes in each unit and how much larger the binary one is.

Decimal units and their binary counterparts
Decimal unit	Bytes	Binary unit	Bytes	Binary is larger by
kilobyte (kB)	1,000	kibibyte (KiB)	1,024	2.40%
megabyte (MB)	1,000,000	mebibyte (MiB)	1,048,576	4.86%
gigabyte (GB)	1,000,000,000	gibibyte (GiB)	1,073,741,824	7.37%
terabyte (TB)	1,000,000,000,000	tebibyte (TiB)	1,099,511,627,776	9.95%
petabyte (PB)	1,000,000,000,000,000	pebibyte (PiB)	1,125,899,906,842,624	12.59%

When comparing sizes, check which system a figure uses. Decimal units are used for storage capacities, mobile data plans and most network figures, while binary units are used for computer memory and by some operating systems when they report file and disk sizes.

File Size Units from Bit to Yottabyte

Bits and Decimal Byte Units

Bit (bit or b): The smallest unit of digital information, representing either zero or one. Internet speeds are commonly expressed in bits per second.
Byte (B): A unit containing eight bits. File sizes and storage capacities are normally measured using bytes and their larger multiples.
Kilobyte (kB): Equal to 1,000 bytes. Kilobytes are commonly used for small text files, simple documents, icons and configuration files. Some software displays the symbol as KB.
Megabyte (MB): Equal to 1,000,000 bytes. Megabytes are commonly used for photographs, music files, documents, applications and shorter video files. Convert MB to GB.
Gigabyte (GB): Equal to 1,000,000,000 bytes. Gigabytes are used for application sizes, games, videos, mobile data and device storage. Convert GB to MB.
Terabyte (TB): Equal to 1,000,000,000,000 bytes. Terabytes are commonly used for SSDs, hard drives, backups, servers and cloud-storage plans.
Petabyte (PB): Equal to 1,000 terabytes, or 10¹⁵ bytes. Petabytes are used for data centres, large scientific datasets and enterprise storage systems.
Exabyte (EB): Equal to 1,000 petabytes, or 10¹⁸ bytes. Exabytes are useful for describing extremely large networks, global data traffic and storage at an industry-wide scale.
Zettabyte (ZB): Equal to 1,000 exabytes, or 10²¹ bytes. Zettabytes are mainly used when discussing worldwide data creation, traffic and storage.
Yottabyte (YB): Equal to 1,000 zettabytes, or 10²⁴ bytes. A yottabyte is an exceptionally large decimal unit used mainly for theoretical or global-scale data quantities.

IEC Binary File Size Units

Kibibyte (KiB): Equal to 1,024 bytes. It is the binary counterpart to the decimal kilobyte.
Mebibyte (MiB): Equal to 1,024 KiB, or 1,048,576 bytes. It is the binary counterpart to the decimal megabyte.
Gibibyte (GiB): Equal to 1,024 MiB, or 1,073,741,824 bytes. It is often encountered when operating systems report memory or storage capacity using binary calculations.
Tebibyte (TiB): Equal to 1,024 GiB, or 1,099,511,627,776 bytes. It is the binary counterpart to the decimal terabyte.
Pebibyte (PiB): Equal to 1,024 TiB, or 2⁵⁰ bytes. It is used for very large binary storage quantities.
Exbibyte (EiB): Equal to 1,024 PiB, or 2⁶⁰ bytes. It is mainly used for extremely large technical or computing datasets.

Decimal symbols and IEC binary symbols follow the IEC unit guidance, and NIST explains the binary prefixes in its guide to binary prefixes.

Converting Between MB and GB

The two conversions that come up most often are megabytes and gigabytes. The MB to GB converter turns a total in megabytes into gigabytes by dividing by 1,000, and the GB to MB converter does the reverse by multiplying by 1,000. Both use decimal units, so 4 MB is 0.004 GB. Binary units are kept separate: 4 MiB is 0.00390625 GiB, which is not the same figure.

Two Stories That Explain Modern Data Storage

A file-size unit tells us how much digital information is stored, but it does not explain the technology holding that information or the quality of the content inside the file. These two stories show why a few megabytes once required an enormous machine and why two images of equal size can still look different.

When a Few Megabytes Required a Room-Sized Disk System

In 1956, IBM introduced the 305 RAMAC, a computer system that used the IBM 350 disk storage unit. It is widely recognised as the first commercial computer system to provide random access to information stored on magnetic disks.

The storage unit contained 50 magnetic disks, each about 24 inches across. Together, they stored five million 6-bit characters, which the Computer History Museum equates to about 3.75 MB. IBM's own figure of five million characters is sometimes rounded to 5 MB, but the characters were 6 bits, not today's 8-bit bytes.

The equipment was approximately the size of two refrigerators and weighed over a ton. Today, a single 4 MB photograph from a smartphone is larger than the RAMAC's entire disk unit (4 ÷ 3.75 ≈ 1.07).

Capacity was not the system's most important achievement. Earlier business records were often kept on punched cards or magnetic tape, where finding one item could require working through information in sequence. RAMAC's moving read-and-write heads could travel directly to the required disk location. This ability to retrieve a record without reading every record before it changed how organisations could manage inventories, accounts and transactions, and it established the principle of magnetic random-access storage used by later hard disk drives.

Read more about the system in IBM's history of the RAMAC.

Why Two 4 MB Images Can Have Very Different Quality

Suppose two photographs are both listed as 4 MB. Their equal file sizes tell us how much storage they occupy, but they do not guarantee equal resolution, sharpness or colour quality. Image quality can be influenced by pixel dimensions, colour depth, compression settings, camera noise, metadata, previous editing and the file format.

PNG normally uses lossless compression, so the encoded image data can be decompressed without losing its original sample information, although two PNG files of the same picture need not be the same size. The familiar form of JPEG usually uses lossy compression, which approximates or discards some image information to reduce storage. Moderate JPEG compression may produce a much smaller photograph with little visible change, while aggressive compression can introduce softened detail, colour smearing, ringing around edges or block-shaped artefacts. File size alone is therefore not a complete measurement of visual quality.

The W3C PNG specification defines PNG as a lossless compressed format, and the Library of Congress description of the JPEG family covers JPEG's lossy and lossless modes.

Converting a size from MB to GB changes only the unit, so 4 MB is 0.004 GB and the file itself is unchanged. Compressing an image is a separate process: an image compressor might reduce a 4 MB photograph to 600 kB, which changes the data, not just the label.