Qu'est-ce qu'un/une Kilobyte (KB) ?
Formal Definition
The kilobyte (symbol: KB) is a unit of digital information equal to 1,000 bytes in the decimal (SI) definition, or 1,024 bytes in the traditional binary computing convention. The International Electrotechnical Commission (IEC) introduced the term "kibibyte" (KiB) in 1998 to specifically denote 1,024 bytes, reserving "kilobyte" for the exact 1,000-byte meaning. However, in practice, both meanings persist depending on context.
The kilobyte represents the first step up from the byte in the hierarchy of data measurement units. It is a relatively small unit by modern standards — sufficient for a few paragraphs of plain text but far too small for images, audio, or video files. Despite its small size, the kilobyte remains relevant in computing for measuring configuration files, simple text documents, small code files, and network packet sizes.
Relationship to Other Data Units
In the decimal system: 1 KB = 1,000 bytes = 8,000 bits. In the binary system: 1 KiB = 1,024 bytes = 8,192 bits. Moving up the scale: 1,000 KB = 1 MB (megabyte), 1,000,000 KB = 1 GB (gigabyte), 1,000,000,000 KB = 1 TB (terabyte). In binary: 1,024 KiB = 1 MiB, 1,048,576 KiB = 1 GiB. The 2.4% difference between KB and KiB is small at the kilobyte level but compounds to a 10% discrepancy at the terabyte level.
Etymology
Construction of the Term
The word "kilobyte" combines the Greek-derived SI prefix "kilo-" (from Greek "chilioi," meaning thousand) with "byte." The prefix "kilo-" officially denotes a factor of exactly 1,000 in the SI system. However, in computing, "kilo-" was informally adopted to mean 1,024 (2¹⁰) because binary powers are more natural for computer memory architectures. This dual meaning has been a source of confusion since the early days of computing.
The abbreviation "KB" uses an uppercase K (unlike the SI convention of lowercase k for kilo-) and uppercase B for byte. This inconsistency with SI conventions — where "k" means 1,000 and "K" means kelvin — reflects the computing industry's independent development of terminology. Some style guides insist on "kB" for the decimal kilobyte, but "KB" is overwhelmingly dominant in practice.
The KiB Alternative
In 1998, the IEC introduced binary prefixes to resolve the ambiguity: kibi- (Ki) for 2¹⁰ = 1,024, mebi- (Mi) for 2²⁰, gibi- (Gi) for 2³⁰, and so on. Under this standard, 1,024 bytes is a kibibyte (KiB), not a kilobyte. While technically precise, the binary prefixes have seen limited adoption outside of certain Linux distributions, technical specifications, and scientific computing. Most consumers, developers, and media continue to use "kilobyte" ambiguously.
Histoire
Early Computing and the 1024 Convention
In the earliest computers, memory was organized in binary — powers of 2 were natural boundaries. A memory chip with 10 address lines could address 2¹⁰ = 1,024 locations. Engineers called this "1K" of memory because 1,024 was close to 1,000, and the Greek prefix kilo (thousand) was convenient shorthand. This conflation of 1,000 and 1,024 was established in the 1960s and 1970s and became deeply embedded in computing culture.
The Intel 1103 (1970), the first commercially successful DRAM chip, held 1,024 bits — described as "1K" of memory. The subsequent Intel 2102 SRAM chip held 1,024 bits as well. When chips grew to hold bytes rather than bits, "1KB" naturally meant 1,024 bytes. The convention was so ingrained that challenging it was nearly impossible.
The Confusion Era
Problems arose when storage manufacturers began using the SI definition (1 KB = 1,000 bytes) for marketing purposes while operating systems continued using the binary definition (1 KB = 1,024 bytes). A floppy disk labeled "1.44 MB" actually held 1,474,560 bytes — neither 1.44 × 1,000,000 (SI) nor 1.44 × 1,048,576 (binary). The 3.5-inch floppy's capacity used its own unique calculation, making the confusion even worse.
Modern Resolution Attempts
The IEC's 1998 introduction of binary prefixes (kibibyte, mebibyte, gibibyte) attempted to resolve the ambiguity. The IEEE and ISO also endorsed these prefixes. However, adoption has been slow. As of the 2020s, hard drive and SSD manufacturers use decimal prefixes (1 TB = 10¹² bytes), macOS uses decimal units for display, Windows uses binary units (showing a "1 TB" drive as approximately 931 GB), and Linux varies by distribution. The kilobyte remains ambiguous in common usage.
Utilisation actuelle
In Software Development
The kilobyte is a relevant unit in software development for measuring small files. A typical HTML page is 20-100 KB. A CSS stylesheet might be 10-50 KB. A minified JavaScript library ranges from a few KB to several hundred KB. Configuration files (JSON, YAML, XML) are typically 1-50 KB. Source code files average 5-30 KB each. Frontend developers monitor kilobyte-level file sizes because they directly affect page load times.
In Networking
Network packet sizes are measured in bytes and kilobytes. A standard Ethernet frame carries a maximum of 1,500 bytes (approximately 1.5 KB) of payload data. TCP/IP headers add 20-60 bytes per packet. Network throughput calculations involve converting between kilobits per second (network speed) and kilobytes per second (data volume), using the factor of 8 bits per byte.
In Email and Messaging
Simple text emails are measured in kilobytes — a short email might be 2-5 KB including headers. Email signatures, HTML formatting, and small inline images push emails into the 10-100 KB range. SMS text messages are limited to 140-160 bytes, well under 1 KB. These small-scale measurements keep the kilobyte relevant in everyday digital communication.
In Embedded Systems
Microcontrollers used in IoT devices, automotive systems, and consumer electronics often have memory measured in kilobytes. An Arduino Uno has 32 KB of flash memory and 2 KB of RAM. Small IoT sensors might have 64-256 KB of memory. In these constrained environments, every kilobyte matters, and programmers must carefully optimize code size to fit within kilobyte-level memory limits.
Everyday Use
Understanding Small File Sizes
When you create a simple text document — a shopping list, a short note, a recipe — the file is typically 1-10 KB. A Word document with a few pages of text (no images) might be 20-50 KB. A saved web page might be 50-200 KB. These small files take up negligible space on modern storage devices but add up when there are thousands of them.
Web Browsing
Every web page you visit involves transferring kilobytes of data. A simple text-heavy website might transfer 50-200 KB. A well-optimized modern website might transfer 500 KB to 2 MB including images, scripts, and styles. Each individual resource (HTML file, CSS file, small icon) is measured in kilobytes. Web developers work hard to minimize kilobyte counts because every KB affects loading speed, especially on mobile connections.
Smartphone Usage
Small data transfers on smartphones are measured in kilobytes. Loading a new email: 2-50 KB. Sending a text message: under 1 KB. Loading a social media post (text only): 5-20 KB. These kilobyte-level transactions happen hundreds of times per day and form the backbone of mobile data usage, though they are dwarfed by image and video transfers measured in megabytes.
Cache and Cookies
Browser cookies typically range from a few bytes to 4 KB per cookie. Local storage allocations in web browsers are measured in kilobytes per domain. Understanding these kilobyte-level limits helps web developers design efficient applications and helps users manage their browser data.
Interesting Facts
The famous '640 KB ought to be enough for anybody' quote, often attributed to Bill Gates in 1981, is almost certainly apocryphal. Gates has repeatedly denied saying it. However, the IBM PC's original 640 KB conventional memory limit did constrain DOS programs for over a decade.
A standard 3.5-inch floppy disk held 1,440 KB (1.44 MB) — a capacity that seemed generous in 1986 but cannot even hold a single modern smartphone photo today.
The entire source code of the Apollo 11 guidance computer — which navigated astronauts to the Moon and back — was approximately 73 KB, smaller than a typical webpage today.
The first commercial hard drive (IBM 350, 1956) held 5 MB — roughly 5,000 KB. It weighed over 900 kg and cost $3,200 per month to rent (approximately $35,000 per month in 2024 dollars).
The original Super Mario Bros. game (1985) for the Nintendo NES was only 40 KB — smaller than a typical email attachment today, yet it provided hours of entertainment and became one of the best-selling games in history.
A single pixel in a 24-bit color image requires 3 bytes. A 1 KB image at this color depth could contain only about 341 pixels — roughly an 18×19 pixel thumbnail, smaller than most app icons.