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Kilometre — Grokipedia Fact-checked by Grok 3 months ago Kilometre Ara Eve Leo Sal 1x The kilometre (km), symbol km, is a unit of length in the International System of Units (SI), equal to one thousand metres (1 km = 1000 m). [1] It derives from the SI prefix kilo- , which denotes a multiplication factor of 10³, applied to the base unit of length, the metre (m). [2] The metre itself is defined as the distance travelled by light in vacuum in 1/299 792 458 of a second, fixing the speed of light at exactly 299 792 458 m/s. [2] The kilometre originated during the development of the metric system in France amid the French Revolution , with the decimal-based system formalized by law on 7 April 1795 , introducing the metre as one ten-millionth of the distance from the equator to the North Pole along a meridian. [3] The term kilomètre was coined around 1795 from the Greek chilioi (thousand) and the French mètre , first appearing in English in 1810 to denote 1000 metres for practical measurement of larger distances. [4] By 1799, platinum standards for the metre were deposited in the French National Archives , establishing the foundational metric prototypes, including provisions for multiples like the kilometre through decimal prefixes . [2] The modern SI framework, including standardized prefixes such as kilo- , was ratified at the 11th General Conference on Weights and Measures (CGPM) in 1960, building on the 1875 Metre Convention that created the International Bureau of Weights and Measures (BIPM) to maintain global uniformity. [2] In contemporary usage, the kilometre is the primary unit for expressing road distances, vehicle speeds (often as km/h), and geographical scales in most countries worldwide, except the United States , where the mile predominates for such purposes despite official SI adoption. [5] It equates to approximately 0.621371 miles or 0.539957 nautical miles, facilitating conversions in international contexts like aviation and science . [6] The unit's adoption has been promoted globally through treaties like the 1960 SI establishment, ensuring coherence in fields from engineering to everyday navigation . [2] Definition and Notation Definition The kilometre is a unit of length in the International System of Units (SI), equal to exactly 1,000 metres ( 1 k m = 1 0 3 m 1\ \mathrm{km} = 10^3\ \mathrm{m} 1 km = 1 0 3 m ). [2] As a decimal multiple of the metre —the SI base unit of length—the kilometre leverages the metric system's prefix "kilo-" to denote a factor of one thousand, promoting straightforward calculations and scalability for measuring distances ranging from urban scales to geographical features. [2] The metre itself is defined as the length of the path travelled by light in vacuum during a time interval of 1 / 299 792 458 1/299\,792\,458 1/299 792 458 of a second. [7] This fixed definition ensures universal reproducibility, independent of physical artifacts or Earth's varying geometry. Historically, the metre was conceived as one ten-millionth of the distance from the equator to the North Pole along a meridian, rendering the kilometre approximately one ten-thousandth of that distance, though modern standards no longer rely on such geodetic measurements. [7] Symbol and Abbreviation The official symbol for the kilometre in the International System of Units (SI) is "km", consisting of the lowercase prefix symbol "k" for kilo- followed by the lowercase symbol "m" for metre, with no period at the end. [2] This symbol is printed in upright roman type and remains unchanged in the plural form; for example, it is written as "5 km" rather than "5 kms". [5] According to SI guidelines, the symbol "km" is used in technical and scientific writing, particularly when accompanied by numerical values, while the full name "kilometre" is preferred in general prose or when the unit stands alone without a number. [2] A non-breaking space must separate the numerical value from the symbol (e.g., "10 km", not "10km"), and the symbol is not capitalized unless it begins a sentence, in which case the full name is typically spelled out instead. [8] The prefix "kilo-", denoted by "k", originates from the Greek word "khilioi", meaning "thousand", and multiplies the base unit by a factor of 10³, so that 1 km equals 1,000 m . [2] Conversions and Equivalences To Metric Units The kilometre, defined as exactly 1,000 metres, facilitates seamless conversions to other metric length units through the system's decimal-based structure. [6] This scalability stems from the International System of Units (SI), where the kilometre serves as a multiple of the base unit, the metre . Conversions to smaller metric units are straightforward: 1 km equals 1,000 metres (m), 100,000 centimetres (cm), or 1,000,000 millimetres (mm). [6] For larger units, 1 km corresponds to 0.001 megametres (Mm) or 10 hectometres (hm). [6] These equivalences highlight the metric system's reliance on powers of ten, enabling quick adjustments without complex factors. The metric prefixes for length measurements denote these powers of ten, promoting uniformity across scales. Key prefixes include kilo- for 1 0 3 10^3 1 0 3 , hecto- for 1 0 2 10^2 1 0 2 , deca- for 1 0 1 10^1 1 0 1 , deci- for 1 0 − 1 10^{-1} 1 0 − 1 , centi- for 1 0 − 2 10^{-2} 1 0 − 2 , and milli- for 1 0 − 3 10^{-3} 1 0 − 3 . [9] For instance, the " kilo " in kilometre indicates multiplication by 1,000 relative to the metre, while " milli " divides by 1,000 to reach the millimetre. [1] In practice, this decimal coherence simplifies everyday calculations, such as converting a 5 km distance to 5,000 metres by simply shifting the decimal point or multiplying by 1,000. [6] Similarly, expressing 2 km in centimetres yields 200,000 cm, underscoring the efficiency of metric internal conversions for fields like engineering and navigation. [6] To Imperial and US Customary Units The primary conversion from the kilometre to imperial and US customary units centers on the statute mile, defined exactly as 1 statute mile = 1.609344 kilometres through an international agreement in 1959 between the United States, United Kingdom, and other Commonwealth nations. [10] Thus, 1 kilometre ≈ 0.621371 statute miles, providing a direct bridge for distance measurements in systems still prevalent in the US and UK. [10] This equivalence is foundational for applications requiring interoperability between metric and non-metric standards. Additional equivalences include 1 kilometre ≈ 3,280.84 feet (using the international foot of exactly 0.3048 metres) and 1 kilometre = 0.539957 nautical miles (based on the nautical mile of exactly 1,852 metres). [10] In finer subdivisions, 1 kilometre ≈ 39,370.08 inches (from the exact inch of 0.0254 metres) or ≈ 1,093.61 yards (from the yard of exactly 0.9144 metres). [10] Unit Conversion from 1 km Basis Statute mile ≈ 0.621371 1 mi = 1.609344 km exactly Foot (international) ≈ 3,280.84 1 ft = 0.3048 m exactly Yard ≈ 1,093.61 1 yd = 0.9144 m exactly Inch ≈ 39,370.08 1 in = 0.0254 m exactly Nautical mile ≈ 0.539957 1 nmi = 1,852 m exactly In US customary units, these conversions generally align with imperial standards but exhibit minor variances due to the US survey foot (≈ 0.3048006096 metres) versus the international foot. For instance, the US survey mile totals 1,609.347218694437 metres, making 1 kilometre ≈ 0.621370 survey miles—a difference of about 0.000001 miles, negligible for most practical purposes but relevant in precise surveying . [10] These conversions derive from the 1959 redefinition anchoring the inch to exactly 2.54 centimetres, scaled upward through successive units (yard = 36 inches, mile = 1,760 yards) to ensure consistency with the metre's definition. No exhaustive derivation is needed here, as the factors are codified in international standards. For quick mental estimates in everyday scenarios, such as travel , 1 kilometre is often approximated as 5/8 mile (0.625 miles), which introduces an error of less than 0.6% and facilitates rapid conversions without calculators. Pronunciation and Spelling Pronunciation In British English, "kilometre" is typically pronounced with stress on the first and third syllables, transcribed in the International Phonetic Alphabet (IPA) as /ˈkɪl.əˌmiː.tər/, which sounds like "KIL-uh-MEE-tuh". [11] An alternative variant places primary stress on the second syllable, /kɪˈlɒm.ɪ.tər/, sounding like "ki-LOM-i-tuh", though the first form is more standard in formal contexts. [12] In non-rhotic accents common to British English , the final "r" is not pronounced, rendering the ending as /tə/ rather than a full /tər/, and the terminal "e" in "metre" remains silent, avoiding any schwa or vowel sound there. [11] In American English , the pronunciation closely mirrors the British primary variant but features a flapped "t" sound in the third syllable and often a more open vowel in the second, given in IPA as /kɪˈlɑː.mə.t̬ɚ/ with stress primarily on the second syllable , sounding like "ki-LAH-muh-ter", or less commonly /ˈkɪl.əˌmiː.t̬ɚ/ like "KIL-uh-MEE-ter". [11] The second-syllable stress is the most prevalent in North American speech. [13] The final "e" is likewise silent, and the word ends with a rhotic "r" sound in rhotic American accents. Australian English generally aligns with British pronunciation, favoring /ˈkɪl.əˌmiː.tər/ ("KIL-uh-MEE-tuh") as per broadcasting standards, though the second-syllable-stressed variant /kɪˈlɒm.ɪ.tər/ ("ki-LOM-i-tuh") is increasingly heard in casual speech. [14] A common mispronunciation in both varieties places undue stress on the second syllable as "kill-OM-eter" (/kɪlˈɒmɪtər/), diverging from the metric prefix "kilo-" pattern in words like "kilogram". [12] Dictionaries recommend avoiding this to maintain consistency with related units. [11] Spelling Variations The spelling of the unit of length equivalent to one thousand metres varies by regional conventions in English. In British English and most Commonwealth countries, inc