Prefix · quantity · thousand
Means "thousand" in the metric system: kilometer, kilogram, kilowatt, kilobyte.
Written by Bryan López, English teacher · Updated
Kilo- comes from Greek "khilioi" (thousand) and is the SI prefix that multiplies any base unit by 1,000. Kilometer = 1,000 metres; kilogram = 1,000 grams; kilowatt = 1,000 watts; kilobyte = 1,000 bytes (or 1,024 in computing); kilohertz = 1,000 hertz. The same prefix works identically in Spanish: kilómetro, kilogramo, kilovatio, kilobyte, kilohercio. It is the most commonly used metric prefix in everyday life.
SI prefixes go in powers of 10: milli- (÷1,000), centi- (÷100), deci- (÷10), base, deca- (×10), hecto- (×100), kilo- (×1,000), mega- (×1,000,000), giga- (×1,000,000,000). Kilo- sits between the base unit and mega-. In everyday speech, "a kilo" of rice = 1,000 grams; speed is measured in km/h. In computing, kilobyte can be 1,000 or 1,024 bytes depending on context (decimal vs. binary).
Combines with SI base units to multiply them by 1,000:
Pronunciation: /ˈkɪl.oʊ/. Official abbreviation: k (lowercase). Exact Spanish cognates: kilómetro, kilogramo, kilovatio (kilowatt), kilobyte, kilohercio. In everyday speech, "a kilo" = 1 kilogram.
kilo-/ˈkɪl.oʊ/
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The prefix kilo- never changes: it always means ×1,000. The confusion arises because the US uses the imperial system (miles, pounds, gallons) while most of the world uses the metric system (kilometres, kilograms, litres). Quick conversion: km × 0.6 ≈ miles; miles × 1.6 ≈ km.
A trick to remember it
Kilo- = thousand (×1,000). Same prefix in Spanish: kilómetro, kilogramo, kilovatio, kilobyte. Abbreviation: k. Nutrition note: food "calories" are actually kilocalories (kcal). A "100-calorie" yoghurt contains 100,000 small calories.
Exercise 1 · Form the word
Exercise 2 · Pick the right one
Why do nutritionists say "calorie" when they technically mean "kilocalorie"?
The prefix kilo- means "thousand" in the metric system: kilometer, kilogram, kilowatt, kilobyte. In Spanish it usually maps to kilo-.
The prefix kilo- is pronounced /ˈkɪl.oʊ/. For example, "kilometer".
"meter" becomes "kilometer". It is a typical example of the kilo- prefix.
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