Kilo is a prefix meaning one thousand. When you see “kilo” at the start of a unit of measurement, it multiplies that unit by 1,000. A kilogram equals 1,000 grams. A kilometer equals 1,000 meters (about six-tenths of a mile). A kilowatt equals 1,000 watts of electrical power.

The prefix works with many different measurements. People sometimes talk about kilobytes (1,000 bytes) to measure computer data, though in computing it can also mean 1,024 instead. Engineers measure force in kilonewtons (1,000 newtons). If you've ever run a 5K race, that “K” stands for kilometers, so you ran 5,000 meters.

People often shorten “kilogram” to just “kilo” in everyday speech, especially outside the United States. If someone at a market says the apples cost five dollars per kilo, they mean per kilogram. The abbreviation for kilo in measurements is a lowercase k, so you might see “5k” meaning 5,000 or “km” meaning kilometers.