A kilobyte is 2 to the 10th power, or 1,024 bytes.
"But doesn't 'kilo' mean one thousand?" you ask. Well yes, but in the computer world, certain rules and guidelines don't have the same influence they do in other areas of life. Just ask any computer programmer. A kilobyte is technically 1,024 bytes because it is measured by 2^10, which equals 1,024. However, kilobytes are often estimated as 10 to the 3rd power, or 1,000 bytes. While this makes it easier to add kilobytes together, estimating can throw off larger measurements. This is because 1,024 kilobytes equal one megabyte, 1,024 megabytes equal one gigabyte, and so on.
Most small files on your computer are measured in kilobytes. For example, thumbnail images might use only 5 to 10KB of space. A larger 900x600 pixel JPEG image can take up 250KB of space. Text files are often less than 1KB. Most documents you save on your computer should be between 1 and 1,024KB. Anything larger than 1,024KB is measured in megabytes.
For a list of all the different units of measurements, view this Help Center article.
Abbreviation: K or KB