What is 21 kg in grams?

21000 grams. The prefix "kilo" (symbol k) in the International System of Units (SI) and other systems, which denotes 103, it means one thousand. Therefore, there are 1,000 grams (g) in a kilogram (kg) and 0.001 kilogram equal a gram. Kilograms and grams are units of mass not volume such as liters, gallons, quarts, pints, and cups, and can only be conveted to each other if you have the specific item being measured, as each item has a different density.