Symbols always are written in the singular form (even when more than one is meant). RIGHT: 1 mm, 500 mm, 1 kg, 36 kg WRONG: 500 mms, 36 kgs BUT: It is correct to pluralize written-out metric unit names: 25 kilograms, 250 milliliters Source: http://lamar.colostate.edu/~hillger/correct.htm

Something you could be referred to as ` a 10kg weight` kg is not showing plural because it is an adjective in this sense

1kg = 1 kilogram 2kgs = 2 kilograms 3kgs = 3 kilograms etc. You use kg when you are referring to 1 kilogram. Every other number you use kgs.