Convert kg to g (kilograms to grams)

One kilogram is exactly 1,000 grams, by the meaning of the prefix. This is one of the few conversions here where there is nothing to look up: 'kilo' is the SI prefix for a thousand, so the factor is part of the unit's name rather than a relationship between two units.

Worked through: 1 kg x 1,000 = 1,000 g.

Going the other way, one gram is 0.001 kilograms.

For mental arithmetic: Add three zeros.

The oddity worth knowing is that the kilogram, not the gram, is the SI base unit of mass, the only base unit with a prefix built into its name. That is a historical accident from the 1795 definition of the gram as the mass of a cubic centimeter of water, which turned out to be too small to realize accurately as a physical standard, so the kilogram prototype became the reference instead. Since 2019 the kilogram has been defined from the Planck constant and no physical artifact is involved at all.

Precision is worth matching to the input. A measurement given as 0.1 kg carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.