What is equal to one billionth of a millimeter?

Answer 1

Using the short scale, 1 billionth of a millimetre = 10⁻⁹ × 10⁻³ m = 10⁻¹² m = 1 pm = 1 picometre

Using the long scale, 1 billionth of a millimetre = 10⁻¹² × 10⁻³ m = 10⁻¹⁵ m = 1 fm = 1 femtometre

Answer 2

It is a picometre.