The mass of atoms can not be measured directly; the number would just be too small. So instead the masses are compared using a relative mass. Carbon-12 has been chosen as this reference isotope.
You can find the masses of elements on the periodic table.
So if I wanted to find out the relative mass of H2O I would look for Hydrogen and Oxygen on the periodic table.
Because H2O is a formula, 18 would be it’s relative formula mass.
Chemistry. 