Hii Karthik!
The molecular weight of water is 18.01528 grams per mole. It is calculated as- H2O - 1*2 + 16 = 18. Since H2O is a molecule, we call it's weight as molecular weight. But, the formula for calculating the atomic or molecular weight remains the same. It is just adding up the individual atomic masses of the atoms present in a molecule.
Hope this helps.
Good luck!