Water [H2O] Molecular Weight Calculation

The molecular weight of Water [H2O] is 18.0154.

Water [H2O] Molecular Weight CalculationWater (H2O) is an inorganic compound of four elements: Hydrogen and Oxygen. The molecular weight of Water is 18.0154 which can be calculated by adding up the total weight (atomic weight multiplied by their number) of all its elements.

CALCULATION PROCEDURE: Water [H2O] Molecular Weight Calculation

Step 1: Find out the chemical formula and determine constituent atoms and their number in a Water molecule.
From the chemical formula, you will know different atoms and their number in a Water molecule. Chemical formula of Water is H2O. From the chemical formula of Water, you can find that one molecule of Water has two Hydrogen (H) atoms and one Oxygen (O) atom.

Step 2: Find out atomic weights of each atom (from periodic table).

Atomic weight of Hydrogen (H) : 1.008 (Ref: Lanl-1)
Atomic weight of Oxygen (O) : 15.9994 (Ref: Jlab-ele008)

Step 3: Calculate the total weight of each atom present in a Water molecule by multiplying its atomic weight by its number.

Number of Hydrogen atoms in Water: 2
Atomic weight of Hydrogen: 1.008
Total weight of Hydrogen atoms in Water: 1.008 x 2 = 2.016

Number of Oxygen atoms in Water: 1
Atomic weight of Oxygen: 15.9994
Total weight of Oxygen atoms in Water: 15.9994 x 1 = 15.9994

Step 4: Calculate the molecular weight of Water by adding up the total weight of all atoms.

Molecular weight of Water: 2.016 (Hydrogen) + 15.9994 (Oxygen) = 18.0154

So the molecular weight of Water is 18.0154.

Water [H2O] Molecular Weight Calculation

Molecular weight of Water
Constituent atoms
Number of each atom
Atomic weightTotal weight
Hydrogen (H)
Oxygen (O)
Molecular weight of Water:


  • Lanl-1: https://periodic.lanl.gov/1.shtml 
  • Pubchem-Hydrogen : https://pubchem.ncbi.nlm.nih.gov/element/Hydrogen 
  • Jlab-ele006; https://education.jlab.org/itselemental/ele006.html
  • Pubchem-6: https://pubchem.ncbi.nlm.nih.gov/element/6
  • Jlab-ele008: https://education.jlab.org/itselemental/ele008.html
  • Pubchem-8: https://pubchem.ncbi.nlm.nih.gov/element/8

See also:

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