Reference Guides

Calculating Molecular Weight


Molecular weight is the complete atomic weight of a molecule. The MW (molecular weight) is found by adding the mass of all individual atoms that are in a molecule. In this guide, we will show the steps needed to calculate MW.


Step 1:


The key is to find the molecular formula for the compound you are calculating. This means the number of atoms that the compound consists of. (You can find this information in any chemistry book or even online.)


As an example, the molecular formula for Vitamin D3 is C27H44O. The molecular formula can be used to find the number of atoms of each element that form the compound.


  • For C27H44O there are 27 atoms of Carbon, 44 atoms of Hydrogen, and 1 atom of Oxygen.


Step 2:


Locate the atomic mass of all elements that are in the compound. All periodic tables will have the atomic mass listed below the symbol for the element. Once you have located the atomic mass on the periodic table, multiply those masses by 1gram/mole.


  • The atomic masses for the elements in Vitamin D3 are Carbon, 12.011 g/mol, Hydrogen 1.008 g/mol, and Oxygen 15.999 g/mol.


Step 3:


Calculate the total molar mass of the individual elements in the compound. Do this by multiplying the molar mass by the number of atoms of the element in the compound. This way you will be able to see how much each element contributes to the compound.


  • Carbon, 12.011 g/mol x 27, Hydrogen 1.008 g/mol x 44, and Oxygen 15.999 g/mol x 1.


Step 4:


Add the molar mass of all elements in the compound. Now you will be able to determine the molar mass for the whole compound. Using the numbers obtained in Step 3, you can add them together to calculate the molar mass.


  • Carbon, 324.297 g/mol + Hydrogen 44.352 g/mol + Oxygen 15.999 g/mol = 384.64


Molecular weight is an important factor in many different research fields. For example, in pharmaceutical research, the molecular weight will determine the time it will take to absorb the chemical. In chemistry, molecular weight is used to determine the theoretical yield of a chemical reaction. This type of calculation (stoichiometry) uses the molecular weights of chemicals and reactants to find the amount of product that will be produced or how much reactant would be needed in order to produce the required amount of product.

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