Molar Mass Calculator

Calculate the molar mass of a compound or element by selecting it from the lists or entering the chemical formula below.

Molar Mass:

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What is Molar Mass?

Molar mass is the mass of a compound in grams per mole of substance, and it’s expressed in g/mol. Molar mass, sometimes referred to as molecular weight or molecular mass, is numerically equal to the combined atomic mass of each element in the compound.[1]

How to Calculate Molar Mass

You can find molar mass of a chemical compound using two different methods.

The first method is to calculate the molecular weight by adding the relative atomic mass for each element in the compound using the periodic table. The National Institute of Standards and Technology published the atomic weights for each element, which is a good resource to consult.[2]

For example, let’s calculate the molar mass for water (H2O).

H2O is composed of two hydrogen (H) atoms and one oxygen (O) atom.

The atomic mass of hydrogen is 1.00794 and oxygen is 15.9994. The formula to calculate the molecular weight should look like this:

molar mass = 1.00794 + 1.00794 + 15.9994

Therefore, the molar mass of H2O is 18.01528.

Graphic showing how to calculate the molar mass of a compound such as H2O

Learn more about calculating the weight of H2O on our water weight calculator.

Molecular Weight Formula

The second way to calculate molecular weight is using the molar mass formula. To use the formula, you’ll need the total mass of the substance in grams and the quantity in moles.

M = m / n

Thus, the molar mass M is equal to the mass of substance m in grams divided by the quantity n in moles.[3]

Of course, the calculator above is the easiest way to find molar mass of an element or compound.


  1. LibreTexts, Molar Mass, 2020, August 13,
  2. National Institute of Standards and Technology | Physical Measurements Laboratory, Atomic Weights and Isotopic Compositions for All Elements,
  3. National Institute of Standards and Technology, NIST Guide to the SI, Chapter 8,