Molar Mass Calculator

What Is Molar Mass?

Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). It is numerically equal to the molecular weight (or formula weight) of a compound. Molar mass is a fundamental concept in chemistry that connects the microscopic world of atoms and molecules to the macroscopic quantities we measure in the laboratory.

How to Calculate Molar Mass

To calculate the molar mass of a compound, follow these steps:

1. Write the chemical formula — identify every element and its subscript count. For example, water is H₂O (2 hydrogen atoms, 1 oxygen atom).
2. Look up atomic masses — find each element's atomic mass on the periodic table (H = 1.008 g/mol, O = 15.999 g/mol).
3. Multiply and sum — multiply each element's atomic mass by its count, then add the results: (2 × 1.008) + (1 × 15.999) = 18.015 g/mol.

Common Examples

Why Molar Mass Matters

Molar mass is essential for converting between grams and moles — the bridge between what you weigh on a balance and the number of particles in a sample. It is used in stoichiometry to determine how much of each reactant is needed and how much product will form. Pharmacists use it to calculate drug dosages, environmental scientists use it to measure pollutant concentrations, and materials engineers use it to design new compounds.

Molecular Weight vs. Molar Mass

While often used interchangeably, molecular weight (MW) technically refers to the mass of a single molecule measured in atomic mass units (amu or Da), whereas molar mass refers to the mass of one mole of molecules in grams per mole (g/mol). The numerical values are the same — water has a molecular weight of 18.015 amu and a molar mass of 18.015 g/mol — but the units differ.

Handling Parentheses in Formulas

Parentheses group atoms that repeat as a unit. In Ca(OH)₂, the (OH) group appears twice, giving 1 calcium, 2 oxygen, and 2 hydrogen atoms. Our calculator handles nested parentheses automatically — just type the formula as you would write it.