Last updated: June 2026
Use this molar mass calculator to find the mass of one mole of a substance from its chemical formula. It is written for O-Level, IGCSE, A-Level and Singapore H2 Chemistry students who need to calculate molar mass, relative molecular mass, formula mass or molecular weight clearly and accurately.
What is molar mass?
Molar mass tells us the mass of one mole of a substance. It is usually measured in g mol−1. For example, if the molar mass of water is 18.0 g mol−1, this means that one mole of water molecules has a mass of 18.0 g.
Molar mass is useful because it connects mass to amount of substance. Once you know the molar mass, you can convert between grams and moles using the mole formula.
n = m / Mr
where n is the amount of substance in mol, m is the mass in g, and Mr is the relative molecular mass or relative formula mass.
Molar mass vs molecular weight
In many school chemistry questions, molar mass, molecular weight, molecular mass and relative molecular mass are used in very similar ways. The numerical value is often the same, but the meaning is slightly different.
Relative molecular mass, Mr, has no unit because it is a relative value. Molar mass has the unit g mol−1. For example, water has Mr = 18.0, while its molar mass is 18.0 g mol−1.
For ionic compounds such as NaCl, MgO and CaCO3, it is more accurate to say relative formula mass instead of relative molecular mass, because ionic compounds do not exist as simple molecules.
How to find molar mass for any compound
To calculate molar mass, count the number of atoms of each element in the formula. Then multiply each number by the relative atomic mass of that element. Finally, add all the values together.
Molar mass = sum of each element’s relative atomic mass × number of atoms
For example, carbon dioxide has formula CO2. It contains one carbon atom and two oxygen atoms. Using C = 12.0 and O = 16.0:
Molar mass of CO2 = 12.0 + 2(16.0) = 44.0 g mol−1
Worked examples
Example 1: Molar mass of water, H2O
Water contains two hydrogen atoms and one oxygen atom.
H = 1.0 and O = 16.0
Molar mass of H2O = 2(1.0) + 16.0 = 18.0 g mol−1
Therefore, one mole of water has a mass of 18.0 g.
Example 2: Molar mass of sodium chloride, NaCl
Sodium chloride contains one sodium ion and one chloride ion in its formula unit.
Na = 23.0 and Cl = 35.5
Molar mass of NaCl = 23.0 + 35.5 = 58.5 g mol−1
Example 3: Molar mass of calcium hydroxide, Ca(OH)2
Calcium hydroxide contains one calcium atom, two oxygen atoms and two hydrogen atoms. The bracket is important because the subscript 2 outside the bracket applies to both O and H.
Ca = 40.1, O = 16.0 and H = 1.0
Molar mass of Ca(OH)2 = 40.1 + 2(16.0) + 2(1.0) = 74.1 g mol−1
Example 4: Molar mass of aluminium sulfate, Al2(SO4)3
Aluminium sulfate is a good example because it contains brackets. Al2(SO4)3 contains two aluminium atoms, three sulfur atoms and twelve oxygen atoms.
Al = 27.0, S = 32.1 and O = 16.0
Molar mass of Al2(SO4)3 = 2(27.0) + 3(32.1) + 12(16.0) = 342.3 g mol−1
Common exam traps
Frequently Asked Questions
Is this the same as a molecular weight calculator?
For school chemistry purposes, yes. This molar mass calculator also helps with molecular weight, molecular mass, relative molecular mass and formula mass calculations. The page uses “molar mass” as the main term because it is the most useful term for mole calculations.
Is molar mass the same as Mr?
The numerical value is usually the same, but the unit is different. Mr has no unit, while molar mass is written in g mol−1.
How do I calculate molar mass when there are brackets?
Multiply everything inside the bracket by the number outside the bracket. For example, Mg(NO3)2 contains one magnesium atom, two nitrogen atoms and six oxygen atoms.
Why do ionic compounds use formula mass?
Ionic compounds form giant ionic lattices instead of simple molecules. That is why formula mass is more accurate than molecular mass for compounds such as NaCl, MgO and CaCO3.
Why is molar mass important?
Molar mass allows you to convert between mass and amount of substance. This is essential for mole calculations, reacting mass questions, titration calculations and many other chemistry topics.