What this calculator does
Moles are mass divided by molar mass. Eighteen grams of water, molar mass 18.02, is 0.9989 moles, just short of one because the molar mass is not exactly 18.
This is the most-used conversion in chemistry, because balanced equations are written in moles while balances weigh in grams. Almost every stoichiometry problem starts here, and the accuracy of everything downstream depends on getting the molar mass right.
The formula
The mass is divided by the molar mass. The molar mass comes from summing the atomic masses in the formula, and it is where most errors originate: a miscounted hydrogen or a forgotten water of crystallisation carries straight through into the answer.
| Term | Meaning |
|---|---|
| Mole | An amount containing 6.02214076×10²³ particles, by definition since 2019. |
| Molar mass | Grams per mole, numerically equal to the formula mass in amu. |
| Formula mass | The sum of atomic masses in the formula. |
| Water of crystallisation | Water bound into a crystal, which must be included in the molar mass. |
The inputs explained
| Field | What to enter |
|---|---|
| Mass (g) | Mass in grams. |
| Molar mass (g/mol) | Molar mass in g/mol. Water is 18.02, sodium chloride 58.44, glucose 180.16. |
When to use it
Starting a stoichiometry calculation
Balanced equations work in moles, so a weighed mass must be converted before anything else.
Preparing a solution
A target molarity translates to moles, which translate back to a mass to weigh out.
Checking a reaction
Converting reactant and product masses to moles shows whether the stoichiometry is consistent.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How many moles is each mass of water?
A range of masses of the same substance.
| Mass | Amount of substance | Mass used | Molar mass used |
|---|---|---|---|
| 1 g | 0.0555 mol | 1.00 g | 18.02 g/mol |
| 18 g | 0.9989 mol | 18.00 g | 18.02 g/mol |
| 18.02 g | 1.000 mol | 18.02 g | 18.02 g/mol |
| 100 g | 5.549 mol | 100.00 g | 18.02 g/mol |
Questions
How do I find the molar mass?
Sum the atomic masses of every atom in the formula. Water is two hydrogens at 1.008 plus one oxygen at 15.999, giving 18.015. For anything complex use a molecular weight calculator, since a single slip propagates through every subsequent step.
Do I include water of crystallisation?
Yes, if it is present in what you are weighing. Copper sulfate pentahydrate has a molar mass of 249.7 against 159.6 for the anhydrous salt, a difference of over 50%. Using the wrong one is a substantial error and a common one.
What exactly is a mole?
Since the 2019 redefinition, a mole is exactly 6.02214076×10²³ particles, fixed by definition rather than derived from a kilogram of carbon-12. The practical consequence is nil, since the number was chosen to match the previous definition.
Is molar mass the same as molecular weight?
Numerically yes, but the units differ. Molecular weight is dimensionless or in amu; molar mass is in g/mol. Water has a molecular weight of 18.02 and a molar mass of 18.02 g/mol. The terms are used interchangeably in practice.
For the particle count, see the moles to atoms calculator. For working out the molar mass, see the molecular weight calculator.