What this calculator does
A mole is the chemist's counting unit: one mole of any substance contains the same fixed number of particles, roughly 6.022 x 10²³ of them (Avogadro's number). The most basic mole calculation converts between the mass of a sample you can actually weigh and the number of moles it represents, using the substance's molar mass as the conversion factor: moles = mass ÷ molar mass.
Molar mass is specific to each substance, so it has to be looked up or calculated from the periodic table rather than assumed; water's molar mass (about 18.02 g/mol) is very different from sodium chloride's (about 58.44 g/mol). This calculator takes the molar mass as an input, so it works for any compound once that figure is known.
The formula
Divide the mass of the sample by the substance's molar mass to get the amount in moles. To go the other way, multiply the amount in moles by the molar mass to recover the mass.
| Term | Meaning |
|---|---|
| Mole (mol) | The SI unit for an amount of substance, equal to Avogadro's number of particles. |
| Molar mass (M) | The mass of one mole of a substance, in grams per mole, specific to that substance. |
| Amount (n) | The quantity of substance in moles: n = mass ÷ molar mass. |
The inputs explained
| Field | What to enter |
|---|---|
| Direction | Choose from Mass (g) to moles, Moles to mass (g). |
| Mass (g) or amount (mol) | Enter the mass in grams, or the amount in moles, depending on the direction selected above. |
| Molar mass of the substance (g/mol) | The molar mass of the substance in grams per mole. This varies by substance and has to be looked up or calculated from atomic masses; it is not a fixed constant. |
When to use it
Preparing a solution from a solid
Before a solution can be made to a target concentration, the mass of solid to weigh out is worked back from the number of moles needed, using this same relationship.
Working from a chemical equation
Balanced equations relate substances by mole ratios, not by mass, so masses measured on a scale need converting to moles before those ratios can be applied.
Checking a lab result
Converting a measured mass to moles, or a stated mole quantity to the mass that should be weighed out, is a routine check before running a reaction or preparing a reagent.
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 are in common masses of water?
A range of water masses converted to moles.
How much does 1 mole of common substances weigh?
A fixed 1 mole converted to mass using different molar masses.
| Molar mass | Mass |
|---|---|
| 18.02 g/mol | 18.020 g |
| 58.44 g/mol | 58.440 g |
| 44.01 g/mol | 44.010 g |
| 98.08 g/mol | 98.080 g |
Questions
Where do I find the molar mass of a substance?
Add up the atomic masses of every atom in the chemical formula, using the values from a periodic table. Water (H₂O) is roughly 2(1.008) + 16.00 = 18.02 g/mol. Many reference tables also list molar masses directly for common compounds.
How is this different from the molarity calculator?
This calculator converts between mass and moles for a substance on its own. The molarity calculator goes a step further, dividing that same mole figure by a solution volume to get a concentration in mol/L.
How is this different from converting moles to atoms?
This calculator relates mass to moles using molar mass. The Avogadro's number calculator relates moles to a count of individual atoms or particles, using Avogadro's constant instead of molar mass.
Why is the mole such a large number?
It is defined so that one mole of a substance has a mass in grams numerically equal to its atomic or molecular mass, which keeps everyday lab quantities (grams) linked to a workable number of moles, rather than working directly with atom counts in the 10²³ range.
To turn this mole figure into a solution concentration, see the molarity calculator. To convert moles into an actual particle count, see the Avogadro's number calculator.