Molar Mass Calculator
Molar mass is the mass of exactly one mole of a substance, measured in grams per mole (g/mol). It equals the sum of the atomic masses of every atom in a molecule, where atomic mass values come from the NIST Atomic Weights and Isotopic Compositions table. For example, water (H2O) has a molar mass of 2 x 1.008 + 15.999 = 18.015 g/mol. Knowing molar mass is fundamental to stoichiometry: it connects measurable mass in grams to the chemist's counting unit of moles, via the relationship moles = mass / molar mass. One mole contains 6.02214076 x 10^23 particles, so dividing molar mass by Avogadro's number gives the mass of a single molecule in grams. This calculator lets you build a compound from up to 8 element types using a dropdown of the 25 most common elements, each with its NIST-sourced standard atomic weight. Enter counts for each element and the tool instantly computes total molar mass. You can also enter a sample mass to convert it to moles. All atomic masses are standard atomic weights as published by NIST SP 330 and the IUPAC Commission on Isotopic Abundances and Atomic Weights.
Molar mass: 18.02 g/mol
Select an element and enter the atom count for each row (set count to 0 to skip).
How molar mass is calculated
For a compound with multiple element types, the molar mass is the sum of each element's atomic mass multiplied by the number of atoms of that element in one formula unit.
M = sum of (atomic_mass_i x count_i) for each element i
Moles = mass (g) / M (g/mol)
Worked example: water (H2O)
- Hydrogen (H): atomic mass = 1.008 g/mol, count = 2. Contribution = 2 x 1.008 = 2.016 g/mol
- Oxygen (O): atomic mass = 15.999 g/mol, count = 1. Contribution = 1 x 15.999 = 15.999 g/mol
- Molar mass of H2O = 2.016 + 15.999 = 18.015 g/mol
- Mass per molecule = 18.015 amu (numerically equal to molar mass in g/mol)
- If sample mass = 36.030 g: moles = 36.030 / 18.015 = 2.00 mol
Reference: atomic masses of common elements (NIST)
| Symbol | Element | Atomic Mass (g/mol) |
|---|---|---|
| H | Hydrogen | 1.008 |
| He | Helium | 4.003 |
| Li | Lithium | 6.941 |
| Be | Beryllium | 9.012 |
| B | Boron | 10.811 |
| C | Carbon | 12.011 |
| N | Nitrogen | 14.007 |
| O | Oxygen | 15.999 |
| F | Fluorine | 18.998 |
| Ne | Neon | 20.18 |
| Na | Sodium | 22.99 |
| Mg | Magnesium | 24.305 |
| Al | Aluminium | 26.982 |
| Si | Silicon | 28.086 |
| P | Phosphorus | 30.974 |
| S | Sulfur | 32.065 |
| Cl | Chlorine | 35.453 |
| Ar | Argon | 39.948 |
| K | Potassium | 39.098 |
| Ca | Calcium | 40.078 |
| Fe | Iron | 55.845 |
| Cu | Copper | 63.546 |
| Zn | Zinc | 65.38 |
| Br | Bromine | 79.904 |
| I | Iodine | 126.904 |
Values are standard atomic weights as published by NIST and the IUPAC Commission on Isotopic Abundances and Atomic Weights. These reflect the natural isotopic composition of each element on Earth.
Molar mass calculator: frequently asked questions
What is molar mass?
Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). It equals the sum of the atomic masses of all atoms in a molecule. For example, water (H2O) has a molar mass of 2 x 1.008 + 15.999 = 18.015 g/mol. Molar mass allows chemists to convert between the mass of a substance and the number of moles, which is essential for stoichiometric calculations.
What is an atomic mass unit (amu)?
An atomic mass unit (amu), also called a dalton (Da), is defined as one-twelfth the mass of a carbon-12 atom. It equals approximately 1.66054 x 10^-27 kg. The atomic mass of an element in amu is numerically equal to its molar mass in g/mol. For example, carbon has an atomic mass of 12.011 amu and a molar mass of 12.011 g/mol. NIST defines the unified atomic mass unit formally in NIST SP 330.
What is Avogadro's number?
Avogadro's number (Na) is 6.02214076 x 10^23 mol^-1. It defines the number of elementary entities (atoms, molecules, ions) in one mole of a substance. This value was fixed exactly by the 2019 redefinition of SI units. Combined with molar mass, it allows you to calculate the mass of a single molecule: divide the molar mass in g/mol by Avogadro's number to get mass in grams per molecule.
Why are molar masses expressed in g/mol?
The gram per mole unit arises directly from the definition of the mole. One mole of any substance contains exactly 6.02214076 x 10^23 particles. Because atomic masses are defined relative to carbon-12 in atomic mass units, and because one amu x Avogadro's number equals one gram, the molar mass in g/mol is numerically identical to the atomic or molecular mass in amu. This makes unit conversion between mass and moles straightforward.
How do I find the molar mass of a compound?
To find the molar mass of a compound, identify each element in the formula and how many atoms of each are present, then multiply each element's atomic mass (from the periodic table or NIST data) by its atom count, and sum all the results. For example, glucose (C6H12O6): 6 x 12.011 + 12 x 1.008 + 6 x 15.999 = 72.066 + 12.096 + 95.994 = 180.156 g/mol. This calculator performs that summation for up to 8 element types.
Official sources
- NIST Atomic Weights and Isotopic Compositions: nist.gov/pml/atomic-weights.
- NIST SP 330 (SI Units): NIST SP 330, 2019 edition.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. For educational use only.