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Molar Mass Calculator with Step-by-Step Working

Enter a compound to see its molar mass in g/mol and every step of the calculation, the way you would write it in an exam. Add a mass to convert grams to moles in the same place.

Quick answer: to calculate molar mass, multiply each element's atomic mass by its subscript and add them. NaCl = 22.990 + 35.45 = 58.440 g/mol.

Compound formula Mass in grams (optional, for grams → moles)

Enter a formula.

How to use the molar mass calculator

Write the correct chemical formula (for example H2SO4).
Look up the atomic mass of each element on the periodic table. The calculator does this for you.
Multiply each atomic mass by how many of that atom are in the formula. Remember that a subscript after a bracket multiplies everything inside it.
Add the values. The total is the molar mass in g/mol. Enter a mass to convert grams to moles with n = m / M.

Worked examples

Sulfuric acid, H₂SO₄

H: 2 × 1.008 = 2.016
S: 1 × 32.06 = 32.060
O: 4 × 15.999 = 63.996

M = 98.072 g/mol

Calcium carbonate, CaCO₃

40.078 + 12.011 + 3 × 15.999

M = 100.086 g/mol

So 25.0 g CaCO₃ = 25.0 ÷ 100.086 = 0.2498 mol.

Carbon dioxide, CO₂

12.011 + 2 × 15.999

M = 44.009 g/mol

How to calculate molar mass

Molar mass (M) is the mass of one mole (6.022 × 10²³ particles) of a substance. Its units are grams per mole. Because the atomic mass unit is defined so that one mole of atoms of mass 1 u weighs 1 g, the molar mass in g/mol equals the relative formula mass.

M = Σ (subscript × atomic mass) n = m / M m = n × M

Tips that avoid common mistakes

Brackets multiply. In Mg₃(PO₄)₂ there are 2 P and 8 O atoms, so M = 3(24.305) + 2(30.974) + 8(15.999) = 262.855 g/mol.
Diatomic elements. Oxygen gas is O₂ (31.998 g/mol), not O (15.999 g/mol).
Hydrates include the water: CuSO₄·5H₂O is 249.677 g/mol, while anhydrous CuSO₄ is 159.602 g/mol.
Rounding. Keep at least 3 decimal places until the final answer, then round to your data's significant figures.

Why is the molar mass of H₂O 18?

Two hydrogen atoms (2 × 1.008 = 2.016) plus one oxygen (15.999) give 18.015 g/mol, which rounds to 18. One mole of water weighs about 18 g, and 18 mL of liquid water at room temperature is roughly one mole.

For a full element-by-element percent composition table use the molecular weight calculator.

Working from a structure? Sketch it in MolDraw and the editor shows the molecular formula and molar mass as you draw.

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Frequently asked questions

How do you calculate molar mass?

Multiply each element's atomic mass by the number of times it appears in the formula, then add all the values. The answer is in g/mol.

What is the formula for calculating molar mass?

M = Σ(n × A), where n is the number of atoms of each element and A is its atomic mass. To link mass and moles use n = m / M.

What is the molar mass of water?

18.015 g/mol (2 × 1.008 + 15.999).

How do I find molar mass without knowing the compound?

Measure a mass and the number of moles (for a gas, from PV = nRT; for a solution, from a titration) and divide: M = m / n.

What is the molar mass of 4 H2O?

The molar mass of H2O is still 18.015 g/mol. '4 H2O' means four moles (or molecules), which have a mass of 4 × 18.015 = 72.06 g.

What is the difference between molar mass and molecular weight?

They have the same numerical value. Molar mass carries units of g/mol; molecular weight is a dimensionless relative mass.

Related chemistry tools

Need to sketch molecules for a report? Draw chemical structures online with MolDraw, a free online ChemDraw alternative (see the ChemDraw vs MolDraw comparison).