Molar Mass Calculator (chemical formula)

Understanding the molar mass calculator — concept and principle

Molar mass is the mass of one mole of a substance, in grams per mole (g/mol). It links what you can weigh on a balance to the number of particles taking part in a reaction, so you need it for almost every stoichiometry, solution or yield problem. Type a chemical formula and the calculator returns the molar mass, the percentage of the total mass contributed by each element, and the atom count.

Twelve hydrogens are the most atoms but only 6.7% of the mass; oxygen carries more than half.

How it is calculated

M = Σ (nᵢ × Aᵢ), where nᵢ is the number of atoms of element i in the formula and Aᵢ is its standard atomic weight (IUPAC abridged values, such as C = 12.011, H = 1.008, O = 15.999).

Parentheses and brackets multiply everything inside: Ca(OH)₂ contains 1 Ca, 2 O and 2 H.
A hydrate dot (· or *) adds the water: CuSO₄·5H₂O contains 5 extra H₂O units.
Mass percent of element i = nᵢAᵢ ÷ M × 100.

The principle

A mole is defined as exactly 6.02214076 × 10²³ entities. Atomic weights are expressed relative to the atomic mass constant (1/12 of the mass of a carbon-12 atom), and the mole is scaled so that an amount with relative mass X weighs about X grams. So adding the atomic weights of all atoms in a formula gives grams per mole directly. Standard atomic weights are averages over the natural isotope mix found on Earth, which is why chlorine is 35.45 rather than a whole number.

Worked example

Default formula: C₆H₁₂O₆ (glucose).

Carbon: 6 × 12.011 = 72.066 g/mol.
Hydrogen: 12 × 1.008 = 12.096 g/mol.
Oxygen: 6 × 15.999 = 95.994 g/mol.
Sum: 72.066 + 12.096 + 95.994 = 180.156 g/mol.
Composition: C 72.066 ÷ 180.156 = 40.00% (shown as 40%), H 6.714%, O 53.28%. Atoms: C6 H12 O6.

How to read the result

Use the molar mass to convert grams to moles (n = m ÷ M) or to weigh out a specific amount. Mass percentages let you check an elemental analysis or compare how much of a nutrient or metal a compound supplies. Small differences from a textbook value (180.16 versus 180.156) are just rounding.

Common mistakes

Writing element symbols in the wrong case: Co is cobalt, CO is carbon monoxide.
Forgetting to multiply a subscript after parentheses across every atom inside.
Leaving out the water of crystallisation when the label says pentahydrate or similar.

Keep in mind: The values use standard atomic weights for natural isotopic abundance. Isotopically labelled compounds, very precise mass spectrometry work or some elements with variable natural composition (such as lithium or boron) may need exact isotopic masses instead.

Frequently asked questions

What is the difference between molar mass and molecular weight?

Numerically they are the same. Molecular weight (relative molecular mass) is dimensionless; molar mass carries the unit g/mol.

Is molar mass the same as monoisotopic mass?

No. Monoisotopic mass uses only the most abundant isotope of each element and is used in mass spectrometry; molar mass uses the natural average.

How do I enter a hydrate like copper sulfate pentahydrate?

Type CuSO4·5H2O or CuSO4*5H2O. The result should be about 249.7 g/mol.

Sources: IUPAC Commission on Isotopic Abundances and Atomic Weights, Standard atomic weights (abridged) · BIPM, The International System of Units (SI Brochure), 9th edition — definition of the mole

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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