Molar Mass Calculator

How to use this calculator

Type a chemical formula into the first box. Element symbols are case-sensitive, parentheses and square brackets can be nested, and hydrates can be written with a middle dot, an asterisk or a period: CuSO4·5H2O, CuSO4*5H2O or CuSO4.5H2O. The molar mass fills in immediately, and a table shows how many atoms of each element there are and what share of the mass they contribute — the percent composition.

Then enter a mass to get moles and number of particles, or a number of moles to find how many grams to weigh out. If you already know the molar mass, skip the formula and type it straight into the molar mass field; from a measured mass and amount, the calculator works the molar mass out instead.

The formula

M = Σ (atoms of element × atomic weight)

n = m / M

N = n × N_A, N_A = 6.02214076 × 10²³ mol⁻¹

M: molar mass
m: mass of sample
n: amount of substance
N: number of particles

Worked example

How many moles and how many molecules are in 10 g of water (H₂O)?

Molar mass of H₂O = 2 × 1.008 + 15.999 = 18.015 g/mol.
n = m / M = 10 / 18.015 = 0.5551 mol.
N = n × N_A = 0.5551 × 6.02214076 × 10²³ = 3.343 × 10²³ molecules.

Ten grams of water is 0.555 mol, or about 334 sextillion molecules.

Molar masses of common compounds

Compound	Formula	g/mol
Water	H₂O	18.015
Carbon dioxide	CO₂	44.009
Sodium chloride	NaCl	58.44
Calcium carbonate	CaCO₃	100.086
Glucose	C₆H₁₂O₆	180.156
Sulfuric acid	H₂SO₄	98.072

Values use IUPAC abridged standard atomic weights, the same figures printed in most textbook periodic tables.

Assumptions and limits

Atomic weights are IUPAC abridged standard values for natural isotopic abundance. Isotopically enriched samples (e.g. heavy water, D₂O) need exact isotope masses.
For elements with no stable isotopes (Tc, Pm, Po and heavier), the mass number of the longest-lived isotope is used.
Charges are ignored: enter SO4 for sulfate; the electron mass is negligible.

Frequently asked questions

What is the difference between molar mass and molecular weight?

Numerically they are the same. Molar mass has units of g/mol; molecular weight (relative molecular mass) is dimensionless, or expressed in daltons per molecule.

How do I find percent composition?

Divide each element’s mass contribution by the total molar mass. The table under the formula box does this for you — water is 11.19% hydrogen and 88.81% oxygen by mass.

Why does CO give a different answer than Co?

Symbols are case-sensitive. Co is cobalt (58.933 g/mol); CO is carbon monoxide, one carbon plus one oxygen (28.010 g/mol).