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Molar Mass Calculator

How do I calculate molar mass?

How it is calculated

Molar mass is the mass of one mole of a substance, in grams per mole. The tool reads the chemical formula, counts the atoms of each element, and adds up M = Σ (atomic weight × number of atoms) using standard atomic weights stored in the tool, for example H = 1.007, O = 15.999, Ca = 40.078. It understands element symbols (a capital letter plus an optional lower-case letter), subscript numbers written as plain digits, and groups in round or square brackets followed by a multiplier. Element symbols are case-sensitive: Co is cobalt but CO is carbon and oxygen. The result is rounded to two decimals and shown in g/mol, with a breakdown of the atom counts. An unrecognised symbol or bracket mismatch gives an error message.

Worked example

Enter Ca(OH)2, calcium hydroxide. The bracket (OH) is repeated twice, so the formula contains 1 calcium, 2 oxygen and 2 hydrogen atoms, which the breakdown shows as Ca + O2 + H2. Calcium: 1 × 40.078 = 40.078. Oxygen: 2 × 15.999 = 31.998. Hydrogen: 2 × 1.007 = 2.014. Total = 74.09 g/mol. So 74.09 g of calcium hydroxide contains one mole of formula units.

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Common mistakes

  • Getting the capital letters wrong: "CO" is carbon monoxide, but "Co" is cobalt, and a lowercase first letter is rejected.
  • Leaving out the multiplier of the water in a hydrate: CuSO4·5H2O (or CuSO4.5H2O) means five water molecules; writing CuSO4·H2O counts only one and gives 177.62 g/mol instead of 249.67 g/mol.
  • Using Unicode subscript digits such as H₂O rather than ordinary digits (H2O).
  • Forgetting the multiplier after a bracket, such as Al2(SO4)3, and leaving out the 3.
  • Using the result as an exact isotopic mass; these are average standard atomic weights, so the figure can differ in the last digit from other tables.

About this calculator

This tool calculates the molar mass of a chemical compound — the mass in grams of one mole (6.022×10²³ particles) of that substance — by parsing a formula like H2O or CaCO3, looking up the atomic weight of each element involved, and summing them according to how many atoms of each appear. Getting this number right depends on correctly reading subscripts and parentheses in the formula, which is where manual calculation most often goes wrong.

It is used by chemistry students working through stoichiometry homework, and by lab technicians and researchers who need to convert between a target mass and a target number of moles when preparing a solution of a specific concentration. Molar mass is also the starting point for calculating things like molarity, percent composition and reaction yields, so it's typically the first number worked out in any quantitative chemistry problem.

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Comments (1)

  • Editorial team

    Molar mass is mostly a stepping stone: moles equal grams divided by molar mass, so this figure feeds straight into stoichiometry and solution-concentration work. The tool rounds atomic weights to three decimals, so water shows as 18.01 g/mol where some textbooks print 18.02. For lab work, use the weights your course or supplier specifies. The breakdown line lists element counts, a quick check that the formula was read as you meant. If a formula was rejected or parsed oddly, post it here.

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