Molar Mass Calculator
Enter any chemical formula — parentheses supported.
Free chemistry formula cheat sheet (PDF)
One page with the formulas you use most — molar mass, grams ⇄ moles, pH and PV = nRT — each with a worked example. Leave your email and we'll send it over.
Worked examples
Water has 2 hydrogen atoms and 1 oxygen atom. From the periodic table: H = 1.008 u, O = 16.00 u. So M(H₂O) = 2 × 1.008 + 1 × 16.00 = 18.02 g/mol. The calculator also shows that oxygen makes up 88.8% of the mass — hydrogen contributes only 11.2%.
Parentheses mean "multiply everything inside": Ca(OH)₂ contains 1 Ca, 2 O and 2 H. M = 40.08 + 2 × (16.00 + 1.008) = 74.09 g/mol. Type Ca(OH)2 into the calculator — the parenthesis parser handles the grouping for you.
6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 72.07 + 12.10 + 95.99 = 180.16 g/mol. Notice how carbon, despite only 6 atoms, contributes 40% of the mass — a quick way to check your answer makes sense before a test.
How it works
The molar mass of a compound is the sum of the atomic masses of every atom in one formula unit, taken straight from the periodic table. Subscripts multiply an element's contribution, and a subscript after parentheses multiplies everything inside them — that's why Ca(OH)₂ counts two of each atom in the OH group.
The result is expressed in g/mol: one mole of the substance (6.022 × 10²³ formula units) has exactly that many grams. Mass percentages tell you which element dominates the mass — useful for empirical-formula problems and hydrate calculations.
Take it with you: download the one-page formula cheat sheet (PDF) — print it and keep it on your desk.
Next steps: convert grams to moles with the Mole Converter, check yourself with the practice quiz, or study the full method in the The Mole & Stoichiometry lesson.