Molecular weight calculator
Enter a chemical formula to get its molar mass (relative formula mass) and how much of that mass each element makes up.
molar mass = Σ (atoms × standard atomic weight)
How to calculate molar mass
Molar mass is the mass of one mole of a substance, in grams per mole (g/mol). To calculate it, multiply each element's atomic weight by the number of its atoms in the formula, then add the results. For glucose, C₆H₁₂O₆: 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.16 g/mol.
Relative formula mass (Mr) and molecular weight
Relative formula mass, written Mr, is the same number as the molar mass without the unit, because it compares the mass of a formula unit with one twelfth of a carbon-12 atom. Molecular weight is an older name for the same thing. To find the Mr of calcium carbonate, CaCO₃: 40.08 + 12.01 + 3 × 16.00 = 100.09. The term "relative formula mass" is usual for ionic compounds such as salts, and "relative molecular mass" for molecules, but they are worked out the same way.
Brackets and hydrates
A number after an element or a bracket multiplies it, so Ca(OH)₂ has one calcium, two oxygens and two hydrogens. A number after a dot multiplies everything that follows it: the ·5H₂O in copper sulfate pentahydrate adds five waters, 90.08 g/mol. Hydrates matter in the lab. The bottle's label says which form it holds, and weighing the wrong one gives the wrong concentration.
Atomic weights used
The calculator uses IUPAC standard atomic weights, the same values as our peptide sequence properties tool. Natural isotope ratios vary slightly between sources, so figures past the second decimal place are estimates. For elements with no stable isotope, the mass number of the longest-lived isotope is used. This is an average mass. Mass spectrometry measures the monoisotopic mass instead, which the sequence tool gives for peptides.
Once you know the molar mass, the molarity calculator works out how much to weigh for a solution.