Molecular Weight Calculator

What molecular weight is

The molecular weight of a compound is the sum of the atomic weights of all the atoms in its formula. Water, H2O, has two hydrogen atoms at 1.008 and one oxygen atom at 15.999, so its molecular weight is 18.015. In grams per mole it is the molar mass: 18.015 grams of water contain one mole, or 6.02214076 × 10²³ molecules. The two terms are used interchangeably here, although molar mass is strictly the more correct name.

How to write a formula

Write each element symbol with a capital first letter, followed by a lowercase letter if it has one, and then the number of atoms as a plain number: NaCl, H2SO4, C6H12O6. Use round, square or curly brackets for groups, and put the number of groups after the closing bracket, as in Ca(OH)2 or Fe2(SO4)3. For a hydrate, put a dot between the parts and the number of water molecules before the water: CuSO4.5H2O. A number at the very start multiplies the whole part, so 2H2O has four hydrogen atoms and two oxygen atoms.

Reading the results

The table lists each element in the order it appears, with how many atoms there are, its atomic weight, and the mass it adds. The percentage beside each subtotal is the mass percent of that element in the compound, which is useful for finding the composition of a compound or checking a purity. The total row gives the number of atoms and the molecular weight.

About the atomic weights

Atomic weights are weighted averages over the natural isotopes of an element, which is why they are not whole numbers. This calculator uses the abridged standard atomic weights published by IUPAC, rounded as in the table above. For elements with no stable isotope, such as technetium, the value is the mass number of the longest-lived isotope. If you need more precision for a specific sample, use the exact isotopic masses instead.

Common questions