Molecular Weight Calculator

How to calculate molecular weight

1. Enter the chemical formula.
2. Use the correct uppercase and lowercase letters for each element symbol.
3. If the formula has a group, use parentheses or brackets.
4. If the formula has water of hydration, put a dot before the water.
5. If the formula has an isotope label or an ionic charge, include it.
6. Select Calculate.

Molecular weight formula

M = Σ(nᵢ × Aᵢ)

Worked example

For water, the formula is H2O. The H2O formula has two hydrogen atoms and one oxygen atom. The calculation is (2 x 1.0080) + (1 x 15.999). The molar mass is 18.015 g/mol.

Chemical formula formats

Enter the chemical formula. The calculator reads groups, hydrates, isotope labels, and ionic charges.

Molecular weight and molar mass

For one chemical formula, molecular weight and molar mass have the same numerical value. Molecular weight uses the unit u or Da. Molar mass uses the unit g/mol. Monoisotopic mass uses one specified isotope of each element. The monoisotopic mass can be different from the average molecular weight.

Common mistakes

• Do not enter a compound name in the Chemical formula field.
• Use the correct uppercase and lowercase letters. CO is carbon monoxide. Co is cobalt.
• Use a dot before the water in a hydrate.

Frequently asked questions

Atomic mass data

The table contains data for all 118 elements. The calculator uses average atomic masses for natural elements. It uses listed isotope masses for radioactive elements.

Sources

• PubChem Periodic Table
• NIST Atomic Weights and Isotopic Compositions