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Molecular Weight Calculator

Type a chemical formula. You get its molar mass in g/mol, the number of atoms and a table showing how much each element contributes.

Your numbers

Result

How we calculate it

The molar mass of a compound is the sum of the atomic weights of all its atoms. The calculator reads the formula (element symbols with a capital first letter, counts, brackets and a dot or · for water of crystallization), counts the atoms of each element and multiplies by the standard atomic weight.

For example, glucose C6H12O6 is 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.156 g/mol, and the table shows that carbon makes up 40.0% of it. In CuSO4·5H2O, the part after the dot is multiplied by 5, giving 249.685 g/mol.

Assumptions and limits

  • Symbols are case sensitive: Co is cobalt, but CO is carbon and oxygen. Brackets ( ) and [ ] can be nested, and a number after a bracket multiplies everything inside.
  • The atomic weights are the conventional standard values, rounded as in common periodic tables, so the answer can differ in the last digit from a table that uses more digits.
  • Elements with no stable isotope have no standard atomic weight. Their value here is the mass number of the longest lived isotope. Charges and isotope labels are not supported.

Frequently asked questions

How do I calculate the molecular weight of H2O?

Add two hydrogen atoms (2 × 1.008) and one oxygen atom (15.999). That gives 18.015 g/mol.

What is the difference between molecular weight and molar mass?

They have the same number. Molecular weight is a relative mass with no unit, and molar mass is the mass of one mole in grams per mole. For water both are 18.015.

How do I enter a hydrate?

Write the water after a dot or the · sign with its number in front, such as CuSO4·5H2O or CuSO4.5H2O. The number multiplies everything after it.

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Worked example (a fixed example using the starting numbers, it does not change as you type)

Inputs: Chemical formula = C6H12O6.

  • Molar mass 180.156 g/mol
  • Formula C6H12O6
  • Total atoms 24

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Last reviewed: . Formulas and assumptions are described on our methodology page. For information only; not professional advice.