Molecular Weight Calculator

Molecular weight vs molar mass vs molecular mass

MW means molecular weight: the sum of the atomic weights of all atoms in a molecule. Molecular mass and molar mass give the same number with different units.

A mass of 18.015 u for one water molecule equals 18.015 g for one mole of water molecules.

Calculate molecular weight by hand

Molecular weight = Σ (number of atoms × atomic weight)

1. Write the chemical formula, for example C6H12O6.
2. Count the atoms of each element. Multiply out brackets and hydrate water.
3. Multiply each atom count by the atomic weight of the element in g/mol.
4. Add the results. The sum is the molecular weight in g/mol.

Molecular weight examples

Molecular weight of hydrates and complex salts

Treat each part after a dot as a separate unit with its own coefficient. Treat each bracket as a group, then add everything.

Grams, moles, and molecules

moles = mass (g) ÷ molecular weight (g/mol)

mass (g) = moles × molecular weight (g/mol)

molecules = moles × 6.02214076 × 10^23 (the Avogadro constant)

Example: 36.0 g of water is 36.0 ÷ 18.015 = 2.00 mol, or 1.20 × 10^24 molecules. Use the converter below each result for these steps.

Molecular weight of DNA, RNA, and proteins

Biomolecules are long chains with thousands of atoms. Enter their sequence instead of a formula in a sequence calculator.

• DNA Molecular Weight Calculator
• RNA Molecular Weight Calculator
• Protein Molecular Weight Calculator

Atomic weight data

Average atomic weights come from the PubChem periodic table (snapshot 2026-08-28). Exact isotope masses come from NIST Atomic Weights and Isotopic Compositions, version 2.4.1.

IUPAC gives the atomic weight of some elements as a range, because their isotope ratios vary in nature. The table shows the single value this calculator uses for each.

Embed this calculator

Teachers and course sites can embed the calculator for free. Copy this code into any web page.

Add ?formula=CuSO4.5H2O to the address to open the calculator with a formula.

<iframe src="https://chemcalculator.com/molecular-weight-calculator?embed=1" title="Molecular Weight Calculator" width="100%" height="640" loading="lazy" style="border:0"></iframe>

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