Molecular Weight Calculator
Calculate Molecular Weight values for science, physics, chemistry, and engineering problems.
This molecular weight calculator finds the molar mass of a chemical compound by adding up the atomic weights of every atom in its formula, weighted by how many times each element appears.
Enter a chemical formula, and the calculator returns its total molecular weight in grams per mole.
Worked Calculation Examples
| Scenario | Result | Calculation Step |
|---|---|---|
| Water (H₂O) | 18.02 g/mol | 2 × 1.008 (H) + 1 × 16.00 (O) = 2.016 + 16.00 = 18.02 g/mol. |
| Carbon dioxide (CO₂) | 44.01 g/mol | 1 × 12.01 (C) + 2 × 16.00 (O) = 12.01 + 32.00 = 44.01 g/mol. |
How Molecular Weight Is Calculated
Molecular weight is the sum of each element's atomic weight multiplied by the number of times it appears in the formula. For water (H2O): 2 x (1.008, hydrogen's atomic weight) + 1 x (16.00, oxygen's atomic weight) = 18.02 g/mol.
Molecular Weight vs. Molar Mass
In practice, molecular weight and molar mass are used interchangeably for most purposes - both describe the mass of one mole of a substance in grams per mole - though molecular weight technically refers to molecules specifically, while molar mass applies more generally, including to ionic compounds that don't form discrete molecules.
How to Use the Molecular Weight Calculator
- Open the Molecular Weight Calculator and enter the values requested in the input fields.
- Check the units, percentages, dates, or time periods before reading the answer.
- Review the instant result and adjust any value to compare another scenario.
- Use the formula, example, and FAQs below to understand how the molecular weight calculator works.
Frequently Asked Questions
How do you calculate molecular weight?
Add up the atomic weight of every atom in the compound's chemical formula, multiplying each element's atomic weight by how many atoms of that element are present.
What is the molecular weight of water?
Water (H2O) has a molecular weight of approximately 18.02 g/mol: 2 hydrogen atoms (2 x 1.008) plus 1 oxygen atom (16.00).
Is molecular weight the same as molar mass?
They're used interchangeably in most contexts and share the same units (g/mol) - molecular weight technically applies to molecules, while molar mass is the broader term that also covers ionic compounds.
How do I use this molecular weight calculator?
Enter the known values, review the units or settings, and the calculator updates the result instantly. The formula and example on this page show how the answer is produced.
What does the Molecular Weight Calculator calculate?
Calculate Molecular Weight values for science, physics, chemistry, and engineering problems. It is designed for fast browser-based calculations without sign-up, downloads, or manual spreadsheet setup.
What formula does this molecular weight use?
The formula is: Molecular Weight = Σ (Atomic Weight × Number of Atoms). Modern atomic weight values trace back to 19th-century chemist John Dalton's atomic theory and were refined through decades of painstaking experimental measurement, standardized internationally by IUPAC (the International Union of Pure and Applied Chemistry) so that a molecular weight calculation gives the same answer regardless of which country or laboratory performs it - a standardization essential for reproducible chemistry worldwide.
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Last updated: September 27, 2026.