Molecular Weight (MW) Calculator
Use this tool to calculate molecular weight, sample mass, or moles using standard stoichiometry relationships.
What is an MW calculator?
An MW calculator is a tool that helps you solve relationships between three core chemistry values: mass, moles, and molecular weight (also called molar mass). In many chemistry classes and labs, you constantly move between these values, so having a quick calculator saves time and reduces arithmetic mistakes.
In this page, “MW” means molecular weight in g/mol. For practical lab work, molecular weight and molar mass are usually used interchangeably.
Core formula behind this calculator
The calculator uses one relationship:
- Mass (g) = Molecular Weight (g/mol) × Moles (mol)
From that equation, you can rearrange to solve anything:
- Molecular Weight (g/mol) = Mass (g) ÷ Moles (mol)
- Moles (mol) = Mass (g) ÷ Molecular Weight (g/mol)
How to use the MW calculator
1) Choose your calculation mode
Pick whether you want to calculate molecular weight, mass, or moles. The field labels update automatically.
2) Enter two known values
Type the values you already have. Use decimal values when needed (for example, 0.025 mol). Both numbers must be greater than zero.
3) Click Calculate
The tool computes the missing value and displays it with units in the result box. Use the Clear button to reset and start a new problem.
Worked examples
Example A: Find molecular weight
You measured 9.8 g of a sample and found it contains 0.2 mol. MW = 9.8 ÷ 0.2 = 49 g/mol.
Example B: Find required mass
You need 0.15 mol of a compound with MW 180.16 g/mol. Mass = 180.16 × 0.15 = 27.024 g.
Example C: Find moles from mass
You have 5.00 g of NaCl (MW 58.44 g/mol). Moles = 5.00 ÷ 58.44 = 0.0856 mol (rounded).
Why molecular weight matters
- Lab preparation: Make solutions at exact concentrations.
- Stoichiometry: Predict product yield and reagent needs.
- Pharmaceutical work: Dose and formulation accuracy.
- Environmental chemistry: Convert ppm and concentration data correctly.
- Food and materials science: Track composition and reaction efficiency.
Quick molecular weight reference table
| Compound | Formula | Approx. MW (g/mol) |
|---|---|---|
| Water | H₂O | 18.015 |
| Carbon Dioxide | CO₂ | 44.009 |
| Sodium Chloride | NaCl | 58.44 |
| Glucose | C₆H₁₂O₆ | 180.16 |
| Ethanol | C₂H₆O | 46.07 |
Common mistakes to avoid
- Mixing units (mg vs g, mmol vs mol) without converting first.
- Using zero for moles in a division calculation.
- Rounding too early, which can amplify final error.
- Confusing molecular weight with concentration (they are different concepts).
Final takeaway
A reliable MW calculator makes day-to-day chemistry faster and cleaner. If you consistently track units and use the correct formula direction, you can solve most mass-mole-molecular weight problems in seconds.