Molecular Weight & Exact Mass Calculator
Compute molar mass and monoisotopic exact mass from a formula. View element composition with mass percentages. Export results as CSV and PDF. Fast and reliable calculations.
Developed by: Nohman Habib
Example Data
Use these sample formulas to test the calculator and compare average molar mass with monoisotopic exact mass.
| Formula | Compound | Avg Molar Mass (g/mol) | Exact Mass (Da) |
|---|---|---|---|
| H2O | Water | 18.015280 | 18.010565 |
| CO2 | Carbon dioxide | 44.009500 | 43.989829 |
| C6H12O6 | Glucose | 180.155880 | 180.063388 |
| Al2(SO4)3 | Aluminum sulfate | 342.150877 | 341.818266 |
| CuSO4·5H2O | Copper(II) sulfate pentahydrate | 249.685000 | 248.934151 |
| Ca3(PO4)2 | Calcium phosphate | 310.176724 | 309.794614 |
Formula Used
Average molar mass is computed by summing each element’s standard atomic weight multiplied by its atom count.
M = Σ(ni × Ai) where ni is the number of atoms and Ai is the average atomic weight.
Exact mass (monoisotopic mass) uses the mass of the most abundant isotope for each element.
Mexact = Σ(ni × mi) where mi is the monoisotopic mass.
How to Use This Calculator
- Enter a chemical formula such as
C6H12O6orAl2(SO4)3. - For hydrates, use a middle dot
·or period., for exampleCuSO4·5H2O. - Select how many decimals you want in the output.
- Enable element composition to see counts, contributions, and mass percentages.
- Press the calculate button to view results above the form.
- Use the CSV or PDF buttons to download your report.
FAQs
What is the difference between molar mass and exact mass?
Molar mass uses average atomic weights based on natural isotopes. Exact mass uses monoisotopic masses for the most abundant isotope of each element, which is commonly used in mass spectrometry work.
Does the calculator support parentheses in formulas?
Yes. Formulas like Al2(SO4)3 and Ca3(PO4)2 are parsed by expanding grouped terms and applying multipliers after the closing bracket.
Can I calculate hydrates and adducts?
Yes. Use the middle dot or a period, such as CuSO4·5H2O. The calculator treats each part as a separate formula and adds them together.
Why do my results differ from another website or textbook?
Small differences can come from using different atomic weight standards, rounding rules, or a different definition of monoisotopic mass. Set decimals consistently when comparing sources.
What elements are supported?
The calculator supports common chemical elements used in general chemistry and many lab formulas. If an element symbol is not recognized, it will display a clear error so you can correct the formula.
Are the CSV and PDF exports identical to the on-page output?
They include the same inputs and totals, plus a composition table when available. The PDF is a clean text report that works well for saving and sharing.