1
Enter the formula
Use correct element capitalization, numeric subscripts, and grouping brackets when needed.
Free chemistry tool
Free Molecular Weight Calculator
A molecular weight calculator adds the atomic masses of every atom in a chemical formula. Enter a compound to get its molar mass, atom counts, element subtotals, and mass percentages.
Enter a chemical formula
Capitalization matters. Use numbers for subscripts and parentheses or brackets for repeated groups.
Supported: element symbols, whole-number subscripts, nested (), [],, and hydrate dots such as CuSO4·5H2O.
Try an example
Calculated molar mass
H2O
18.015g/mol
Elements
2
Total atoms
3
| Element | Atoms | Atomic mass | Subtotal | Mass % |
|---|---|---|---|---|
| HHydrogen | 2 | 1.008 | 2.016 | 11.19% |
| OOxygen | 1 | 15.999 | 15.999 | 88.81% |
Atomic masses use the values in the app's 118-element periodic table and are rounded for classroom use.
How to Calculate Molecular Weight
2
Check atom counts
Review how the calculator expands groups and hydrate coefficients into each element count.
3
Use the total
Read the molar mass in g/mol and verify each atomic-mass subtotal in the table.
Molecular Weight Calculator FAQ
What is a molecular weight calculator?
A molecular weight calculator adds the atomic masses of every atom in a chemical formula and reports the total as molar mass in grams per mole.
Are molecular weight and molar mass the same?
They use the same numerical sum for a formula. Molecular weight is technically a unitless relative value, while molar mass is expressed in grams per mole.
How do I enter parentheses and subscripts?
Type ordinary numbers after an element or group, such as Ca(OH)2. The calculator multiplies every atom inside the parentheses by the following subscript.
Can this calculator handle hydrates?
Yes. Separate the compound and water with a dot, such as CuSO4·5H2O. A regular period is automatically converted to a hydrate dot.
Why might my answer differ from another source?
Small differences usually come from rounding atomic masses. This tool uses standard classroom atomic-mass values and shows each value in the breakdown.
Related Free Tools
Interactive Periodic Table
Explore the atomic masses and properties of all 118 elements.
Standard Deviation Calculator
Calculate sample or population spread with clear steps.
Graphing Calculator
Plot and compare equations in a classroom-ready graph.
Turn chemistry practice into an accessible lesson
SPED Lesson Planner connects standards, IEP goals, accommodations, and differentiated instruction in one plan.