Molecular Weight Calculator
Adjust the inputs below. Results update as you type.
How it works
Sum the atomic masses of all atoms in the formula. H₂O: 2(1.008) + 15.999 = 18.015 g/mol. Parentheses multiply: Ca(OH)₂ = 40.078 + 2(15.999 + 1.008) = 74.093 g/mol. Molecular weight is used to convert between mass and moles. Molar mass numerically equals molecular weight but has units of g/mol. Isotope-specific masses differ from standard atomic weights.
Input guidance
- Confirm date/time/unit settings before comparing outputs.
- If a task has optional fields, run both with and without them to understand impact.
- Save scenario variants when making practical decisions.
The formula
Molecular weight = sum of the atomic weights of all atoms in the formula. Multiply each element's atomic mass by its count and add.
Worked example
Water (H₂O): 2 × 1.008 + 16.00 = 18.02 g/mol.
More examples to test
- Quick-pass example: run default assumptions for a first estimate.
- Refined example: update one assumption at a time to isolate impact.
How to interpret results
Use outputs as operational estimates and pair them with local constraints, business rules, or provider requirements.
When this can be inaccurate
Utilities can miss local policy details, special-case rules, and environment-specific constraints.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
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General recommendation, not professional advice.
Frequently asked questions
What is molecular weight?+
The mass of one mole of a substance, found by adding the atomic weights of every atom in its chemical formula, in grams per mole.
How is it different from molar mass?+
They are numerically the same for a given substance; molar mass carries the units g/mol and applies to moles of the compound.
Why do I need it?+
It converts between mass and moles, which is essential for preparing solutions and balancing chemical reactions.
Why are outputs slightly different from another tool?+
Different tools can use different rounding rules, default assumptions, and treatment of edge cases.
How can I improve estimate reliability?+
Use verified inputs from real records and re-run calculations after major assumption changes.