Molecular weight calculator
Compute molar mass from a chemical formula, including hydrates.
Molar mass
Enter a formula to calculate.
M = Σ (atomic weight × atom count)- Standard atomic weights of the elements 2021 — Pure and Applied Chemistry, 2021
Values follow IUPAC’s current abridged atomic weights, so a result may differ from a supplier’s catalogue in the last digit. Ammonium sulfate, for instance, comes to 132.13 here against the 132.14 printed on most bottles — the difference is sulfur.
When to use this
Use this to turn a chemical formula into a molar mass, including hydrates and nested groups. It is the step before the molarity calculator when the bottle gives a formula but no molecular weight, and it is the honest way to handle a hydrate rather than guessing.
Worked example
Your bottle says copper(II) sulfate pentahydrate, CuSO4·5H2O.
- Formula
- CuSO4·5H2O
Result
The anhydrous salt is 159.61, so weighing by that figure would leave you 36% short with nothing about the solution looking wrong.
What people get wrong
- Dropping the water of crystallisation. The hydrate is a different compound with a different mass, and it is the commonest silent error in solution preparation.
- Confusing case: Co is cobalt, CO is carbon monoxide. The parser reads element symbols strictly, which is why it refuses rather than guessing.
- Expecting an exact match to a supplier’s catalogue. Atomic weights are periodically revised and some are intervals, so the last digit can legitimately differ.
Questions
+How do I write a hydrate?
Either CuSO4·5H2O with a middle dot, or CuSO4.5H2O with a full stop. Both parse the same way.
+Does it handle brackets?
Yes, including nesting — Ca(NO3)2 and K3[Fe(CN)6] both parse, with the multipliers applied outward.
+Is this the average or the monoisotopic mass?
The average, from standard atomic weights, which is what solution preparation needs. For mass spectrometry the peptide analyser reports monoisotopic mass as well.
Related tools
- Molarity calculator — Convert between mass, molar concentration and volume for a solution.
- Dilution calculator — Work out how much stock and diluent to combine for a target concentration.
Science last reviewed .