Molecular Weight Calculator
Compute molar mass from a chemical formula with a per-element breakdown.
Inputs
Use standard element symbols with correct capitalisation. Subscripts are numbers. Hydrates use a dot.
Enter moles to also calculate the mass of that quantity.
Molar Mass
18.01500g/mol
Total Atom Count
3
Mass for Given Moles
18.0150g
Step by step
Parsed formula
= H: 2, O: 1
Molar mass = Σ (atoms × atomic weight)
2×1.008 + 1×15.999
= 18.015 g/mol
Element breakdown
| Element | Atoms | Atomic Weight (g/mol) | Contribution (g/mol) |
|---|---|---|---|
| H | 2 | 1.0080 | 2.0160 |
| O | 1 | 15.9990 | 15.9990 |
How it works
Molar mass is the sum of the atomic weights of all atoms in a formula unit. Atomic weights are weighted averages of naturally occurring isotope masses, published by IUPAC. The calculator parses parentheses recursively (e.g., Ca(OH)₂ = Ca + 2×O + 2×H), handles subscripts, and supports hydrate dot notation (CuSO₄·5H₂O).
Formula
Molar mass
Molar mass = Σ (number of atoms of each element × atomic weight of that element)
- n_i
- Number of atoms of element i
- A_i
- Standard atomic weight of element i (g/mol)
Frequently Asked Questions
What is the molar mass of water (H₂O)?
H₂O: 2 × H (1.008) + 1 × O (15.999) = 2.016 + 15.999 = 18.015 g/mol.
How do I enter CuSO₄·5H₂O (copper sulfate pentahydrate)?
Type "CuSO4.5H2O" using a period as the dot separator. The calculator splits on the dot, applies the coefficient 5 to H₂O, and adds up all atoms: Cu + S + 4O + 5×(2H + O) = 249.685 g/mol.
Why does capitalisation matter?
"CO" is carbon monoxide (C + O), while "Co" is cobalt. "NO" is nitric oxide (N + O), not nobelium. Capitalising only the first letter of each element symbol is essential.
What elements are included in the atomic weight table?
All 103 stable and well-characterised elements through Lawrencium (Lr=103), using IUPAC 2021 standard atomic weights. Transactinides (Z > 103) are not included.