Molecular Weight Calculator
Compute molar mass from a chemical formula with a per-element breakdown.
About Molecular Weight Calculator
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)
Worked example
With the default values below, Molecular Weight Calculator returns Molar Mass of 18.01500. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Chemical Formula
- H2O
- Quantity (optional)minimum 0 mol
- 1 mol
Results
- Molar Mass
- 18.01500 g/mol
- Total Atom Count
- 3
- Mass for Given Moles
- 18.0150 g
Step by step
- 1Parsed formula= H: 2, O: 1
- 2Molar mass = Σ (atoms × atomic weight)= 18.015 g/mol
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.