Formula
How molecular weight is calculated
Molar mass =Σ(number of atoms × atomic mass)
Each element symbol is identified from the formula. Its atomic mass is multiplied by its atom count, and all contributions are added together.
Worked example
Molecular weight of water
Water has the formula H2O. It contains two hydrogen atoms and one oxygen atom.
- Hydrogen: 2 × 1.008 = 2.016
- Oxygen: 1 × 15.999 = 15.999
- Total: 2.016 + 15.999 = 18.015 g/mol
Supported notation
Chemical formulas this calculator supports
- Standard formulas such as H2O and NaCl
- Multi-digit subscripts such as C12H22O11
- Parentheses such as Ca(OH)2
- Nested groups such as K4(ON(SO3)2)2
- Hydrates such as CuSO4·5H2O
Common mistakes
Molecular-weight calculation mistakes
- Using incorrect capitalization for element symbols.
- Forgetting to multiply grouped atoms by a parenthesis subscript.
- Ignoring hydrate water molecules.
- Using atomic number instead of atomic mass.
- Rounding individual element masses too early.
Model assumptions
Calculator assumptions and limitations
- The chemical formula must use valid element symbols and correct capitalization.
- Subscripts, grouped compounds, and hydrate coefficients must be entered accurately.
- The calculation uses standard tabulated atomic masses.
- Isotopic composition and sample-specific abundance differences are not modeled.
- The result represents molar mass and does not verify the compound's identity or purity.
Questions and answers
Molecular weight calculator FAQ
How do you calculate molecular weight?
Multiply the atomic mass of each element by the number of atoms shown in the chemical formula, then add all element contributions.
Are molecular weight and molar mass the same?
They use the same numerical value for a substance. Molecular weight is often treated as a relative value, while molar mass is expressed in grams per mole.
Does this calculator support parentheses?
Yes. It supports grouped formulas such as Ca(OH)2 and nested groups such as K4(ON(SO3)2)2.
Can this calculator handle hydrates?
Yes. Hydrates can be entered using a middle dot, for example CuSO4·5H2O.