Chemical formula calculator

Molecular Weight Calculator

Calculate molecular weight and molar mass from chemical formulas, grouped compounds, and hydrate notation.

Enter chemical formula

Calculate molecular weight

Free tool

Supports element symbols, subscripts, parentheses, nested groups, and hydrates.

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Enter a valid chemical formula, then calculate its molecular weight.

Calculator guide

How this calculator works

Molecular Weight Calculator determines the total mass of a molecule by adding the atomic masses of all elements in a chemical formula. It helps students, researchers, and laboratory professionals calculate molecular weight and molar mass values accurately.

Formula explanation

Molecular weight is calculated by adding the atomic masses of all atoms present in a chemical compound.

Formula

Molecular Weight = Sum of (Number of Atoms × Atomic Mass)

Worked example

Example: Water molecular weight is calculated by combining hydrogen and oxygen atomic masses.

Assumptions

  • The chemical formula is entered correctly.
  • Atomic masses are based on accepted periodic table values.
  • Element quantities in the formula represent the actual compound composition.

Examples

  • Example: Water (H₂O) molecular weight is calculated by adding two hydrogen atoms and one oxygen atom masses.
  • Example: Chemists use molecular weight calculations to determine quantities needed for experiments and chemical reactions.

Common mistakes

  • Counting atoms incorrectly.
  • Using outdated atomic mass values.
  • Ignoring chemical subscripts.

Variables

  • Chemical formula
  • Element symbols
  • Number of atoms
  • Atomic masses
  • Molecular weight

Limitations

  • Accuracy depends on correct chemical formulas and atomic mass values.
  • Different atomic mass standards may produce small variations in calculated results.

Scientific references

  • OpenStax Chemistry: Chemical Formulas and Composition
  • NIST Chemistry WebBook
  • Periodic table atomic mass reference data

Content review

Reviewed by: ScienceCalcHub Chemistry Review Team | Last reviewed: 2026-08-30

Applications

  • Chemical compound analysis
  • Laboratory preparation
  • Stoichiometry calculations
  • Molecular research

Frequently asked questions

How is molecular weight calculated?

Molecular weight is calculated by adding the atomic masses of every atom present in a chemical formula.

What is the difference between molecular weight and molar mass?

Molecular weight describes the mass of a molecule, while molar mass represents the mass of one mole of a substance.

Why is molecular weight important in chemistry?

Molecular weight helps scientists calculate chemical quantities, prepare solutions, and perform accurate laboratory experiments.

Accuracy and transparency

Created and maintained by our editorial team

This chemistry calculator is maintained by the ScienceCalcHub Editorial Team. Its calculation logic is tested with representative inputs, while the supporting guidance is checked for formula clarity, units, assumptions, and common mistakes.

Written by
ScienceCalcHub Editorial Team
Reviewed by
ScienceCalcHub Scientific Review Team
Review standard
Formula accuracy, units, examples, and educational clarity

Learn more about our formula-review and correction process, explore our calculation methodology, or view our scientific references.

  • Calculation logic tested
  • Variables and units explained
  • Assumptions stated clearly
  • Corrections handled transparently

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.

  1. Hydrogen: 2 × 1.008 = 2.016
  2. Oxygen: 1 × 15.999 = 15.999
  3. 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.

Related chemistry tools

Use the Mass to Moles Calculator to convert between grams and moles using the calculated molar mass.

Use the Molarity Calculator to calculate solution concentration.

Questions and answers

Calculator FAQ

How is molecular weight calculated?

Molecular weight is calculated by adding the atomic masses of every atom present in a chemical formula.

What is the difference between molecular weight and molar mass?

Molecular weight describes the mass of a molecule, while molar mass represents the mass of one mole of a substance.

Why is molecular weight important in chemistry?

Molecular weight helps scientists calculate chemical quantities, prepare solutions, and perform accurate laboratory experiments.