Estimate molecular weight from a chemical formula using a built-in element parser.
This tool provides estimates for informational purposes only. It is not a substitute for professional advice. Individual results vary based on your inputs and assumptions, so review important decisions with a qualified professional.
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Molecular weight (also called molecular mass or molar mass) is a fundamental concept in chemistry, essential for everything from balancing chemical equations to dosing pharmaceuticals. Whether you are a UK A-level chemistry student working through stoichiometry problems, a US AP Chemistry student preparing for exams, a pharmacist calculating drug doses, or a researcher in materials science, understanding how to calculate molecular weight is a core skill. This comprehensive guide covers the calculation method, key atomic masses, worked examples, and the connections to moles, molarity, and stoichiometry.
Molecular weight (MW) is the sum of the atomic masses of all atoms in a molecule. It is expressed in atomic mass units (amu or u) or, equivalently, in grams per mole (g/mol) — because numerically these are the same. For example:
These three terms are often used interchangeably but have subtle distinctions:
In practice, all three terms are used interchangeably in most educational and professional contexts.
| Element | Symbol | Atomic Number | Atomic Mass (g/mol) |
|---|---|---|---|
| Hydrogen | H | 1 | 1.008 |
| Carbon | C | 6 | 12.011 |
| Nitrogen | N | 7 | 14.007 |
| Oxygen | O | 8 | 16.00 |
| Fluorine | F | 9 | 19.00 |
| Sodium | Na | 11 | 22.99 |
| Magnesium | Mg | 12 | 24.31 |
| Phosphorus | P | 15 | 30.97 |
| Sulfur | S | 16 | 32.06 |
| Chlorine | Cl | 17 | 35.45 |
| Potassium | K | 19 | 39.10 |
| Calcium | Ca | 20 | 40.08 |
| Iron | Fe | 26 | 55.85 |
| Copper | Cu | 29 | 63.55 |
| Bromine | Br | 35 | 79.90 |
| Iodine | I | 53 | 126.90 |
To calculate molecular weight from a chemical formula:
Worked example — Sulfuric acid (H₂SO₄):
Paracetamol (UK) / Acetaminophen (US) is one of the most commonly used drugs worldwide:
Avogadro's number (6.022 × 10²³) defines how many particles (atoms, molecules, or formula units) are in one mole of a substance. The mole connects the macro world (grams you can weigh in a lab) to the micro world (atoms and molecules).
Number of moles = Mass (g) ÷ Molar Mass (g/mol)
Example: How many moles are in 36.03 g of water (MW = 18.015 g/mol)? Moles = 36.03 ÷ 18.015 = 2 moles
Molarity (M) = Moles of solute ÷ Volume of solution (litres)
Example: Dissolving 58.44 g of NaCl (1 mole) in 1 litre of water gives a 1 M (1 molar) NaCl solution.
Stoichiometry — the quantitative relationship between reactants and products in a chemical reaction — relies entirely on molecular weights. For the combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O. The molecular weights tell you exactly how many grams of each substance are involved: 16.04 g methane + 64.00 g oxygen → 44.01 g CO₂ + 36.03 g water.
This is central to both UK A-level Chemistry (AQA, OCR, Edexcel syllabuses) and US AP Chemistry curriculum. Stoichiometry questions on both exams require students to convert between grams, moles, and numbers of particles using molecular weight as the conversion factor.
The empirical formula gives the simplest whole-number ratio of atoms. The molecular formula gives the actual number of atoms. Glucose has the molecular formula C₆H₁₂O₆ but empirical formula CH₂O. To find the molecular formula from an empirical formula: divide the molecular weight by the empirical formula weight. Glucose empirical mass = 12+2+16 = 30 g/mol. 180.16 ÷ 30 = 6. Molecular formula = (CH₂O)₆ = C₆H₁₂O₆.
Molecular weight is critical in drug formulation and dosing. Drug doses are often specified in mg/kg of body weight, which requires knowing the molecular weight to prepare solutions of known molarity. The concept of drug "equivalents" — comparing different salts of the same drug — also requires molecular weight calculations. For example, lithium carbonate (Li₂CO₃, MW = 73.89 g/mol) vs lithium chloride (LiCl, MW = 42.39 g/mol): a 300 mg dose of lithium carbonate contains 2 × 6.94/73.89 × 300 = 56.3 mg of elemental lithium.
In the UK, the British Pharmacopoeia (BP) and in the US, the United States Pharmacopeia (USP) both require molecular weight information for drug substance monographs. Analytical chemists use MW constantly in HPLC, mass spectrometry, and NMR calibrations.
For polymers (plastics, biological macromolecules like proteins), molecular weight is not a single number but a distribution. Two key averages are used: number-average molecular weight (Mn) and weight-average molecular weight (Mw). The ratio Mw/Mn is called the polydispersity index (PDI) — a measure of chain length uniformity. Proteins have a single defined molecular weight (monodisperse), while synthetic polymers like polyethylene have a distribution. Protein molecular weights are commonly expressed in daltons (Da) or kilodaltons (kDa): human insulin is 5,808 Da; haemoglobin is 64,500 Da.
Identify all elements in the chemical formula and their subscripts. Multiply each element's atomic mass by its subscript count. Sum all results. For H₂SO₄: (2 × 1.008) + (1 × 32.06) + (4 × 16.00) = 2.016 + 32.06 + 64.00 = 98.08 g/mol. Our molecular weight calculator does this automatically from any formula.
Water (H₂O) has a molecular weight of 18.015 g/mol. Calculated as: 2 × 1.008 (hydrogen) + 1 × 16.00 (oxygen) = 2.016 + 16.00 = 18.016 g/mol (commonly rounded to 18.015 or 18.02 in different contexts).
Molecular weight is the mass of one molecule in atomic mass units (amu). Molar mass is the mass of one mole of a substance in grams per mole (g/mol). They are numerically identical — the molecular weight of H₂O is 18.015 amu per molecule, and the molar mass is 18.015 g/mol. In practice, both terms are used interchangeably.
Moles = Mass (g) ÷ Molar Mass (g/mol). For example, to find moles in 90 g of water: 90 ÷ 18.015 = 4.996 ≈ 5 moles. Conversely, Mass = Moles × Molar Mass.
Glucose (C₆H₁₂O₆) has a molecular weight of 180.16 g/mol. Calculated as: 6 × 12.011 (C) + 12 × 1.008 (H) + 6 × 16.00 (O) = 72.066 + 12.096 + 96.00 = 180.16 g/mol.
Empirical formula gives the simplest whole-number ratio of atoms. Molecular formula gives the actual number. Glucose molecular formula is C₆H₁₂O₆; empirical formula is CH₂O. To find molecular formula: divide the known molecular weight by the empirical formula weight and multiply the subscripts by that ratio.
Molecular weight is used to prepare solutions of known molarity (concentration), to compare different salt forms of the same drug, and to calculate drug doses per kilogram body weight. The British Pharmacopoeia (UK) and US Pharmacopeia (US) include molecular weight in all drug substance monographs as a key quality parameter.
A dalton (Da) is the same as an atomic mass unit (amu) — it equals 1/12 the mass of a carbon-12 atom. In biochemistry and protein science, molecular weights are commonly expressed in daltons or kilodaltons (kDa). Human haemoglobin is ~64,500 Da (64.5 kDa). The dalton is named after John Dalton, the English chemist who developed atomic theory in the early 19th century.