Molecular Weight Calculator (Molar Mass)
Find molecular weight (molar mass) in g/mol from a chemical formula or atom counts, with element breakdown and mass percent composition.
Type a chemical formula with case-sensitive element symbols — for example H2SO4, Al2(SO4)3 or CuSO4.5H2O. Parentheses, brackets and hydrate dots are supported.
Pick a compound to fill the formula.
When entered, the sample is converted to moles and molecules.
Molecular weight
18.015 g/mol
Molar mass of the molecule
Element breakdown
| Element | Count | Mass (g/mol) | Mass (%) |
|---|---|---|---|
| H | 2 | 2.016 | 11.19% |
| O | 1 | 15.999 | 88.81% |
Last updated: January 2026
How this calculator is verified
Checked by Rahim Virani on
- Atomic masses checked against current IUPAC standard atomic weights
- Parentheses in formulas parsed before multiplication so nesting resolves correctly
- Hydrates and multi-unit compounds sum all atoms exactly once
The full verification method is on our how we verify page. Found an error? Tell us and we will re-check it.
When to Use This Calculator
Use this calculator whenever a chemistry question, lab manual or industrial recipe asks for the molar mass of a compound. It covers the 14 elements that appear in the vast majority of school-level and undergraduate formulas — carbon, hydrogen, oxygen, nitrogen, halogens, alkali and alkaline earth metals, plus iron, copper and zinc — so most problems are solvable without a periodic table. Pharmacists use molecular weights to convert between moles and mass when compounding; fertiliser and pesticide formulators do the same when blending nutrient percentages. For stoichiometry, the molecular weight is the bridge that turns a balanced equation into grams on a balance. If the compound contains an element outside this list, add its atomic mass to the result manually.
How to Use This Calculator
- Step 1: Choose Formula mode and type a case-sensitive formula, for example Al2(SO4)3 — or pick a compound from the Common chemicals list.
- Step 2: Alternatively, switch to Atom counts mode and enter how many atoms of each element the molecule contains.
- Step 3: Optional: enter a sample mass in grams to convert it to moles and molecules.
- Step 4: Read the molecular weight in g/mol and the element breakdown with mass percentages.
Worked Example
Aluminium sulfate, Al₂(SO₄)₃, contains 2 aluminium, 3 sulfur and 12 oxygen atoms. Using IUPAC standard atomic weights — aluminium 26.982, sulfur 32.06 and oxygen 15.999 — the calculation is 2 × 26.982 = 53.964, 3 × 32.06 = 96.18 and 12 × 15.999 = 191.988. Adding these gives 342.132 g/mol, exactly what this calculator returns for Al2(SO4)3. Water (H₂O) comes to 18.015 g/mol, while copper sulfate pentahydrate, CuSO₄·5H₂O, which carries five water molecules, weighs 249.677 g/mol.
Tips and Common Mistakes
- •Type formulas case-sensitively — CO2 is carbon dioxide, but co2 is not a valid formula.
- •For hydrates, write the waters into the formula: CuSO4.5H2O includes five water molecules.
- •Enter a sample mass to convert grams to moles and molecules in one step.
- ✗Do not round atomic masses to whole numbers — the difference between 35.45 and 35 for chlorine matters in lab work.
- ✗Do not confuse molecular weight with molarity — molarity also depends on the solution volume.
Expert Note
Formulas must be case-sensitive: Co is cobalt, CO is carbon monoxide. For hydrates like CuSO₄·5H₂O, include the water molecules in the formula. The calculator uses IUPAC 2021 standard atomic weights — these are the values used in competitive exams (JEE, NEET) and professional chemistry.
Sources and References
- •IUPAC Technical Report: Standard Atomic Weights 2021 (Pure and Applied Chemistry)
- •NIST: Atomic Weights and Isotopic Compositions
- •NCERT Class 11 Chemistry, Chapter 1: Some Basic Concepts of Chemistry
Official sources are linked so you can confirm the current rate yourself. Check the linked page for the latest notification before relying on these figures.
Frequently Asked Questions
What is molecular weight?
Molecular weight (molar mass) is the mass in grams of one mole of a molecule, found by adding the atomic masses of all atoms. Water (H₂O) is 18.02 g/mol, glucose (C₆H₁₂O₆) is 180.16 g/mol.
How is molecular weight calculated?
Multiply each element's atomic mass by its atom count and add them up. Glucose has 6 carbon (12.011), 12 hydrogen (1.008) and 6 oxygen (15.999): 6×12.011 + 12×1.008 + 6×15.999 = 180.16 g/mol.
Why does this matter in chemistry?
Molecular weight converts grams to moles, which is needed for molarity, stoichiometry and balancing reactions. It is also used in pharmacy, fertiliser mixing and lab reagent preparation.
Which elements are included?
Fourteen common elements: H, C, N, O, Na, Mg, P, S, Cl, K, Ca, Fe, Cu and Zn. Elements outside this list, such as iodine or manganese, need to be added to the totals separately.
Is molecular weight the same as molar mass?
Yes — the terms are used interchangeably. Molecular weight is often stated in g/mol (or the older 'amu per molecule'), and numerically the two are identical.
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