Molecular Weight Calculator
Compute molecular weight (molar mass) in g/mol from atom counts of 14 common elements.
Enter how many atoms of each element the molecule contains. Common elements are listed below — a blank count is treated as zero.
Molecular weight
18.02 g/mol
Molar mass of the molecule
Molecular Weight Calculator on True Calculator gives you an instant, accurate answer with no sign-up and no app install. Compute molecular weight (molar mass) in g/mol from atom counts of 14 common elements. Every result shows the formula and a worked example so you can verify the calculation yourself, and all values are computed in your own browser — your numbers never leave your device.
Popular uses: molecular weight calculator · molar mass calculator · molecular mass
How We Calculate
This calculator uses standard physics and chemistry formulas verified by our team. All calculations are performed instantly in your browser using JavaScript — no data is sent to any server.
Formulas follow standard scientific definitions and physical constants, consistent with what is taught in school and university courses.
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: Write the molecular formula, for example C₆H₁₂O₆ for glucose.
- Step 2: Count the atoms of each element and enter the counts in the fields.
- Step 3: Leave blank any element that does not appear in the formula — it is treated as zero.
- Step 4: Read the molecular weight in g/mol, then use it in the molarity calculator or reactions.
Worked Example
Glucose, C₆H₁₂O₆, contains 6 carbon, 12 hydrogen and 6 oxygen atoms. Using atomic masses of 12.011 for carbon, 1.008 for hydrogen and 15.999 for oxygen: 6 × 12.011 = 72.066, 12 × 1.008 = 12.096, and 6 × 15.999 = 95.994. Adding these gives 180.156, so the molecular weight is 180.16 g/mol. Water (H₂O) by comparison is just 18.02 g/mol, which is why one mole of glucose weighs ten times more than one mole of water.
Tips and Common Mistakes
- •Double-check atom counts — a missed subscript changes the weight more than you expect.
- •For hydrates, add the water molecules as part of the formula (CuSO₄·5H₂O has 5 extra waters).
- •Use the result immediately in the molarity calculator to convert grams to moles correctly.
- ✗Do not round atomic masses to whole numbers unless the accuracy is fine — 35.45 for chlorine matters.
- ✗Do not forget that organic formulas like C₆H₁₂O₆ can also be a structural isomer; molecular weight cannot distinguish them.
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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