Free Chemistry Tool · Tech Serve Solutions
Molecular Weight Calculator
Calculate the molecular weight (molar mass) of any compound instantly. Enter a chemical formula — including nested brackets like K₃[Fe(CN)₆] and hydrates like CuSO₄·5H₂O — and get the molecular weight, monoisotopic mass, and a full element-by-element mass-percent breakdown. Free, no sign-up, IUPAC 2021 atomic weights, all 118 elements.
How to calculate molecular weight
Molecular weight is the sum of the atomic weights of every atom in a molecule, expressed in grams per mole (g/mol). To calculate it by hand:
- Write the chemical formula of the compound.
- Look up the standard atomic weight of each element from the periodic table.
- Multiply each element’s atomic weight by the number of atoms of that element in the formula.
- Add all the subtotals together.
Worked example — caffeine (C₈H₁₀N₄O₂)
Caffeine has the molecular formula C₈H₁₀N₄O₂. Calculating its molecular weight:
| Element | Atoms | Atomic weight | Subtotal |
|---|---|---|---|
| Carbon (C) | 8 | 12.011 | 96.088 |
| Hydrogen (H) | 10 | 1.008 | 10.080 |
| Nitrogen (N) | 4 | 14.007 | 56.028 |
| Oxygen (O) | 2 | 15.999 | 31.998 |
| Molecular weight | 194.19 g/mol | ||
So the molecular weight of caffeine is 194.19 g/mol. You can verify this by entering C8H10N4O2 in the calculator above.
Molecular weights of common chemicals
| Chemical | Formula | CAS | Molecular weight |
|---|---|---|---|
| Caffeine Anhydrous | C8H10N4O2 | 58-08-2 | 194.19 g/mol |
| Water | H2O | — | 18.02 g/mol |
| Sodium Chloride | NaCl | 7647-14-5 | 58.44 g/mol |
| Glucose | C6H12O6 | 50-99-7 | 180.16 g/mol |
| Ethanol | C2H6O | 64-17-5 | 46.07 g/mol |
| Acetic Acid | CH3COOH | 64-19-7 | 60.05 g/mol |
| Sulfuric Acid | H2SO4 | 7664-93-9 | 98.07 g/mol |
| Ascorbic Acid (Vitamin C) | C6H8O6 | 50-81-7 | 176.12 g/mol |
| Aspirin | C9H8O4 | 50-78-2 | 180.16 g/mol |
| Copper(II) Sulfate Pentahydrate | CuSO4·5H2O | 7758-99-8 | 249.68 g/mol |
| Potassium Permanganate | KMnO4 | 7722-64-7 | 158.03 g/mol |
| Calcium Carbonate | CaCO3 | 471-34-1 | 100.09 g/mol |
Tech Serve Solutions supplies many of these chemicals in USP, BP, EP, IP, and FCC grades. Browse the full catalogue →
Frequently asked questions
What is molecular weight?
Molecular weight (also called molar mass or molecular mass) is the sum of the atomic weights of every atom in a molecule, expressed in grams per mole (g/mol). It tells you how much one mole of a substance weighs. For example, the molecular weight of caffeine (C8H10N4O2) is 194.19 g/mol.
What is the difference between molecular weight and molar mass?
In everyday chemistry the terms are used interchangeably. Strictly: molecular weight is a dimensionless relative figure (relative to 1/12 the mass of a carbon-12 atom), while molar mass carries the unit g/mol. Numerically they are identical, so a molecular weight calculator and a molar mass calculator give the same answer.
How do you calculate molecular weight?
Multiply the standard atomic weight of each element by the number of atoms of that element in the formula, then add all the products. For water (H2O): hydrogen 2 × 1.008 + oxygen 1 × 15.999 = 18.015 g/mol.
What is monoisotopic mass?
Monoisotopic mass uses the exact mass of the single most abundant isotope of each element, instead of the abundance-weighted average used for standard molecular weight. It is the value mass spectrometrists need. This calculator reports both.
How do you calculate the molecular weight of a hydrate?
A hydrate such as copper(II) sulfate pentahydrate (CuSO4·5H2O) includes water molecules bound in the crystal. Calculate the molecular weight of the anhydrous part, then add the molecular weight of the water (number of water molecules × 18.015 g/mol). This calculator handles the dot notation automatically — enter CuSO4·5H2O, CuSO4.5H2O, or CuSO4*5H2O.
Is this molecular weight calculator free?
Yes. The Tech Serve Solutions molecular weight calculator is completely free, requires no sign-up, and runs entirely in your browser. It uses IUPAC 2021 standard atomic weights for all 118 elements.
Calculator uses IUPAC 2021 conventional standard atomic weights. Published by Tech Serve Solutions, a global supplier of fine chemicals and laboratory reagents since 1989.