Molecular Weight Calculator
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Molar mass of C₆H₁₂O₆ = 180.156 g/mol
- C
- 40%
- H
- 6.71%
- O
- 53.28%
| Element | Symbol | Atoms | Atomic weight | Mass (g/mol) | Mass % |
|---|---|---|---|---|---|
| Carbon | C | 6 | 12.011 | 72.066 | 40.002% |
| Hydrogen | H | 12 | 1.008 | 12.096 | 6.714% |
| Oxygen | O | 6 | 15.999 | 95.994 | 53.284% |
Atomic weights are the IUPAC (CIAAW) standard atomic weights, using the conventional values for elements such as H, C, N, O and Cl whose weights vary in nature.
How to use the molecular weight calculator
- Type the formula with correct capital letters: NaCl, not NACL or nacl. Co is cobalt, while CO is carbon monoxide.
- Use brackets for repeated groups, as in Ca(OH)₂ or K₄[Fe(CN)₆], and a dot, middle dot or asterisk before water of crystallization: CuSO4.5H2O works.
- Ions are fine: write the charge with a caret (SO4^2-) or at the end (NH4+, Fe3+). The charge is ignored because electrons weigh almost nothing.
- Read the molar mass, the Hill formula and the table of each element's atoms, mass and percentage.
Molar mass formula
mass %ᵢ = nᵢ × Arᵢ ÷ M × 100
The molar mass M of a compound is the sum, over every element i in the formula, of the number of atoms nᵢ times that element's standard atomic weight Arᵢ. Atomic weights have no unit, but numerically they equal the mass in grams of one mole of atoms, so the result is in grams per mole. The same number is the molecular weight (a pure ratio) and the mass of one molecule in daltons. For ionic solids such as NaCl, which have no separate molecules, chemists call it the formula weight; the arithmetic is identical.
Worked examples
- Calcium hydroxide, Ca(OH)₂: one calcium (40.078), two oxygens (2 × 15.999) and two hydrogens (2 × 1.008) give 40.078 + 31.998 + 2.016 = 74.092 g/mol. Calcium makes up 54.09% of the mass.
- Copper(II) sulfate pentahydrate, CuSO₄·5H₂O: the anhydrous salt is 159.602 g/mol and the five waters add 90.075, for 249.677 g/mol in all. Water is 36.1% of the crystals' mass, which matters when you weigh out the blue hydrate to make a solution of a given strength.
Molar masses of common compounds
Calculated with this calculator's engine and the atomic weights described below, rounded to three decimal places.
| Compound | Formula | Molar mass (g/mol) |
|---|---|---|
| Water | H₂O | 18.015 |
| Oxygen gas | O₂ | 31.998 |
| Nitrogen gas | N₂ | 28.014 |
| Methane | CH₄ | 16.043 |
| Ammonia | NH₃ | 17.031 |
| Carbon dioxide | CO₂ | 44.009 |
| Hydrochloric acid (HCl) | HCl | 36.458 |
| Sodium hydroxide | NaOH | 39.997 |
| Ethanol | C₂H₅OH | 46.069 |
| Sodium chloride | NaCl | 58.44 |
| Calcium hydroxide | Ca(OH)₂ | 74.092 |
| Sodium bicarbonate | NaHCO₃ | 84.006 |
| Sulfuric acid | H₂SO₄ | 98.072 |
| Calcium carbonate | CaCO₃ | 100.086 |
| Ammonium sulfate | (NH₄)₂SO₄ | 132.134 |
| Potassium permanganate | KMnO₄ | 158.032 |
| Iron(III) oxide | Fe₂O₃ | 159.687 |
| Glucose | C₆H₁₂O₆ | 180.156 |
| Copper(II) sulfate pentahydrate | CuSO₄·5H₂O | 249.677 |
| Sucrose | C₁₂H₂₂O₁₁ | 342.297 |
Where the atomic weights come from
The calculator uses the standard atomic weights published by the IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW), from its current table based on the "Atomic Weights 2021" report with the 2024 revisions for gadolinium, lutetium and zirconium. For 14 elements the isotope mix varies naturally, so IUPAC gives an interval rather than a single number; for those, the calculator uses IUPAC's conventional values, which are intended for everyday calculations:
| Element | Symbol | Conventional atomic weight |
|---|---|---|
| Hydrogen | H | 1.008 |
| Lithium | Li | 6.94 |
| Boron | B | 10.81 |
| Carbon | C | 12.011 |
| Nitrogen | N | 14.007 |
| Oxygen | O | 15.999 |
| Magnesium | Mg | 24.305 |
| Silicon | Si | 28.085 |
| Sulfur | S | 32.06 |
| Chlorine | Cl | 35.45 |
| Argon | Ar | 39.95 |
| Bromine | Br | 79.904 |
| Thallium | Tl | 204.38 |
| Lead | Pb | 207.2 |
Radioactive elements such as technetium, polonium and plutonium have no standard atomic weight. For them the calculator uses the mass number of the longest-lived known isotope (shown in square brackets), as most periodic tables do, and flags the result as approximate.
Tips and common mistakes
- Capitals matter. "CO" is carbon monoxide (28.010 g/mol); "Co" is cobalt (58.933 g/mol).
- Don't forget the water. Many lab chemicals are sold as hydrates; using the anhydrous molar mass for a hydrate gives a solution that is too weak.
- Subscripts multiply everything inside the bracket. In (NH₄)₂SO₄ there are 2 nitrogens and 8 hydrogens.
- Isotope labels such as D₂O or ¹³C are not supported; enter the ordinary formula and adjust by hand.
With the molar mass in hand, the molarity calculator tells you how many grams to dissolve for a solution of a given concentration. To relate a mass to a volume of a material, see the density calculator.
Frequently asked questions
How do you calculate molecular weight?
Multiply each element’s atomic weight by the number of its atoms in the formula and add the results. For water, H₂O: 2 × 1.008 + 15.999 = 18.015 g/mol. For glucose, C₆H₁₂O₆, the total is 180.156 g/mol.
What is the molar mass of NaCl?
Sodium chloride is 22.98976928 + 35.45 = 58.44 g/mol. Sodium makes up about 39.34% of the mass and chlorine about 60.66%.
How do I enter a hydrate such as CuSO4·5H2O?
Type a middle dot, a period or an asterisk before the water: CuSO4·5H2O, CuSO4.5H2O and CuSO4*5H2O all work. The number after the dot multiplies the water. Copper(II) sulfate pentahydrate is 249.677 g/mol, of which about 36.1% is water.
Is molecular weight the same as molar mass?
They have the same number. Molar mass is in grams per mole, while molecular weight (relative molecular mass) is a pure ratio with no unit. Calcium hydroxide, Ca(OH)₂, has a molar mass of 74.092 g/mol and a molecular weight of 74.092.
Which atomic weights does the calculator use?
IUPAC CIAAW standard atomic weights, based on the Atomic Weights 2021 report with the 2024 updates. For the 14 elements whose weights vary in nature, such as hydrogen (1.008), carbon (12.011) and chlorine (35.45), it uses the IUPAC conventional values.