How to use it
- Type the formula with element symbols and their counts, for example H2O or C6H12O6. Use brackets for a group, as in Ca(OH)2.
- For a hydrate, write the water after a middle dot or a dot, with its count: CuSO4·5H2O or CuSO4.5H2O.
- Read the Molar mass in g/mol. The Elements table gives the atoms of each element and its share of the mass.
- Follow the Working line by line. A note under the result explains a charge or an element with no standard atomic weight.
How the molar mass is calculated
The molar mass of a substance is the mass of one mole of it, in grams per mole. For a formula, add the atomic weight of every atom in it, counting each atom as often as it appears. A hydrate counts the water as part of the formula, multiplied by its count.
The atomic weights are the CIAAW abridged standard atomic weights of 2024, with five significant figures: 1.0080 for hydrogen, 12.011 for carbon, 15.999 for oxygen. Each element's share of the mass is its subtotal divided by the total.
M = Σ nᵢ × Aᵣ(i) (in g/mol, where nᵢ is the number of atoms of element i and Aᵣ(i) its atomic weight)Charges, isotopes and what the result leaves out
- A charge is read and then ignored. Electrons weigh almost nothing, so the molar mass of an ion is taken as that of its neutral atoms.
- An element with no standard atomic weight, such as technetium, uses the mass of one of its isotopes, and the result says so.
- The result is for the formula as you type it. It does not correct for isotope-enriched material, for a mixture or for a polymer with a varying length.
Worked example: glucose, C6H12O6
Glucose has 6 carbon, 12 hydrogen and 6 oxygen atoms. With the atomic weights 12.011 for carbon, 1.0080 for hydrogen and 15.999 for oxygen, the subtotals are 6 × 12.011 = 72.066, 12 × 1.0080 = 12.096 and 6 × 15.999 = 95.994 g/mol.
Together they give a molar mass of 180.156 g/mol. Carbon makes up 40.00 % of the mass, hydrogen 6.71 % and oxygen 53.28 %. The rounded shares add up to 99.99 %, which the page notes.
| Element | Atoms | Atomic weight | Mass (g/mol) | Share of mass |
|---|---|---|---|---|
| Carbon | 6 | 12.011 | 72.066 | 40.00 % |
| Hydrogen | 12 | 1.008 | 12.096 | 6.71 % |
| Oxygen | 6 | 15.999 | 95.994 | 53.28 % |
| Molar mass | 24 | 180.156 |
Frequently asked questions
Why is my textbook's molar mass a little different?
Textbooks use older or rounded atomic weights. This tool uses the CIAAW abridged standard atomic weights of 2024, so the last digit can differ from the value in a book that uses other figures.
How do I write a hydrate such as copper sulfate pentahydrate?
Write the salt, then a middle dot or a dot, then the water with its count: CuSO4·5H2O or CuSO4.5H2O. The count of five multiplies the whole water part, so the tool adds five water molecules to the mass.
Does the charge of an ion change its molar mass?
No. Electrons weigh almost nothing, so an ion's molar mass is taken as that of its neutral atoms. The tool still reads the charge and shows it in a note under the result.
Why does a result for technetium or promethium carry a note?
They have no standard atomic weight, because they have no stable isotope in normal material. The tool uses the mass of one isotope for them, so the molar mass is approximate, and the note says so.
How do I turn grams into moles with the molar mass?
Divide the mass in grams by the molar mass: moles = grams ÷ (g/mol). For example, 10 g of water is 10 ÷ 18.015 = 0.555 mol.
Does the calculator keep my formula or send it anywhere?
No. The molar mass is worked out in your browser, and nothing you type is stored or sent to our server.
Sources
- CIAAW: Abridged Standard Atomic Weights 2024
- PubChem periodic table data (NCBI, US government)
- Wikipedia: Molar mass
Figures and text last checked on Tuesday, November 10, 2026.