Molar Mass Calculator

What molar mass actually is

The molar mass of a substance is the mass of one mole of it, written in grams per mole. Water is 18.015 g/mol, which means a mole of water molecules weighs 18.015 grams. That is about a tablespoon. Glucose is 180.156 g/mol, so a mole of glucose is roughly the mass of a small apple.

Those two comparisons are worth holding onto, because the mole is the part students find abstract and the mass is the part they can picture. A mole is a count, 6.02214076 × 10²³ of something, in the same way that a dozen is a count. Molar mass is what that count weighs for a particular substance. Every calculation you will do with moles runs through that one bridge.

Molar mass, molecular weight and formula mass

Three terms get used for the same number and they are not interchangeable.

Molar mass has units. It is grams per mole, and it belongs to a quantity of substance you could put on a balance.

Molecular weight, more properly relative molecular mass, has no units at all. It is a ratio: how heavy one molecule is compared with one twelfth of a carbon-12 atom. Water’s relative molecular mass is 18.015, full stop, with no g/mol after it. The number is the same as the molar mass, which is why almost nobody distinguishes them and why chemistry teachers keep correcting it.

Formula mass is the term to reach for when the substance has no molecules. Sodium chloride is not made of NaCl molecules. It is a lattice of sodium and chloride ions, and NaCl is the simplest whole-number ratio in that lattice rather than a description of a particle. Calling 58.440 its molecular weight implies a molecule that does not exist. Formula mass, or formula unit mass, says what is actually being counted. The same applies to every ionic compound, which is most of what you will meet in an introductory course.

If you want the distinction in one line: molecular weight is a ratio, molar mass is a mass, and formula mass is what you say instead of molecular weight when the compound is ionic.

Working it out by hand

Three steps, and the calculator above does exactly these.

First, count the atoms of each element. Second, multiply each count by that element’s standard atomic weight. Third, add the results.

Take glucose, C₆H₁₂O₆. Carbon is 12.011, hydrogen is 1.008 and oxygen is 15.999.

• Carbon: 6 × 12.011 = 72.066
• Hydrogen: 12 × 1.008 = 12.096
• Oxygen: 6 × 15.999 = 95.994

72.066 + 12.096 + 95.994 = 180.156 g/mol.

Percent composition falls out of the same three numbers. Divide each element’s contribution by the total and multiply by 100. Carbon is 72.066 / 180.156 = 40.00%, hydrogen is 6.72% and oxygen is 53.28%.

Those three should add to 100, and on this site they always do. Rounded independently they come to 99.99, because hydrogen’s exact share is 6.714% and rounding it down loses more than the other two lose between them. The missing hundredth goes to whichever element gave up the most, which here is hydrogen. Columns that total 99.99 or 100.01 are common on other calculators and they are a rounding artefact rather than a chemistry result.

Brackets, and the dot in a hydrate

A subscript after a closing bracket multiplies everything inside it. In calcium nitrate, Ca(NO₃)₂, the 2 applies to the whole nitrate group, so there are two nitrogens and six oxygens rather than two nitrogens and three oxygens. Brackets nest, which is why potassium ferrocyanide is written K₄[Fe(CN)₆] and contains six carbons and six nitrogens.

The raised dot in a formula like CuSO₄·5H₂O is not multiplication and it is not a decimal point. It marks water of crystallisation: water molecules built into the crystal structure itself, in a fixed ratio. Copper(II) sulfate pentahydrate is 249.677 g/mol, of which 90.08 is the five waters. Heat it and the water leaves, the blue crystals turn white and you are left with anhydrous copper(II) sulfate at 159.602 g/mol. Weigh out a hydrate as though it were anhydrous and every mole calculation downstream is wrong by more than a third.

Why two calculators give you different answers

Look up the molar mass of aluminium sulfate and you will find 342.13 in some places and 342.15 in others. Neither is a typo.

Fourteen elements have no single atomic weight. Hydrogen, lithium, boron, carbon, nitrogen, oxygen, magnesium, silicon, sulfur, chlorine, argon, bromine, thallium and lead all vary measurably in isotopic composition depending on where the sample came from. Carbon from a limestone deposit is not quite the same as carbon from the atmosphere. IUPAC publishes a range for these elements rather than a value, and separately publishes a conventional value for when one number is required. Carbon’s range is 12.0096 to 12.0116 and its conventional value is 12.011.

Lead is the extreme case. Its range runs from 206.14 to 207.94, which is nearly two whole mass units, because lead is the end point of three different radioactive decay chains and an ore body’s lead carries the isotopic fingerprint of whatever decayed into it.

Sulfur is where the aluminium sulfate discrepancy comes from. The older value was 32.065. The current conventional value is 32.06, and three sulfurs in the formula turn that small revision into two hundredths of a gram per mole. A calculator built on a 2007 table and one built on the current table will disagree, and both are internally consistent.

This tool uses the CIAAW Standard Atomic Weights 2024, which is the 2021 report with revisions to gadolinium, lutetium and zirconium. Where a compound contains one of the fourteen variable elements, the page says so underneath the table and gives the published range.

A separate wrinkle: since the SI redefinition in 2019 fixed the Avogadro constant by definition, the molar mass of carbon-12 is no longer exactly 12 g/mol. It is 11.9999999958 g/mol. The difference is far below anything that matters in a school laboratory, but it is the reason molar mass now carries an experimental uncertainty at all.

Turning grams into moles

Molar mass is the conversion factor between a mass you can weigh and a number of particles you can put into an equation.

To get moles from grams, divide by the molar mass. 25 g of water is 25 / 18.015 = 1.388 mol. To go the other way, multiply. 2 mol of water is 2 × 18.015 = 36.03 g.

That is the whole of it, and it is the step that stoichiometry rests on. A balanced equation tells you the ratio in moles, never in grams, so a mass has to become moles before the equation can be used and the answer has to become a mass again afterwards.

Where this goes wrong

Four mistakes account for most wrong answers.

Capital letters. CO is carbon monoxide. Co is cobalt. A chemical symbol is one capital letter, sometimes followed by one lowercase letter, and the difference between CO at 28.010 g/mol and Co at 58.933 g/mol is not subtle.

Subscripts outside brackets. Writing Fe₂SO₄3 instead of Fe₂(SO₄)₃ attaches the 3 to the last oxygen rather than to the whole sulfate group.

Forgetting the hydrate. If the bottle says pentahydrate, the five waters are part of the mass.

Using atomic mass instead of atomic weight. Carbon-12 has an atomic mass of exactly 12. Carbon as it occurs on Earth has a standard atomic weight of 12.011, because roughly 1.1% of it is carbon-13. Unless a question specifies an isotope, the abundance-weighted value is the one you want.

For more on the groups that travel together through reactions, see the full list of polyatomic ions. For worked examples across a wider range of substances, see our list of chemical compounds and the guide to binary compounds.