How to Calculate Molar Mass: Steps and Examples

To calculate molar mass, write out the chemical formula, look up the atomic weight of each element, multiply each atomic weight by the number of atoms of that element in the formula, and add the results. The total is the molar mass in grams per mole (g/mol). The part that is easy to overlook is which atomic weight to use: the published values for hydrogen, carbon, nitrogen, oxygen and chlorine are intervals, so the third or fourth decimal place depends on the table you pick, and a lab record should say which one was used.

What you need

The correct chemical formula of the substance as it is actually weighed, including any water of hydration (for example Na₂CO₃·10H₂O is not the same material as Na₂CO₃).
A table of standard atomic weights. This page uses the CIAAW 2024 table and the NIST atomic weights and isotopic compositions table.
A calculator, and a decision on how many decimal places you will report.

Steps to calculate molar mass

Write the formula and list each element once. For H₂O the list is hydrogen and oxygen.
Count the atoms of each element. The subscript after a symbol is the count; no subscript means one atom. For a group in parentheses, multiply every subscript inside by the number outside the bracket: Ca(OH)₂ contains one calcium, two oxygen and two hydrogen atoms.
Look up the atomic weight of each element. Use one table for the whole calculation and note its name and year.
Multiply each atomic weight by its atom count.
Add the products. The sum is the molar mass in g/mol.
Round last, not during the calculation. Carry all digits through the sum and round the final figure to the precision your work needs.

Which atomic weights to use

CIAAW lists standard atomic weights as the values recommended for all normal materials. For hydrogen, carbon, nitrogen, oxygen and chlorine it gives an interval rather than a single number, because the isotopic make-up of these elements varies in nature; the value for a normal material is expected to lie inside the interval (CIAAW, Standard Atomic Weights 2024). NIST publishes the same intervals for hydrogen, carbon and oxygen (NIST Atomic Weights and Isotopic Compositions).

Element	Standard atomic weight (CIAAW 2024)
Hydrogen (H)	[1.00784, 1.00811]
Carbon (C)	[12.0096, 12.0116]
Nitrogen (N)	[14.00643, 14.00728]
Oxygen (O)	[15.99903, 15.99977]
Sodium (Na)	22.98976928(2)
Chlorine (Cl)	[35.446, 35.457]

The figure in parentheses after sodium is the uncertainty in the last digit. Square brackets mark an interval. When you need a single number, pick one value from a table, state it, and use it consistently. The worked examples below carry both ends of each interval through the sum so you can see how far the answer can move.

Worked examples

Water, H₂O

Two hydrogen atoms and one oxygen atom.

Low end: 2 × 1.00784 + 15.99903 = 18.01471 g/mol
High end: 2 × 1.00811 + 15.99977 = 18.01599 g/mol

Rounded to three decimals, the low end gives 18.015 g/mol and the high end 18.016 g/mol, so the third decimal depends on the table value used.

Sodium chloride, NaCl

One sodium atom and one chlorine atom.

Low end: 22.98976928 + 35.446 = 58.43577 g/mol
High end: 22.98976928 + 35.457 = 58.44677 g/mol

The chlorine interval is the whole spread here, so the second decimal place is not fixed by the table; reporting about 58.44 g/mol, with the table named, is reasonable.

A hydrate, Na₂CO₃·10H₂O

A hydrate is calculated as the anhydrous formula plus the water: add the molar mass of the compound to ten times the molar mass of water.

Na₂CO₃: 2 × 22.98976928 + C + 3 × O gives 105.98623 (low end) to 105.99045 (high end) g/mol
10 H₂O: 10 × 18.01471 = 180.1471 (low end) to 10 × 18.01599 = 180.1599 (high end) g/mol
Total: 286.1333 to 286.1503 g/mol, which is 286.14 g/mol to two decimals

Leaving out the water would give about 106 g/mol, roughly 2.7 times too small for the decahydrate.

Carbon dioxide, CO₂

One carbon and two oxygen atoms: 12.0096 + 2 × 15.99903 = 44.00766 g/mol (low end) to 12.0116 + 2 × 15.99977 = 44.01114 g/mol (high end), or 44.01 g/mol to two decimals.

Check your answer against a database

Compare your sum with a curated value. PubChem lists the molecular weight of water (formula H₂O) as 18.015, which sits inside the range calculated above (PubChem compound property record for water). If your figure differs in the first or second decimal, recheck the atom counts before suspecting the atomic weights; differences in the fourth decimal are usually just the choice of table.

Sources may label the same sum molecular weight, formula weight or molar mass. The arithmetic above is the same; for converting between grams and moles, the unit to attach is g/mol.

Using molar mass in the lab

Molar mass converts a weighed mass into moles: moles = mass in grams / molar mass in g/mol. That is the first step of making a solution of known molarity; see mol to molarity for the next step, molarity and solution calculations for the lab for worked preparations, and mass percent solutions when the recipe is given by mass instead. For a single worked reagent, DMSO molar mass shows the same method applied to one solvent. Weighing itself is covered in analytical balance calibration and weighing technique, and the full preparation workflow in buffer and solution preparation.

What can go wrong

Ignoring the interval. Using the low end of an interval in one calculation and the high end in another gives answers that differ for no chemical reason. Choose one table and record it.
Misreading subscripts. In CO₂ the 2 applies only to oxygen. In a bracketed group the number outside multiplies every atom inside.
Dropping the water of hydration. The hydrate example above is about 2.7 times heavier than the anhydrous salt; weighing a hydrate with the anhydrous molar mass gives far too little compound.
Rounding too early. Round only the final sum.
Mixing units. Molar mass in g/mol times moles gives grams; if the mass is in milligrams, convert first.

When to get professional help

For a critical preparation, use the molecular weight printed on the label or certificate of analysis of the product you are actually weighing, since it describes that exact material including any hydration, and treat your own sum as a cross-check. If the two disagree, ask the supplier or your lab’s chemical hygiene or quality officer before preparing the solution.

FAQ

How to calculate molar mass?

Multiply the atomic weight of each element in the formula by its number of atoms and add the products. For water, 2 × 1.00784 + 15.99903 = 18.01471 g/mol at the low end of the published intervals, about 18.015 g/mol when rounded (18.016 g/mol at the high end).

Why do different sources give slightly different molar masses?

Because the standard atomic weights of hydrogen, carbon, nitrogen, oxygen and chlorine are published as intervals, and sources choose different single values or rounding. Cite the table you used.

Do I include water for a hydrate?

Yes. Add the molar mass of each water of hydration, as in the Na₂CO₃·10H₂O example.

Sources

CIAAW, Standard Atomic Weights 2024, https://ciaaw.org/atomic-weights.htm (checked 2026-10-29).
NIST, Atomic Weights and Isotopic Compositions for All Elements, https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl (checked 2026-10-29).
PubChem (NCBI), compound property record for water, molecular weight 18.015, https://pubchem.ncbi.nlm.nih.gov/compound/water (checked 2026-10-29).

Ask about How to Calculate Molar Mass: Steps, Examples and Which Atomic Weights to Use

Ask your first 2 questions free below. Subscribers get 150 a day for $29 a month.

An AI assistant specialized in research administration. It cites the sources behind every answer, labels web answers and says when it can't answer.

Answers draw on CASRAI's guides and dictionary plus the federal and funder documents we index: Federal Register, Grants.gov, Regulations.gov and UKRI.

Everything CASRAI publishes — this page, the dictionary, the guides and the news — stays free to read, with no account and no card.