Molecular Weight Calculator
Calculate molar mass from formulas with parentheses and bracket groups.
Enter a formula directly, choose a common compound, or insert any element from the periodic table. Hydrate dot formulas such as C2HCl3O.H2O are supported.
Common compounds
Element insert
How to Use the Molecular Weight Calculator
1
Enter the formula. Type a chemical formula such as Ca(OH)2, pick one from the common compound library, or insert an element from the periodic table grid.
2
Calculate. Run the calculation to expand every parenthesis, bracket, and hydrate group into individual atom counts.
3
Read the breakdown. Check the total molar mass in g/mol along with each element's atom count, contribution in g/mol, and mass percent.
What is the Molecular Weight Calculator?
The molecular weight calculator converts any chemical formula into its molar mass in grams per mole. It reads element symbols, subscripts, parentheses, and bracket groups, so formulas like Ca(OH)2, Al2(SO4)3, and complex organic structures all resolve to an accurate total using standard atomic masses.
Molar mass is the bridge between the mass you weigh on a balance and the moles you use in equations. Nearly every stoichiometry problem, from solution preparation to limiting reagent checks and percent yield, starts with this conversion, which is why it is the most-used calculation in general chemistry.
Common Uses
- Convert grams to moles before a stoichiometry problem
- Check the molar mass of a compound for lab solution prep
- Verify a formula you drew matches its expected mass
- Compare molecular weights of similar compounds
How to Solve It by Hand
Manual calculation is still important: identify the known variables, convert units before substitution, apply the equation, and check whether the result is chemically reasonable. The most common mistakes are inconsistent units, constants rounded too early, and skipping the interpretation step.
Practice Prompt
Try changing the default values and ask the lower-right chemistry chat why the result increased or decreased. That turns the calculator from a number machine into a study loop.
Multiply each element's atomic mass by how many atoms of it the formula contains, then add the products together. For Ca(OH)2 that is 40.078 + 2 × (15.999 + 1.008) = 74.09 g/mol. The calculator does this expansion for you and shows each element's contribution and mass percent.
Yes. Nested groups such as Al2(SO4)3 and K3[Fe(CN)6] are expanded correctly, and hydrate dot notation such as CuSO4.5H2O or C2HCl3O.H2O is treated as separate units multiplied by their coefficient.
Numerically they are the same for a given substance. Molecular weight is a relative, unitless number; molar mass is the mass of one mole in g/mol; formula mass is the preferred term for ionic compounds such as NaCl that have no discrete molecules.
Textbooks round atomic masses differently. This calculator uses standard IUPAC atomic weights to three or four decimals, so an answer may differ by a few hundredths of a g/mol from a table rounded to one decimal. That difference is not a mistake.
Divide the mass in grams by the molar mass in g/mol. 18.02 g of water divided by 18.02 g/mol gives 1 mole. To go the other way, multiply moles by molar mass.