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Molecular Weight Calculator

Type a formula like Ca(OH)2 or CuSO4·5H2O and get its molar mass in g/mol, broken down element by element.

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Answer

Enter your numbers or pick an example — the answer and the steps appear here instantly.

What it reads and what it returns

  • Formulas as you write them

    Nested brackets ( ) and [ ], subscripts typed as plain digits or as subscript characters, and a leading coefficient like 2H2O.

  • Hydrates and adducts

    Join the parts with a middle dot, an asterisk or a period: CuSO4·5H2O, CuSO4*5H2O and CuSO4.5H2O are read the same way.

  • Case-sensitive symbols

    Co is cobalt, while CO is carbon monoxide. A capital letter always starts a new element, so check capitalization when a result looks wrong.

  • Full breakdown

    Atoms of each element, their mass contribution in g/mol and their mass percent of the whole compound.

  • Standard atomic weights

    Values come from the CIAAW standard atomic weights in abridged form; radioactive elements use the mass number of their longest-lived isotope.

  • What it does not do

    It does not balance equations or run stoichiometry. Use the molar mass it gives as the input for those calculations.

How to calculate molar mass by hand

Molar mass is the mass of one mole of a substance, in grams per mole. To find it, count the atoms of each element in the formula, multiply each count by that element's atomic weight, and add the products. A subscript applies only to the element or bracket directly before it.

Take calcium hydroxide, Ca(OH)2. The 2 multiplies everything inside the bracket, so there is 1 Ca, 2 O and 2 H. Using Ca = 40.078, O = 15.999 and H = 1.008, the sum is 40.078 + 2 × 15.999 + 2 × 1.008 = 74.092 g/mol.

Hydrates add their water. CuSO4·5H2O is CuSO4 plus five H2O units: Cu 63.546 + S 32.06 + 4 × 15.999 gives 159.602 for the salt, and 5 × 18.015 = 90.075 for the water, so the total is about 249.68 g/mol.

Mass percent follows directly: divide each element's contribution by the total and multiply by 100. In water, oxygen supplies 15.999 of 18.015 g/mol, which is about 88.81 percent of the mass.

Getting a molar mass in three steps

  1. Type the formula

    Write element symbols with the correct capital letters, e.g. C6H12O6 for glucose or Ca(OH)2 for slaked lime.

  2. Calculate

    The molar mass appears in g/mol with atom counts, contributions and mass percent; rounded figures carry ≈.

  3. Check or copy

    Copy the result into your problem set, or send it to the AI Math Solver for a narrated explanation of the arithmetic.

Frequently asked questions

They are numerically the same. Strictly, molecular weight is a relative, unitless quantity, while molar mass carries units of grams per mole, which is what this calculator reports.

Textbooks often use rounded atomic weights, such as 1.01 for hydrogen or 32.07 for sulfur. This calculator uses the abridged CIAAW standard values, so small differences in the second decimal place are normal.

Write a middle dot, an asterisk or a period between the parts, as in CuSO4·5H2O, CuSO4*5H2O or CuSO4.5H2O. The number after the separator multiplies the whole group that follows it.

No. It works on one formula at a time and returns its molar mass and composition; balancing and stoichiometry are outside what it does. The Chemistry AI Solver can help with those.

Elements without a standard atomic weight, such as technetium or plutonium, use the mass number of their longest-lived isotope, which is the usual periodic-table convention.

The molar mass and breakdown are free, need no account and are computed in your browser. An AI explanation is optional: it is included in the Pro plans* (*fair use: up to 300 runs a day across the writing, study and code tools), takes 2 credits without a plan, and signed-out visitors see a free preview of its start. Use it to learn the method, follow your course's rules on AI help, and remember that AI text can contain mistakes.

Chemistry and math helpers

Need a molar mass?

Type the formula and get g/mol with each element's share of the total.

Calculate molar mass
  • Brackets and hydrates
  • Mass percent by element
  • CIAAW atomic weights