Molecular Weight Calculator

How to calculate molecular weight

Molecular weight is the sum of the atomic weights of every atom in a molecule. To find it, count the atoms of each element in the chemical formula, multiply each count by that element's atomic weight, and add the results. The molecular weight calculator does this for any formula you enter, including groups in parentheses or brackets and hydrates.

Molecular weight = Σ (number of atoms × atomic weight)

Molar mass vs. molecular weight

Molecular weight (relative molecular mass) is a ratio with no unit. Molar mass is the mass of one mole of a substance in grams per mole (g/mol). Because atomic weights are defined relative to the same standard, the two have the same number—water has a molecular weight of 18.015 and a molar mass of 18.015 g/mol—so this page also works as a molar mass calculator or molecular mass calculator. For ionic compounds such as NaCl, which do not form discrete molecules, the same sum is called the formula weight.

How to find molar mass: worked example

Glucose, C₆H₁₂O₆, has 6 carbon, 12 hydrogen, and 6 oxygen atoms:

• Carbon: 6 × 12.011 = 72.066
• Hydrogen: 12 × 1.008 = 12.096
• Oxygen: 6 × 15.999 = 95.994
• Total: 72.066 + 12.096 + 95.994 = 180.156 g/mol

The result also shows each element's share of the total mass (mass percent) and of the total atom count (atom percent).

How to enter a chemical formula

• Symbols are case-sensitive: Co is cobalt, while CO is carbon monoxide.
• A number after an element or a closing bracket is a count: Ca(OH)2 has 2 oxygen and 2 hydrogen atoms. Parentheses, square brackets, and braces can be nested, as in [Co(NH3)6]Cl3.
• Separate hydrate or adduct parts with a dot (. or ·), and put a coefficient in front of a part to repeat it: CuSO4·5H2O.
• A leading coefficient multiplies the whole formula: 2H2O gives 36.03 g/mol.

Atomic weights used

The calculator uses the abridged standard atomic weights published by the IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW), with uncertainty ignored. These are averages for normal terrestrial materials, so a sample with an unusual isotopic composition can differ slightly. Radioactive elements such as technetium and plutonium have no standard atomic weight; for them the calculator uses the mass number of a long-lived isotope listed by CIAAW (for example, 244 for plutonium). For a few elements, such as technetium (97 or 98), CIAAW lists two isotopes that cannot be ranked by stability, and the calculator uses one of them.

Frequently asked questions

What is the molar mass of water?

The molar mass of water (H₂O) is 18.015 g/mol: 2 × 1.008 for hydrogen plus 15.999 for oxygen. The molecular weight of water is the same number, 18.015, without a unit.

How do you calculate molar mass?

Multiply the atomic weight of each element by the number of its atoms in the formula, then add the products. The answer is in grams per mole.

Is molecular weight the same as molar mass?

They have the same numerical value but different units. Molecular weight (relative molecular mass) is dimensionless, the mass of a single molecule is expressed in daltons (Da), and molar mass is in g/mol. For water, these are 18.015, 18.015 Da, and 18.015 g/mol.

How do I calculate molecular weight for a hydrate?

Add the water molecules to the formula with a dot. Copper(II) sulfate pentahydrate is CuSO4·5H2O, which gives 249.677 g/mol—the 159.60 g/mol of CuSO₄ plus 5 × 18.015 g/mol of water.

Why does my formula show an error?

Check the capitalization of each symbol, make sure every bracket is closed, and use whole numbers for counts. Charges and isotope labels (such as SO4²⁻ or ¹³C) are not supported.