Molecular Weight Calculator
Find the molecular weight of a compound by entering the number of carbon, hydrogen, oxygen, and nitrogen atoms. Useful for stoichiometry, solution preparation, and mass spectrometry interpretation.
About this calculator
Molecular weight (also called molar mass) is the sum of the atomic weights of all atoms in a molecule, expressed in g/mol. The formula used here is: MW = (C × 12.011) + (H × 1.008) + (O × 15.999) + (N × 14.007), where each coefficient is the standard atomic weight from the IUPAC periodic table. A multiplicative isotope correction factor is then applied: MW_corrected = MW × isotopeCorrection. For standard calculations the correction equals 1.000; selecting a deuterium-enriched or ¹³C-enriched isotopologue shifts the mass accordingly. Molecular weight is essential for converting between grams and moles (moles = mass / MW), preparing solutions of known molarity, and interpreting mass spectra where the molecular ion peak appears at m/z = MW. Remember that this tool covers only C, H, O, and N—elements like sulfur, phosphorus, or halogens must be added separately if present in your molecule.
How to use
Calculate the molecular weight of glucose (C₆H₁₂O₆): enter 6 carbon atoms, 12 hydrogen atoms, 6 oxygen atoms, and 0 nitrogen atoms, with an isotope correction of 1 (standard). MW = (6 × 12.011) + (12 × 1.008) + (6 × 15.999) + (0 × 14.007) = 72.066 + 12.096 + 95.994 + 0 = 180.156 g/mol × 1 = 180.156 g/mol. This matches the accepted value of 180.16 g/mol. To dissolve 1 mol of glucose in 1 L of water, weigh out 180.16 g.
Frequently asked questions
What is molecular weight and how is it different from molecular mass?
Molecular weight is a dimensionless ratio historically expressed in atomic mass units (amu), but in practice it is numerically equal to molar mass in g/mol and the terms are often used interchangeably. Molecular mass refers to the mass of a single molecule in daltons (Da) or amu, while molar mass is the mass of one mole (6.022 × 10²³ molecules) in grams. For glucose, the molecular mass is 180.16 Da and the molar mass is 180.16 g/mol—numerically the same but conceptually distinct quantities.
How do I calculate molecular weight for a compound that contains sulfur or phosphorus?
This calculator covers carbon (12.011), hydrogen (1.008), oxygen (15.999), and nitrogen (14.007). For compounds with other elements, add their contributions manually: sulfur has an atomic weight of 32.06 g/mol and phosphorus 30.974 g/mol. For example, methionine (C₅H₁₁NO₂S) would give MW from this tool for the C/H/N/O portion and then you add 1 × 32.06 g/mol for sulfur. Alternatively, use a full periodic-table molecular weight calculator for complex heterocycles or inorganic salts.
Why does isotope type affect the molecular weight calculation?
Natural atomic weights are averages weighted by the natural abundance of each element's isotopes. When you work with isotopically labeled compounds—such as ¹³C-labeled glucose used in NMR or metabolic flux studies—each ¹³C replaces a ¹²C and adds approximately 1 amu per substitution. The isotope correction factor scales the base molecular weight to account for enrichment. Mass spectrometry experiments in particular must use exact monoisotopic masses (e.g., ¹²C = 12.000 exactly) rather than average atomic weights for precise m/z matching.