In chemistry and laboratory sciences, understanding the mass of individual molecules and bulk chemical compounds is fundamental. Molecular weight represents the sum of the atomic weights of all atoms in a single molecule. Because atoms are too small to weigh individually in a standard laboratory, chemists use the concept of moles to bridge the microscopic world of atoms with the macroscopic world of grams, defining what we call molar mass.
While they are numerically equivalent for any given compound, they represent different concepts:
To convert concentration values in a solution from molarity to other formats, use our chemical solution concentration helper. For basic math checks, use our everyday daily math helper.
To calculate the molecular weight of a compound, look up the atomic weight of each element in the periodic table, multiply by the number of times it appears in the formula, and add them together.
For example, the atomic weight of Hydrogen (H) is roughly 1.008 u, Oxygen (O) is 15.999 u, and Carbon (C) is 12.011 u. To calculate weight to mass ratios, check out our material mass solver or see our material density helper.
To round molecular mass outputs to standard decimal levels, try our decimal rounding utility.
Element symbols are case-sensitive, meaning correct capitalization is essential. For example, "Co" represents Cobalt, whereas "CO" represents Carbon Monoxide (one Carbon atom and one Oxygen atom). Typing "co" or "CO" incorrectly will alter the parsed element types and results.
In addition, this tool supports brackets and parentheses for nested formulas. Grouping parts of a formula allows you to easily specify polyatomic ions like sulfate or ammonium in complex compounds. The calculator expands these groups automatically when computing the final mass.
For checking physical weights across different systems, visit our translating measuring units tool.
Example 1 (Water - H₂O): Water consists of 2 Hydrogen atoms and 1 Oxygen atom. - H: 2 × 1.008 = 2.016 g/mol - O: 1 × 15.999 = 15.999 g/mol - Total: 2.016 + 15.999 = 18.015 g/mol.
Example 2 (Table Salt - NaCl): Salt contains 1 Sodium atom and 1 Chlorine atom. - Na: 1 × 22.990 = 22.990 g/mol - Cl: 1 × 35.453 = 35.453 g/mol - Total: 22.990 + 35.453 = 58.443 g/mol.
Example 3 (Glucose - C₆H₁₂O₆): Glucose contains 6 Carbon, 12 Hydrogen, and 6 Oxygen atoms. - C: 6 × 12.011 = 72.066 g/mol - H: 12 × 1.008 = 12.096 g/mol - O: 6 × 15.999 = 95.994 g/mol - Total: 72.066 + 12.096 + 95.994 = 180.156 g/mol. These examples demonstrate standard molar mass summation.