Molecular Weight Calculator (Molar Mass)
Type in or choose a molecular formula below to obtain its molecular weight (molar mass). Keep in mind that element symbols are case‑sensitive. The tool uses IUPAC’s shortened standard atomic weights, ignoring their uncertainties. Throughout the page, “molecular weight” and “molar mass” refer to the same quantity.
Atomic weight, molecular weight and molar mass form the backbone of chemical calculations. They allow you to quantify reactants, set concentrations, probe molecular characteristics and more. The sections that follow assume you are acquainted with the definitions given next.
Term Definitions
Atom— The simplest unit of an element, comprising a nucleus surrounded by electrons. The nucleus contains protons and neutrons. An element’s identity is set by the number of protons, which is its atomic number.
Mole (mol)— A counting unit for chemical quantities. One mole equals exactly 6.02214076×1023 elementary entities (atoms, molecules, ions, etc.), known as Avogadro’s number.
Sample—A small amount of a material taken from a larger quantity for testing and analysis.
Molecule—A group of two or more atoms held together by chemical bonds.
Atomic Weight
Atomic weight, more accurately called relative atomic mass, expresses the average mass of a representative sample of an element’s atoms relative to the atomic mass constant. Because the ratio cancels the units, the result is dimensionless. It reflects the weighted mean of all isotopic masses in the sample. By contrast, atomic mass denotes the mass of a single atom, typically measured in daltons (Da).
The atomic weight of an element is the weighted mean of the masses of its isotopes. Take hydrogen as an example: it naturally occurs as three isotopes—¹H (≈99.9855% of all hydrogen), deuterium ²H (≈0.0145%) and trace tritium ³H. With masses of 1.007825031898 Da for ¹H and 2.01410177811 Da for ²H, the overall atomic weight can be computed accordingly.
1.007825031898 × 99.9855% + 2.01410177811 × 0.0145% ≈ 1.008 Da
Note that this calculator uses standard atomic weights as stated by IUPAC (International Union of Pure and Applied Chemistry). The table of standard atomic weights for each element is provided at the bottom of this page.
Molecular Weight (relative molecular mass)
Molecular weight, more properly termed relative molecular mass, is the ratio of a molecule’s mass to the atomic mass constant, yielding a dimensionless figure. Unlike atomic weight, it accounts for a collection of atoms. Consequently, adding up the atomic weights of all atoms in a compound gives its molecular weight. For water (H2O), using 1.008 Da for hydrogen and 15.999 Da for oxygen, the calculation proceeds as shown.
2 × 1.008 + 15.999 = 18.015 Da
Although the above is the more accurate definition of molecular weight, for the intents of this calculator, molecular weight is used interchangeably with molar mass, defined below.
Molar Mass
Molar mass denotes the mass of one mole of a substance, usually expressed in grams per mole (g·mol⁻¹). Though often swapped with molecular mass in everyday language, the two are not identical. Molecular weight, defined earlier, is a ratio; molar mass relates the mass of a bulk sample to the amount of substance (in moles).
Even though molar mass and molecular weight stem from different definitions and carry distinct units, their numeric values coincide for most practical purposes. Historically, before recent redefinitions, they were numerically equal but differed in unit labels, allowing educators to treat them as interchangeable in high‑school chemistry.
Molecular Mass
Adding to the confusion, molecular mass differs from molecular weight. The term molecular weight actually denotes the relative molecular mass, which reflects the weighted isotopic composition of a compound. For example, water’s relative molecular mass is 18.015 Da, yet a single water molecule can weigh anywhere from 18.0106 Da to 22.0277 Da.
Calculate Molecular Weight and Molar Mass
Atomic weight underpins the determination of both molecular weight and molar mass. By knowing the mass of each atom and adding them according to the molecule’s formula, we obtain the total mass of the molecule. Thus, molecular weight or molar mass can be calculated by:
- Identifying and counting the number of atoms of each element in the molecule.
- Obtaining the atomic weights of each element using the standard atomic weights from the periodic table or the table provided below.
- Multiplying the atomic weight of each element by the number of atoms of that element, then summing the results.
The following are some examples:
Example: Water (H2O)
- There are 2 Hydrogen (H) atoms and 1 Oxygen (O) atom in the molecule
- Their atomic weights are
H: 1.008 g/mol
O: 15.999 g/mol - The molecular weight of H2O is:
1.008×2 + 15.999×1 = 18.015 g/mol
Some molecules have more complex formulas, including those with parentheses or hydrates.
Example: Aluminum Sulfate Al2(SO4)3
- The elements counts are:
Aluminum (Al): 2 atoms
Sulfur (S): 1×3 = 3 atoms
Oxygen (O): 4×3 = 12 atoms - Their atomic weights are:
Al: 26.982 g/mol
S: 32.06 g/mol
O: 15.999 g/mol - The molecular weight of Al2(SO4)3 is:
26.982×2 + 32.06×3 + 15.999×12 = 342.132 g/mol
Example: Copper(II) Sulfate Pentahydrate CuSO4·5H2O
- Anhydrous compound (CuSO4):
Copper (Cu): 63.546 g/mol
Sulfur (S): 32.06 g/mol
Oxygen (O): 15.999×4 = 63.996 g/mol
Subtotal: 63.546 + 32.06 + 63.996 = 159.602 g/mol - Water of crystallization (5H2O):
Water (H2O): 18.015 g/mol
Total water: 18.015×5 = 90.075 g/mol - The molecular weight of CuSO4·5H2O is:
159.602 + 90.075 = 249.677 g/mol.
Table of abridged standard atomic weights
Below is a table of the abridged standard atomic weights of the elements. The abridged version is commonly used in practical scenarios, as it simplifies calculations by providing values rounded to a fixed number of decimal places, ignoring the typically small natural variations in isotope ratios from different sources or samples. These values are published by the International Union of Pure and Applied Chemistry (IUPAC). The calculations of this calculator are based on this data.