Molecular Weight Calculator (Molar Mass)

Please enter or select the molecular formula of a molecule to calculate its molecular weight/molar mass. Note that the formula is case-sensitive. This calculator utilizes the abridged standard atomic weights published by IUPAC with uncertainty ignored. Also, the terms "molecular weight" and "molar mass" are used interchangeably.

Modify the values and click the calculate button to use
Molecular Formula
Common Chemicals


Atomic weight, molecular weight, and molar mass are fundamental concepts in chemistry. These measurements are essential for calculating the quantities of substances involved in chemical reactions, determining concentrations, analyzing molecular properties, and many other situations. To understand much of the content below, it is helpful to first be familiar with the following definitions.

Term Definitions

Atom—The basic particle of chemical elements that are made up of a nucleus, protons, neutrons, and electrons. Chemical elements are distinguished by the number of protons they contain. The atomic number of an element is its number of protons.

Isotope—An isotope of an element is an atom that has the same number of protons but a different number of neutrons. All atoms that have 12 protons are Magnesium, but Magnesium has three stable isotopes: 24Mg, 25Mg, and 26Mg which have 12, 13, and 14 neutrons respectively.

Mole (mol)—A unit of measurement for measuring the amount of a substance. One mole is an aggregate of exactly 6.02214076×1023 (Avogadro's number) base particles, which can be atoms, molecules, ions, ion pairs, or other particles.

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 precisely referred to as relative atomic mass (and not to be confused with atomic mass), is defined as the ratio of the average mass of a sample of atoms of an element to the atomic mass constant. Since both have units of mass, the resulting quantity is dimensionless. It represents the weighted average of the masses of individual atoms, including all isotopes, in a sample. Atomic mass, on the other hand, is the mass of a single atom which can be measured with precision in units of Dalton (Da).

The atomic weight of a given element is the weighted average of the atomic masses of its different isotopes. For example, the Hydrogen atom has three main isotopes naturally: Hydrogen-2 (2H, also known as deuterium), Hydrogen-3 (3H, also known as tritium), and Hydrogen-1. Hydrogen-1 comprises 99.9855% of naturally occurring Hydrogen while deuterium comprises 0.0145%. Tritium exists naturally in only negligible trace amounts. Since 2H has a mass of 2.01410177811 Da and 1H has a mass of 1.007825031898 Da, its atomic weight can be calculated as:

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)

The molecular weight (more precisely referred to as relative molecular mass) is defined as the ratio of the mass of a molecule to the atomic mass constant. Like the atomic weight, this is a dimensionless quantity since both have units of mass. Molecular weight differs from atomic weight simply by the fact that a molecule is made up of multiple atoms. Thus, the sum of the atomic weights of the atoms that make up a compound is its molecular weight. For example, the molecular weight of a water molecule (H2O) using an atomic weight of 1.008 Da for a hydrogen atom and atomic weight of 15.999 Da for an Oxygen atom is:

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 is defined as the mass of 1 mole of a substance and is typically measured in units of grams per mole (g/mol). Molar mass is a term that is frequently used interchangeably with molecular mass, even though they are not exactly the same. Molecular weight, as defined above, is the ratio of the mass of a molecule to the atomic mass constant. Molar mass can be defined in similar terms as the ratio of the mass of any sample of a compound to the amount of substance (measured in moles).

Although molar mass and molecular weight are defined differently and are usually expressed in different units, for more informal purposes, they have more or less the same value. In the past, before the redefinition of certain values, molecular weight and molar mass were numerically equivalent with different units. Thus, for most purposes, including a high school chemistry class, the terms may be used largely interchangeably.

Molecular Mass

To make things more confusing, molecular mass is also distinct from molecular weight. Molecular weight is most accurately referred to as relative molecular mass, which takes into account the weighted abundance of the various isotopic compositions of a given compound. For example, the relative molecular mass of water is 18.015 Da, but a given water molecule may have a molecular mass ranging from 18.0106 Da to 22.0277 Da.

Calculate Molecular Weight and Molar Mass

Atomic weight serves as the foundation for calculating molecular weight and molar mass. It provides the mass of individual atoms, which, when combined according to a molecule's chemical formula, yields the mass of the entire molecule. Therefore, molecular weight or molar mass can be calculated by:

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.

Atomic NumberSymbolNameAtomic Weight
(g/mol)
Density
(g/cm3)
Phase at Room Temp.
1HHydrogen1.0080.00008988gas
2HeHelium4.00260.0001785gas
3LiLithium6.940.534solid
4BeBeryllium9.01221.85solid
5BBoron10.812.34solid
6CCarbon12.0112.267solid
7NNitrogen14.0070.0012506gas
8OOxygen15.9990.001429gas
9FFluorine18.9980.001696gas
10NeNeon20.180.0009002gas
11NaSodium22.990.968solid
12MgMagnesium24.3051.738solid
13AlAluminium26.9822.7solid
14SiSilicon28.0852.329solid
15PPhosphorus30.9741.823solid
16SSulfur32.062.07solid
17ClChlorine35.450.0032gas
18ArArgon39.950.001784gas
19KPotassium39.0980.89solid
20CaCalcium40.0781.55solid
21ScScandium44.9562.985solid
22TiTitanium47.8674.506solid
23VVanadium50.9426.11solid
24CrChromium51.9967.15solid
25MnManganese54.9387.21solid
26FeIron55.8457.874solid
27CoCobalt58.9338.9solid
28NiNickel58.6938.908solid
29CuCopper63.5468.96solid
30ZnZinc65.387.14solid
31GaGallium69.7235.91solid
32GeGermanium72.635.323solid
33AsArsenic74.9225.727solid
34SeSelenium78.9714.81solid
35BrBromine79.9043.1028liquid
36KrKrypton83.7980.003749gas
37RbRubidium85.4681.532solid
38SrStrontium87.622.64solid
39YYttrium88.9064.472solid
40ZrZirconium91.2246.52solid
41NbNiobium92.9068.57solid
42MoMolybdenum95.9510.28solid
43TcTechnetium9711solid
44RuRuthenium101.0712.45solid
45RhRhodium102.9112.41solid
46PdPalladium106.4212.023solid
47AgSilver107.8710.49solid
48CdCadmium112.418.65solid
49InIndium114.827.31solid
50SnTin118.717.265solid
51SbAntimony121.766.697solid
52TeTellurium127.66.24solid
53IIodine126.94.933solid
54XeXenon131.290.005894gas
55CsCaesium132.911.93solid
56BaBarium137.333.51solid
57LaLanthanum138.916.162solid
58CeCerium140.126.77solid
59PrPraseodymium140.916.77solid
60NdNeodymium144.247.01solid
61PmPromethium1457.26solid
62SmSamarium150.367.52solid
63EuEuropium151.965.244solid
64GdGadolinium157.257.9solid
65TbTerbium158.938.23solid
66DyDysprosium162.58.54solid
67HoHolmium164.938.79solid
68ErErbium167.269.066solid
69TmThulium168.939.32solid
70YbYtterbium173.056.9solid
71LuLutetium174.979.841solid
72HfHafnium178.4913.31solid
73TaTantalum180.9516.69solid
74WTungsten183.8419.25solid
75ReRhenium186.2121.02solid
76OsOsmium190.2322.59solid
77IrIridium192.2222.56solid
78PtPlatinum195.0821.45solid
79AuGold196.9719.3solid
80HgMercury200.5913.534liquid
81TlThallium204.3811.85solid
82PbLead207.211.34solid
83BiBismuth208.989.78solid
84PoPolonium2099.196solid
85AtAstatine210NA
86RnRadon2220.00973gas
87FrFrancium223NA
88RaRadium2265.5solid
89AcActinium22710solid
90ThThorium232.0411.7solid
91PaProtactinium231.0415.37solid
92UUranium238.0319.1solid
93NpNeptunium23720.45solid
94PuPlutonium24419.85solid
95AmAmericium24312solid
96CmCurium24713.51solid
97BkBerkelium24714.78solid
98CfCalifornium25115.1solid
99EsEinsteinium2528.84solid
100FmFermium257NA
101MdMendelevium258NA
102NoNobelium259NA
103LrLawrencium266NA
104RfRutherfordium267NA
105DbDubnium268NA
106SgSeaborgium267NA
107BhBohrium270NA
108HsHassium271NA
109MtMeitnerium278NA
110DsDarmstadtium281NA
111RgRoentgenium282NA
112CnCopernicium285NA
113NhNihonium286NA
114FlFlerovium289NA
115McMoscovium290NA
116LvLivermorium293NA
117TsTennessine294NA
118OgOganesson294NA
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