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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.

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:

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.

Atomic Number	Symbol	Name	Atomic Weight (g/mol)	Density (g/cm³)	Phase at Room Temp.
1	H	Hydrogen	1.008	0.00008988	gas
2	He	Helium	4.0026	0.0001785	gas
3	Li	Lithium	6.94	0.534	solid
4	Be	Beryllium	9.0122	1.85	solid
5	B	Boron	10.81	2.34	solid
6	C	Carbon	12.011	2.267	solid
7	N	Nitrogen	14.007	0.0012506	gas
8	O	Oxygen	15.999	0.001429	gas
9	F	Fluorine	18.998	0.001696	gas
10	Ne	Neon	20.18	0.0009002	gas
11	Na	Sodium	22.99	0.968	solid
12	Mg	Magnesium	24.305	1.738	solid
13	Al	Aluminium	26.982	2.7	solid
14	Si	Silicon	28.085	2.329	solid
15	P	Phosphorus	30.974	1.823	solid
16	S	Sulfur	32.06	2.07	solid
17	Cl	Chlorine	35.45	0.0032	gas
18	Ar	Argon	39.95	0.001784	gas
19	K	Potassium	39.098	0.89	solid
20	Ca	Calcium	40.078	1.55	solid
21	Sc	Scandium	44.956	2.985	solid
22	Ti	Titanium	47.867	4.506	solid
23	V	Vanadium	50.942	6.11	solid
24	Cr	Chromium	51.996	7.15	solid
25	Mn	Manganese	54.938	7.21	solid
26	Fe	Iron	55.845	7.874	solid
27	Co	Cobalt	58.933	8.9	solid
28	Ni	Nickel	58.693	8.908	solid
29	Cu	Copper	63.546	8.96	solid
30	Zn	Zinc	65.38	7.14	solid
31	Ga	Gallium	69.723	5.91	solid
32	Ge	Germanium	72.63	5.323	solid
33	As	Arsenic	74.922	5.727	solid
34	Se	Selenium	78.971	4.81	solid
35	Br	Bromine	79.904	3.1028	liquid
36	Kr	Krypton	83.798	0.003749	gas
37	Rb	Rubidium	85.468	1.532	solid
38	Sr	Strontium	87.62	2.64	solid
39	Y	Yttrium	88.906	4.472	solid
40	Zr	Zirconium	91.224	6.52	solid
41	Nb	Niobium	92.906	8.57	solid
42	Mo	Molybdenum	95.95	10.28	solid
43	Tc	Technetium	97	11	solid
44	Ru	Ruthenium	101.07	12.45	solid
45	Rh	Rhodium	102.91	12.41	solid
46	Pd	Palladium	106.42	12.023	solid
47	Ag	Silver	107.87	10.49	solid
48	Cd	Cadmium	112.41	8.65	solid
49	In	Indium	114.82	7.31	solid
50	Sn	Tin	118.71	7.265	solid
51	Sb	Antimony	121.76	6.697	solid
52	Te	Tellurium	127.6	6.24	solid
53	I	Iodine	126.9	4.933	solid
54	Xe	Xenon	131.29	0.005894	gas
55	Cs	Caesium	132.91	1.93	solid
56	Ba	Barium	137.33	3.51	solid
57	La	Lanthanum	138.91	6.162	solid
58	Ce	Cerium	140.12	6.77	solid
59	Pr	Praseodymium	140.91	6.77	solid
60	Nd	Neodymium	144.24	7.01	solid
61	Pm	Promethium	145	7.26	solid
62	Sm	Samarium	150.36	7.52	solid
63	Eu	Europium	151.96	5.244	solid
64	Gd	Gadolinium	157.25	7.9	solid
65	Tb	Terbium	158.93	8.23	solid
66	Dy	Dysprosium	162.5	8.54	solid
67	Ho	Holmium	164.93	8.79	solid
68	Er	Erbium	167.26	9.066	solid
69	Tm	Thulium	168.93	9.32	solid
70	Yb	Ytterbium	173.05	6.9	solid
71	Lu	Lutetium	174.97	9.841	solid
72	Hf	Hafnium	178.49	13.31	solid
73	Ta	Tantalum	180.95	16.69	solid
74	W	Tungsten	183.84	19.25	solid
75	Re	Rhenium	186.21	21.02	solid
76	Os	Osmium	190.23	22.59	solid
77	Ir	Iridium	192.22	22.56	solid
78	Pt	Platinum	195.08	21.45	solid
79	Au	Gold	196.97	19.3	solid
80	Hg	Mercury	200.59	13.534	liquid
81	Tl	Thallium	204.38	11.85	solid
82	Pb	Lead	207.2	11.34	solid
83	Bi	Bismuth	208.98	9.78	solid
84	Po	Polonium	209	9.196	solid
85	At	Astatine	210		NA
86	Rn	Radon	222	0.00973	gas
87	Fr	Francium	223		NA
88	Ra	Radium	226	5.5	solid
89	Ac	Actinium	227	10	solid
90	Th	Thorium	232.04	11.7	solid
91	Pa	Protactinium	231.04	15.37	solid
92	U	Uranium	238.03	19.1	solid
93	Np	Neptunium	237	20.45	solid
94	Pu	Plutonium	244	19.85	solid
95	Am	Americium	243	12	solid
96	Cm	Curium	247	13.51	solid
97	Bk	Berkelium	247	14.78	solid
98	Cf	Californium	251	15.1	solid
99	Es	Einsteinium	252	8.84	solid
100	Fm	Fermium	257		NA
101	Md	Mendelevium	258		NA
102	No	Nobelium	259		NA
103	Lr	Lawrencium	266		NA
104	Rf	Rutherfordium	267		NA
105	Db	Dubnium	268		NA
106	Sg	Seaborgium	267		NA
107	Bh	Bohrium	270		NA
108	Hs	Hassium	271		NA
109	Mt	Meitnerium	278		NA
110	Ds	Darmstadtium	281		NA
111	Rg	Roentgenium	282		NA
112	Cn	Copernicium	285		NA
113	Nh	Nihonium	286		NA
114	Fl	Flerovium	289		NA
115	Mc	Moscovium	290		NA
116	Lv	Livermorium	293		NA
117	Ts	Tennessine	294		NA
118	Og	Oganesson	294		NA

To calculate molecular weight accurately, sum the standard atomic weights of all atoms in the chemical formula, adjusting for isotopic composition if high precision is required. Do not rely on generic online tools for stoichiometry without verifying hydration states, as water molecules in crystal structures alter mass significantly. For bulk reactions, standard averages suffice; for mass spectrometry, use monoisotopic mass.

Isotopic Variance and Atomic Weight Definitions

The fundamental operation of a molecular weight calculator involves summing the products of atom counts and their respective atomic weights. However, a critical hidden variable often overlooked is the distinction between standard atomic weight and monoisotopic mass. Standard atomic weights found on periodic tables are weighted averages of all naturally occurring isotopes of an element, reflecting terrestrial abundance. This average is sufficient for bulk stoichiometry where macroscopic quantities smooth out isotopic variance. Conversely, mass spectrometry detects specific isotopic configurations, requiring the sum of the most abundant isotope for each element rather than the average.

Consider the element chlorine. Its standard atomic weight is approximately 35.45, reflecting a mix of Chlorine-35 and Chlorine-37. A calculator using standard weights will yield a bulk molar mass useful for weighing reagents on a balance. However, a mass spectrometer will resolve distinct peaks at integer masses corresponding to specific isotopes. Using the average weight in a mass spec context introduces systematic error because no single molecule possesses the average mass; every individual molecule contains specific isotopes. This distinction dictates tool selection. If your output feeds into reaction yield calculations, standard weights are appropriate. If your output validates molecular identity via spectrometry, standard weights are incorrect.

The mathematical notation for standard molecular weight (M_w) is expressed as:

$ M_w = \sum_{i=1}^{n} n_i \times A_r(i) $

Where n_i is the count of element i and A_r(i) is the standard atomic weight. For monoisotopic mass, A_r(i) is replaced by the mass of the most abundant isotope. Most generic calculators default to A_r(i) without explicit toggles. You must verify the algorithm’s source data. Relying on a tool that does not disclose its weight standards risks incompatibility with high-precision instrumentation. The trade-off is between representational accuracy for bulk matter versus exactitude for individual molecular entities.

Hydration States and Significant Figure Propagation

A more frequent source of calculation error than isotopic variance is the misidentification of hydration states. Chemical reagents often exist as hydrates, containing water molecules integrated into their crystal lattice. A molecular weight calculator processes the string you input; it cannot infer physical state. Inputting “CuSO4” calculates the mass of anhydrous copper sulfate. If the reagent in the lab is “CuSO4·5H2O”, the calculated mass will be significantly lower than the actual mass weighed out. This discrepancy leads to incorrect molar concentrations and failed reactions.

You must manually account for water molecules in the input string. The mass contribution of water is substantial; in copper sulfate pentahydrate, water accounts for over 30% of the total molecular weight. Ignoring this variable creates a massive asymmetry in experimental outcomes. Furthermore, significant figures must be managed rigorously. Atomic weights are known to varying degrees of precision. When summing weights, the result should not exceed the precision of the least precise input value, though in practice, standard weights are often treated as exact constants relative to balance precision.

Component	Input String	Impact on Mass
Anhydrous	`Na2CO3`	Baseline Mass
Monohydrate	`Na2CO3.H2O`	+18.015 g/mol
Decahydrate	`Na2CO3.10H2O`	+180.15 g/mol

The table above illustrates how hydration drastically shifts the calculated value. A calculator will not warn you if you omit the hydrate notation. This requires human judgment prior to computation. Additionally, purity levels affect the effective molecular weight in solution. If a reagent is 98% pure, the remaining 2% impurities alter the effective molarity. While calculators compute theoretical weight, practical application demands adjusting for certificate of analysis data. Do not treat the calculator output as the final truth for solution preparation.

Final Strategic Directive

Verify the physical form of your reagent before inputting any formula. The calculator provides a theoretical value based on perfect stoichiometry; it cannot account for lab-grade impurities or hydration unless explicitly told to do so. Always cross-reference the calculator output with the reagent bottle label to ensure hydration states match. For high-precision work, confirm whether your downstream application requires average atomic weights or monoisotopic masses.

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