Standard atomic weights of the elements

The values this site uses for every calculation: the abridged standard atomic weights from IUPAC, for all 118 elements.

An atomic weight is the average mass of the atoms of an element as it is found in nature, compared with one twelfth of an atom of carbon-12. It has no unit. The same number, read in grams per mole, is the molar mass of the element. The table below is the abridged list published by the Commission on Isotopic Abundances and Atomic Weights (CIAAW) of IUPAC, in which each value is given to five significant figures. We read it on 12 November 2026.

The uncertainty column shows how far the value can vary. For most elements it is tiny. For a few, such as lithium, boron and sulphur, natural samples differ enough that the last digit is not fixed. "None" means the element has no stable isotopes and so no standard atomic weight; the calculator does not work out formulas that contain those elements. Why the values are what they are is explained on the page where atomic weights come from.

Tap an element name to open its page on our periodic table site, with its properties, uses and history.

Find an element

No.	Symbol	Element	Atomic weight	Uncertainty
1	H	Hydrogen	1.0080	± 0.0002
2	He	Helium	4.0026	± 0.0001
3	Li	Lithium	6.94	± 0.06
4	Be	Beryllium	9.0122	± 0.0001
5	B	Boron	10.81	± 0.02
6	C	Carbon	12.011	± 0.002
7	N	Nitrogen	14.007	± 0.001
8	O	Oxygen	15.999	± 0.001
9	F	Fluorine	18.998	± 0.001
10	Ne	Neon	20.180	± 0.001
11	Na	Sodium	22.990	± 0.001
12	Mg	Magnesium	24.305	± 0.002
13	Al	Aluminium	26.982	± 0.001
14	Si	Silicon	28.085	± 0.001
15	P	Phosphorus	30.974	± 0.001
16	S	Sulphur	32.06	± 0.02
17	Cl	Chlorine	35.45	± 0.01
18	Ar	Argon	39.95	± 0.16
19	K	Potassium	39.098	± 0.001
20	Ca	Calcium	40.078	± 0.004
21	Sc	Scandium	44.956	± 0.001
22	Ti	Titanium	47.867	± 0.001
23	V	Vanadium	50.942	± 0.001
24	Cr	Chromium	51.996	± 0.001
25	Mn	Manganese	54.938	± 0.001
26	Fe	Iron	55.845	± 0.002
27	Co	Cobalt	58.933	± 0.001
28	Ni	Nickel	58.693	± 0.001
29	Cu	Copper	63.546	± 0.003
30	Zn	Zinc	65.38	± 0.02
31	Ga	Gallium	69.723	± 0.001
32	Ge	Germanium	72.630	± 0.008
33	As	Arsenic	74.922	± 0.001
34	Se	Selenium	78.971	± 0.008
35	Br	Bromine	79.904	± 0.003
36	Kr	Krypton	83.798	± 0.002
37	Rb	Rubidium	85.468	± 0.001
38	Sr	Strontium	87.62	± 0.01
39	Y	Yttrium	88.906	± 0.001
40	Zr	Zirconium	91.222	± 0.003
41	Nb	Niobium	92.906	± 0.001
42	Mo	Molybdenum	95.95	± 0.01
43	Tc	Technetium	none
44	Ru	Ruthenium	101.07	± 0.02
45	Rh	Rhodium	102.91	± 0.01
46	Pd	Palladium	106.42	± 0.01
47	Ag	Silver	107.87	± 0.01
48	Cd	Cadmium	112.41	± 0.01
49	In	Indium	114.82	± 0.01
50	Sn	Tin	118.71	± 0.01
51	Sb	Antimony	121.76	± 0.01
52	Te	Tellurium	127.60	± 0.03
53	I	Iodine	126.90	± 0.01
54	Xe	Xenon	131.29	± 0.01
55	Cs	Caesium	132.91	± 0.01
56	Ba	Barium	137.33	± 0.01
57	La	Lanthanum	138.91	± 0.01
58	Ce	Cerium	140.12	± 0.01
59	Pr	Praseodymium	140.91	± 0.01
60	Nd	Neodymium	144.24	± 0.01
61	Pm	Promethium	none
62	Sm	Samarium	150.36	± 0.02
63	Eu	Europium	151.96	± 0.01
64	Gd	Gadolinium	157.25	± 0.01
65	Tb	Terbium	158.93	± 0.01
66	Dy	Dysprosium	162.50	± 0.01
67	Ho	Holmium	164.93	± 0.01
68	Er	Erbium	167.26	± 0.01
69	Tm	Thulium	168.93	± 0.01
70	Yb	Ytterbium	173.05	± 0.02
71	Lu	Lutetium	174.97	± 0.01
72	Hf	Hafnium	178.49	± 0.01
73	Ta	Tantalum	180.95	± 0.01
74	W	Tungsten	183.84	± 0.01
75	Re	Rhenium	186.21	± 0.01
76	Os	Osmium	190.23	± 0.03
77	Ir	Iridium	192.22	± 0.01
78	Pt	Platinum	195.08	± 0.02
79	Au	Gold	196.97	± 0.01
80	Hg	Mercury	200.59	± 0.01
81	Tl	Thallium	204.38	± 0.01
82	Pb	Lead	207.2	± 1.1
83	Bi	Bismuth	208.98	± 0.01
84	Po	Polonium	none
85	At	Astatine	none
86	Rn	Radon	none
87	Fr	Francium	none
88	Ra	Radium	none
89	Ac	Actinium	none
90	Th	Thorium	232.04	± 0.01
91	Pa	Protactinium	231.04	± 0.01
92	U	Uranium	238.03	± 0.01
93	Np	Neptunium	none
94	Pu	Plutonium	none
95	Am	Americium	none
96	Cm	Curium	none
97	Bk	Berkelium	none
98	Cf	Californium	none
99	Es	Einsteinium	none
100	Fm	Fermium	none
101	Md	Mendelevium	none
102	No	Nobelium	none
103	Lr	Lawrencium	none
104	Rf	Rutherfordium	none
105	Db	Dubnium	none
106	Sg	Seaborgium	none
107	Bh	Bohrium	none
108	Hs	Hassium	none
109	Mt	Meitnerium	none
110	Ds	Darmstadtium	none
111	Rg	Roentgenium	none
112	Cn	Copernicium	none
113	Nh	Nihonium	none
114	Fl	Flerovium	none
115	Mc	Moscovium	none
116	Lv	Livermorium	none
117	Ts	Tennessine	none
118	Og	Oganesson	none

Using the table

Find each element of your formula.
Multiply its atomic weight by the number of its atoms.
Add the results to get the molar mass in g/mol.

For school sums a rounded value is usually expected: 1 for hydrogen, 12 for carbon, 16 for oxygen, 35.5 for chlorine. Use what your textbook or question paper prints.

Source: CIAAW, Abridged Standard Atomic Weights, ciaaw.org. Element names are spelled as IUPAC spells them, except that sulphur is written the Indian way.