Average Atomic Mass Calculator

Enter the percentage and masses for the number of isotops selected and the calculator will show complete work for average atomic mass.

Select No. of isotopes:

% of 1st isotope (f₁):

% ▾

Mass of 1st (m₁):

amu

Mass of 4 (f₂):

% ▾

% of 5 (m₂):

amu

Related

The average atomic mass calculator helps you determine the average atomic mass of an element using the masses of its isotopes and their natural abundances. It quickly calculates the weighted average and provides accurate results.

What is Average Atomic Mass?

“Average atomic mass is the weighted average of all naturally occurring isotopes of an element.”

It is sometimes referred to as the atomic weight of an element. Since most elements exist as a mixture of isotopes, their atomic masses on the periodic table are usually decimal values rather than whole numbers.

Examples:

Hydrogen has three isotopes: Protium (¹H), Deuterium (²H), and Tritium (³H).
Carbon has isotopes: C-12, C-13, and C-14.
Chlorine has two major isotopes: Cl-35 and Cl-37.

Because isotopes have different masses but the same atomic number, we calculate a weighted average based on their relative abundance to find the average atomic mass.

Average Atomic Mass Formula:

Average Atomic Mass = (f₁ × m₁) + (f₂ × m₂) + ... + (fₙ × mₙ)

Where:

f = fractional abundance of each isotope (percentage ÷ 100)
m = isotopic mass (amu)

Example (Chlorine):

Chlorine has two naturally occurring isotopes:

Cl-35 → Mass = 35 amu, Abundance = 75.77%
Cl-37 → Mass = 37 amu, Abundance = 24.23%

Solution:

First, convert percentages into decimal form:

75.77% = 0.7577

24.23% = 0.2423

Average Atomic Mass = (0.7577 × 35) + (0.2423 × 37)

= 26.5195 + 8.9651

= 35.48 amu

Therefore, the average atomic mass of chlorine is approximately 35.48 amu, which matches the value listed on the periodic table.

The online average atomic mass calculator performs these steps automatically and ensures accurate weighted calculations.

Average Atomic Masses Table

The following table lists the average atomic masses of the first 30 elements:

ATOMIC NUMBER	ELEMENT	AVERAGE ATOMIC MASS (amu)
1	Hydrogen	1.008
2	Helium	4.0026
3	Lithium	6.94
4	Beryllium	9.0122
5	Boron	10.81
6	Carbon	12.011
7	Nitrogen	14.007
8	Oxygen	15.999
9	Fluorine	18.998
10	Neon	20.180
11	Sodium	22.990
12	Magnesium	24.305
13	Aluminium	26.982
14	Silicon	28.085
15	Phosphorus	30.974
16	Sulfur	32.06
17	Chlorine	35.45
18	Argon	39.948
19	Potassium	39.098
20	Calcium	40.078
21	Scandium	44.956
22	Titanium	47.867
23	Vanadium	50.942
24	Chromium	51.996
25	Manganese	54.938
26	Iron	55.845
27	Cobalt	58.933
28	Nickel	58.693
29	Copper	63.546
30	Zinc	65.38