Average Atomic Mass Calculator
Find average atomic mass from isotopes.
Results update as you type.
How it works
The atomic mass listed on the periodic table is a weighted average of an element's naturally occurring isotopes, and this calculator computes that average from isotope data. You enter the mass and percent abundance of each isotope — up to three — and it returns the average atomic mass and the total abundance you entered as a check. The calculation multiplies each isotope's mass by its fractional abundance and sums the results, which is a weighted average that gives more influence to the more abundant isotopes. For example, chlorine has two main isotopes, chlorine-35 and chlorine-37, and because chlorine-35 is about three times more common, the average atomic mass is around 35.45, much closer to 35 than 37. This is why periodic table masses are rarely whole numbers even though individual isotopes have masses close to whole numbers. The abundances should total 100 percent; the calculator shows your total so you can verify it. Understanding average atomic mass is fundamental to chemistry, since it links the masses of individual atoms to the molar masses used in weighing and stoichiometry. Enter each isotope's mass and abundance to find the average atomic mass.
Average mass = Σ (isotope mass × abundance) ÷ 100
The average atomic mass is the sum of each isotope's mass multiplied by its fractional abundance. With abundances entered as percentages, multiply each mass by its percent, add them, and divide by 100. The total abundance should add up to about 100 percent for a correct weighted average.
Source:Atomic structure (isotopic abundance)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter isotope 1 mass and isotope 1 abundance.
- 2
Adjust the options
Adjust isotope 2 mass, isotope 2 abundance, isotope 3 mass and isotope 3 abundance if they apply — sensible defaults are already set.
- 3
Read your result
Average atomic mass updates instantly as you type — no button to press.
Frequently asked questions
How do you calculate average atomic mass?
Multiply each isotope's mass by its fractional abundance (its percent divided by 100), then add the results. Equivalently, multiply each mass by its percent, sum, and divide by 100. This weighted average gives more weight to more abundant isotopes, producing the atomic mass shown on the periodic table for the element.
Why are atomic masses not whole numbers?
Because the periodic table lists a weighted average of an element's isotopes, which have different masses and abundances. Even though individual isotope masses are close to whole numbers, averaging them by abundance usually gives a non-integer. Chlorine's 35.45, for instance, reflects a mix of chlorine-35 and chlorine-37 rather than any single isotope's mass.
What is an isotope?
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, giving them different masses. They share chemical properties but differ in mass and sometimes stability. Most elements occur as a mix of isotopes in nature, and their relative abundances determine the element's average atomic mass.
Should the abundances add up to 100 percent?
Yes. Because you are computing a weighted average over all of an element's naturally occurring isotopes, their percent abundances should total 100 percent. The calculator displays your entered total so you can check it. If it does not sum to 100, either an isotope is missing or an abundance is off, which would make the average inaccurate.
How does average atomic mass relate to molar mass?
The average atomic mass in atomic mass units numerically equals the element's molar mass in grams per mole. So once you know the average atomic mass, you know how many grams make up one mole of that element. This connection lets chemists move from the masses of individual atoms to the weighable, mole-based quantities used in the lab.