🧮 Average Atomic Mass Calculator
Enter each isotope’s mass and percent abundance to get the element’s average atomic mass — the weighted average that appears on the periodic table.
Isotopes — mass (u) and natural abundance (%)
Average atomic mass
35.4529 u
Average atomic mass = Σ (isotope mass × fractional abundance). 🔒 Computed in your browser.
How the average atomic mass calculator works
An element’s average atomic mass is the abundance-weighted mean of its isotopes: Σ (isotope mass × fractional abundance). Enter each isotope’s mass (in u) and its natural abundance (%), and the tool sums the contributions. The abundances should total 100%.
This is why atomic masses on the periodic table are not whole numbers — chlorine’s 35.45 comes from ~76% chlorine-35 and ~24% chlorine-37. The tool flags if your abundances don’t add up to 100%.
Frequently asked questions
How do I calculate average atomic mass?
Multiply each isotope’s mass by its fractional abundance (percent ÷ 100) and add them up. For chlorine: 34.969 × 0.7577 + 36.966 × 0.2423 ≈ 35.45 u.
Why isn’t atomic mass a whole number?
Because it is a weighted average over an element’s naturally occurring isotopes, which have different masses. Only a single isotope would give a near-whole number.
What units are isotope masses in?
Atomic mass units (u, also called daltons). Each isotope’s mass is close to its mass number but not exactly, due to nuclear binding energy.
Do the abundances have to add to 100%?
Yes — natural abundances are fractions of the whole, so they should total 100%. The tool warns you if they don’t.
What is the difference between mass number and atomic mass?
Mass number is the whole-number count of protons + neutrons in one isotope; average atomic mass is the weighted average over all isotopes, which is what the periodic table lists.