🧮 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%. For example, boron is about 19.9% boron-10 (10.013 u) and 80.1% boron-11 (11.009 u), giving 10.013 × 0.199 + 11.009 × 0.801 ≈ 10.81 u.
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.
Can I use fractional abundances instead of percentages?
Yes, enter 75.77 as a percent or 0.7577 as a fraction, as long as you are consistent. Percentages should total 100; fractions should total 1.
How do I find an unknown abundance from the average mass?
For two isotopes, let the first fraction be x and the second (1 − x): set mass₁·x + mass₂·(1 − x) equal to the known average and solve for x. Chlorine’s 35.45 average solves to about 76% chlorine-35.
Why does the average mass sit closer to one isotope than the other?
It is weighted toward the more abundant isotope. Copper’s 63.55 lies near copper-63 (62.93 u) because about 69% of copper atoms are that isotope, versus 31% copper-65.
Is average atomic mass the same as atomic weight?
In everyday use, yes, both refer to the abundance-weighted average mass of an element’s isotopes shown on the periodic table. Strictly, “relative atomic mass” is the dimensionless value and this average is what the table reports.