Average Atomic Mass Calculator
Most elements occur as a mixture of isotopes with slightly different masses, so the value on the periodic table is an abundance-weighted average rather than any single isotope mass. This calculator takes up to three isotopes, each with a mass in unified atomic mass units and a percent natural abundance, and returns the weighted average. It also totals your abundances so you can confirm they sum to 100 percent. Enter measured isotope data; the weighted-average arithmetic is exact.
Average atomic mass formula
fractional abundance = percent / 100
average = sum( mass_i * fraction_i )
= m1*(a1/100) + m2*(a2/100) + m3*(a3/100)
total abundance = a1 + a2 + a3 (should equal 100)
For chlorine, 34.969 times 0.7576 plus 36.966 times 0.2424 gives about 35.45 u, the periodic-table value.
Isotope facts
- Isotopes of an element share the same proton count but differ in neutron count.
- The unified atomic mass unit is one twelfth of the mass of a carbon-12 atom.
- Natural abundances should sum to 100 percent across all stable isotopes.
- The periodic-table atomic weight is this abundance-weighted average.
- Leave unused isotope rows at zero to use only the isotopes you need.
Average atomic mass: frequently asked questions
How is average atomic mass calculated?
The average atomic mass is the abundance-weighted mean of the isotope masses. Multiply each isotope mass by its fractional abundance (percent divided by 100) and add the products. The result is the value reported on the periodic table for that element.
Should abundances add up to 100 percent?
Yes. The percent abundances of all naturally occurring isotopes of an element should sum to 100. This calculator shows the total abundance you entered so you can confirm it adds up; if it does not, the weighted average will not be meaningful.
What units does atomic mass use?
Atomic masses are given in unified atomic mass units (u), also called daltons (Da). One unified atomic mass unit is defined as one twelfth of the mass of a carbon-12 atom. Enter isotope masses in u and the average comes out in u.
Where do isotope masses and abundances come from?
Isotope masses and natural abundances are measured values published by bodies such as NIST and IUPAC. Because they are empirical and element specific, you enter them yourself; the calculator performs only the deterministic weighted average.
Why is chlorine about 35.45 rather than a whole number?
Chlorine is a mix of about 75.8 percent chlorine-35 and 24.2 percent chlorine-37. The weighted average, 0.758 times 34.969 plus 0.242 times 36.966, comes to about 35.45 u, which is why the periodic table value is not a whole number.
Official sources
- NIST: NIST Atomic Weights and Isotopic Compositions.
- IUPAC: IUPAC Periodic Table and atomic weights.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.