Average Atomic Mass Calculator
The Average Atomic Mass Calculator finds the weighted average mass of an element from the masses and natural abundances of its isotopes. Enter each isotope's mass (in atomic mass units) and its percent abundance, and you get the element's average atomic mass — the value you see on the periodic table. Heavier or more abundant isotopes pull the average toward their own mass.
Formula
Average atomic mass = Σ(isotope mass × fractional abundance)
- The average atomic mass is the abundance-weighted mean of an element's isotope masses.
- Formula: Average atomic mass = Σ(isotope mass × fractional abundance).
- Convert each percent abundance to a fraction (divide by 100) before multiplying, or let the calculator normalize by the total abundance you enter.
- Input definitions: • Isotope mass: the mass of that isotope in atomic mass units (u). • Isotope abundance: how common that isotope is, entered as a percentage.
- Masses are in unified atomic mass units (u); abundances are percentages that ideally sum to 100%.
- The result matches the standard atomic weight listed on the periodic table for that element.
- Cross-check against a reference table to confirm your isotope data is reasonable.
Example Calculation
Inputs
- Isotope 1 mass: 34.969 u
- Isotope 1 abundance: 75.77 %
- Isotope 2 mass: 36.966 u
- Isotope 2 abundance: 24.23 %
- Isotope 3 mass (optional): 0 u
- Isotope 3 abundance (optional): 0 %
Chlorine has two main isotopes: Cl-35 (mass 34.969 u, 75.77%) and Cl-37 (mass 36.966 u, 24.23%). Average mass = 34.969 × 0.7577 + 36.966 × 0.2423 ≈ 35.45 u, which matches the value on the periodic table.
Frequently asked questions
What is average atomic mass?
It is the weighted average of the masses of all naturally occurring isotopes of an element, weighted by how common each isotope is. This is the atomic mass shown on the periodic table.
How do I calculate it?
Multiply each isotope's mass by its abundance (as a fraction), then add the products together: Average mass = Σ(mass × fractional abundance).
Do abundances need to add up to 100%?
Ideally yes, since they represent all isotopes of the element. This calculator normalizes by the total abundance you enter, so it still works if your figures are slightly off.
What units is atomic mass in?
Atomic mass is given in unified atomic mass units (u), also called daltons (Da). One u is one-twelfth the mass of a carbon-12 atom.
Why isn't the average a whole number?
Because it blends isotopes of different masses. Chlorine, for example, averages about 35.45 u because it mixes chlorine-35 and chlorine-37.
Can I enter more than three isotopes?
This tool handles up to three isotopes, which covers most common elements. For elements with more, group the minor isotopes or use the two or three most abundant.