Atomic Mass Calculator

Chemistry

Calculate the average atomic mass of an element from the masses and natural abundances of two isotopes. The calculator takes the abundance-weighted mean: (m₁·a₁ + m₂·a₂) ÷ (a₁ + a₂). Enter each isotope's mass in atomic mass units (u) and its percentage abundance to get the element's relative atomic mass.

Formula

Atomic mass = (m1 x a1 + m2 x a2) / (a1 + a2)
  • Average atomic mass = (m₁·a₁ + m₂·a₂) ÷ (a₁ + a₂), the abundance-weighted mean of the isotope masses.
  • Enter each isotope's mass in unified atomic mass units (u) and its natural abundance as a percentage.
  • If the two abundances add up to 100%, the denominator is simply 100; the formula still works if they don't sum exactly to 100 because it divides by the total.
  • The heavier isotope pulls the average up; the result lands closer to whichever isotope is more abundant.

Example: chlorine (³⁵Cl and ³⁷Cl)

Inputs
  • Isotope 1 mass (u): 35
  • Isotope 1 abundance (%): 75.77
  • Isotope 2 mass (u): 37
  • Isotope 2 abundance (%): 24.23

Chlorine-35 (75.77%) and chlorine-37 (24.23%) give (35×75.77 + 37×24.23) ÷ 100 ≈ 35.48 u, which matches chlorine's standard atomic weight.

Frequently asked questions

How is average atomic mass calculated?
It is the weighted mean of the isotope masses: multiply each isotope's mass by its fractional abundance, add the results, and divide by the total abundance.
Why isn't chlorine's atomic mass a whole number?
Because it is an average of two isotopes. Chlorine-35 and chlorine-37 occur in roughly a 3:1 ratio, giving an average near 35.45 u rather than a whole number.
What units do I use for isotope mass?
Use unified atomic mass units (u), where carbon-12 is defined as exactly 12 u. Abundances are entered as percentages.
Do the abundances have to add up to 100%?
Ideally yes for a two-isotope element, but the formula divides by the sum of the abundances, so it still returns the correct weighted average if they are slightly off.
What is the difference between mass number and atomic mass?
The mass number is the whole-number count of protons and neutrons in one isotope, while the (average) atomic mass is the abundance-weighted mean across all isotopes of the element.

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