Average Atomic Mass Calculator
Enter isotopic masses and abundances to calculate the weighted average atomic mass.
Tip: Try Cl-35 (34.96885, 75.78%) and Cl-37 (36.96590, 24.22%).
What Is Atomic Mass?
Atomic mass is the average mass of atoms of an element, measured in atomic mass units (amu). Because most elements exist as a mix of isotopes, the value shown on the periodic table is not usually a whole number. Instead, it is a weighted average based on how common each isotope is in nature.
For example, chlorine has two common isotopes: chlorine-35 and chlorine-37. Since chlorine-35 is more abundant, the average atomic mass is closer to 35 than to 37.
How This Atomic Mass Calculator Works
This calculator uses the weighted average formula:
Average atomic mass = Σ (isotope mass × isotope abundance) ÷ Σ(abundance)
If your abundances add to exactly 100% (or 1.0 in fraction mode), the result is the standard average atomic mass. If they do not, the calculator normalizes your values automatically and tells you that it did so.
Input Fields Explained
- Isotope label: A name such as C-12, C-13, Cl-35, or U-238.
- Isotope mass: The isotopic mass in amu (from data tables).
- Abundance: Natural abundance in percent or decimal fraction.
- Abundance type: Choose whether your abundance numbers are entered as % or as 0 to 1 fractions.
Atomic Mass vs. Mass Number
These terms are related but different:
- Mass number: Whole number of protons + neutrons in one specific isotope.
- Atomic mass: Weighted average over all naturally occurring isotopes of an element.
So, carbon-12 has a mass number of 12, but elemental carbon has an average atomic mass around 12.011 amu.
Step-by-Step Example (Chlorine)
Given data
- Cl-35 mass = 34.96885 amu, abundance = 75.78%
- Cl-37 mass = 36.96590 amu, abundance = 24.22%
Calculation
(34.96885 × 75.78 + 36.96590 × 24.22) ÷ 100 = 35.4529 amu (approximately)
This is why chlorine’s periodic-table atomic mass is about 35.45 amu.
Common Mistakes to Avoid
- Using mass numbers (35 and 37) instead of precise isotopic masses.
- Forgetting to convert percentages to decimals when using manual formulas.
- Entering abundances that are negative or missing.
- Rounding too early during intermediate steps.
When You Might Use an Atomic Mass Calculator
- Chemistry homework and exam preparation
- General chemistry lab reports
- Introductory nuclear chemistry practice
- Cross-checking periodic table and isotope table values
Quick FAQ
Why is atomic mass often a decimal?
Because it is an average over isotopes with different masses and natural abundances.
What if abundances do not add to 100%?
This tool normalizes them automatically and reports that adjustment in the result area.
Can atomic mass change?
The natural isotopic composition can vary slightly by sample source, so highly precise values can vary a bit in advanced contexts.