Atomic Mass Calculator

Calculate the average atomic mass of an element from isotopic masses and their percent abundances using a weighted-average calculation.

Isotope Calculator Percent Abundance Multiple Isotopes

Calculate Average Atomic Mass

Enter each isotope's mass and percent abundance. Add more isotope rows whenever needed.

Weighted average: Multiply each isotope mass by its fractional abundance, then add all weighted contributions. For a complete isotope distribution, the entered abundances should total approximately 100%.

Isotope

Isotopic Mass (u)

Abundance (%)

Total Abundance: 0%

Average Atomic Mass Formula
Atomic Mass = Σ [Isotopic Mass × (Percent Abundance ÷ 100)]

Average Atomic Mass

Weighted average based on the isotope data entered.

Isotopes Used -

Total Abundance -

Unit u / amu

Chemistry Guide

What Is Atomic Mass?

The atomic mass commonly listed for an element is a weighted average of the masses of its naturally occurring isotopes. Isotopes of the same element have the same number of protons but can have different numbers of neutrons.

Because different isotopes may occur in different proportions, a simple arithmetic average is usually not appropriate. Each isotope's mass must be weighted according to its relative abundance.

Main Formula

Average Atomic Mass Formula

Convert each percent abundance into fractional abundance by dividing it by 100. Multiply that fraction by the isotope's mass and then add all isotope contributions.

Atomic Mass = Σ(mᵢ × fᵢ) mᵢ = isotopic mass • fᵢ = fractional abundance

How It Works

How to Use the Atomic Mass Calculator

Enter Isotope Masses

Enter the measured isotopic mass for each isotope included in the calculation.

Add Percent Abundance

Enter the corresponding percentage abundance for every isotope. A complete distribution should total approximately 100%.

Calculate Weighted Mass

The calculator converts percentages to fractions, calculates each weighted contribution, and adds them together.

Worked Example

Average Atomic Mass Example

Example with Two Isotopes

Suppose an element has two isotopes:

Isotope 1: mass = 34.969 u, abundance = 75.77%

Isotope 2: mass = 36.966 u, abundance = 24.23%

Weighted contribution 1 = 34.969 × 0.7577

Weighted contribution 2 = 36.966 × 0.2423

Adding the weighted contributions gives an average atomic mass of approximately 35.45 u.

Formula Reference

Atomic Mass Calculation Variables

Term	Meaning	Example
Isotopic Mass	Mass of an individual isotope	34.969 u
Percent Abundance	Percentage of atoms represented by that isotope	75.77%
Fractional Abundance	Percent abundance divided by 100	0.7577
Weighted Contribution	Isotopic mass × fractional abundance	34.969 × 0.7577
Average Atomic Mass	Sum of all weighted isotope contributions	u / amu

Important Difference

Atomic Mass vs Mass Number

Atomic mass and mass number are related but are not the same quantity. A mass number identifies an individual isotope using the total number of protons and neutrons in its nucleus, while an element's average atomic mass reflects the weighted contribution of its isotopes.

Property	Atomic Mass	Mass Number
Meaning	Weighted average isotope mass	Protons + neutrons for an isotope
Usually	Often not a whole number	Whole number
Used For	Average elemental mass	Identifying an isotope

Important: For a complete isotope distribution, abundances should add to approximately 100%. The calculator allows a small rounding tolerance because published abundance values may be rounded.

Frequently Asked Questions

Atomic Mass Calculator FAQs

How do you calculate average atomic mass?

Multiply the mass of each isotope by its fractional abundance and add all of the weighted contributions together.

Why is atomic mass a weighted average?

Different isotopes of an element can occur in different proportions. The more abundant isotopes therefore contribute more strongly to the average.

What unit is used for atomic mass?

Atomic-scale masses are commonly expressed using the unified atomic mass unit, symbol u. The term amu is also commonly used in educational contexts.

Do isotope abundances need to total 100%?

For a complete isotope distribution, the percent abundances should total approximately 100%. Small differences may occur when values have been rounded.

Is atomic mass the same as mass number?

No. Mass number is the total number of protons and neutrons in a particular isotope. Average atomic mass is based on isotope masses and their relative abundances.

Can I calculate atomic mass with more than two isotopes?

Yes. This calculator allows multiple isotope rows. Each isotope contributes to the final weighted average according to its abundance.

What is fractional abundance?

Fractional abundance is percent abundance divided by 100. For example, 75% corresponds to a fractional abundance of 0.75.

Why is average atomic mass often a decimal?

It represents a weighted average of different isotope masses rather than the mass number of one individual isotope.