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Chemistry

Average Atomic Mass Calculator

Calculate a weighted atomic mass from isotope masses and measured percent abundances.

Explicit units and model assumptionsLocal calculations with worked examples
Chemistry workspace
Chemistry workspace

Check the units, concentration basis and assumptions before interpreting the result.

See the method

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Runs on your device

Your chemistry result

Your result

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Use the Calculate button to see your result.

Review the method below for assumptions and conventions.

How to use this tool

  1. Enter isotope masses (u), percent abundances in matching order.
  2. Select Calculate to view the result.
  3. Check the method and assumptions below before using the result.

The method, explained

Average atomic mass = Σ(isotope mass × fractional abundance). Enter percentages totaling 100; small rounding differences within 0.1 percentage point are normalized.

A WORKED EXAMPLE

Using isotope masses (u) = 10.012937, 11.009305, percent abundances in matching order = 19.9, 80.1, the result is 10.8110278 u. Change these example inputs to match your task; use the method above to check each step.

Understanding your result

An element can contain multiple isotopes. The mixture’s weighted mean lies between their masses and depends on their relative abundances.

What to keep in mind

Calculates the supplied sample’s isotopic mixture. Natural abundances can vary between materials. Educational calculation only; use validated procedures for laboratory work.

Reference: OpenStax Chemistry 2e — chemical quantities and models

Common questions

Why is atomic mass not always a whole number?

An element can contain multiple isotopes. The mixture’s weighted mean lies between their masses and depends on their relative abundances.

Are my inputs uploaded?

No. This calculation runs in your browser without an account or an external API.

Methodology maintained by ClarityKit. How these tools are built and checked.