Average Atomic Mass Calculator
Calculate abundance-weighted mean atomic mass from entered isotope masses and percentages.
YOUR CALCULATION
Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Read each isotope mass and nonnegative abundance. Check that abundances sum to one hundred percent within tolerance. Normalize the entered abundances by their actual total. Sum isotope mass times normalized abundance.
Average mass=Σ(mass×abundance)/Σabundance
Worked example
Masses 12 u and 13 u at abundances 75% and 25% give an average mass of 12.25 u.
Assumptions and input details
The abundance total must be within 0.01 percentage points of one hundred. Masses are supplied by you, not assumed equal to mass numbers. Uncertainties, isotope enrichment mechanisms and standard atomic-weight intervals are not modeled.
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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