What is the Atomic Mass Calculator?
Look up standard atomic mass (atomic weight) for any element from hydrogen to oganesson, or calculate weighted average atomic mass from isotope masses and natural abundances. Element lookup shows atomic number and mass in unified atomic mass units (u), equivalent to g/mol. Isotope mode supports percent or fractional abundances with a full contribution breakdown. Uses IUPAC 2021 standard atomic weights. Runs instantly in your browser with no signup.
How to use the Atomic Mass Calculator
- Choose element lookup or isotope average mode.
- For lookup: enter an element symbol (e.g. Fe, Cl) or pick a popular element.
- For isotopes: enter two or more isotope masses and abundances (% or fraction).
- Read atomic mass, atomic number, or weighted average with breakdown.
- Copy the summary for homework or lab reference.
Common use cases
- Quick lookup of Fe atomic mass and atomic number 26
- Verifying Cl average mass from Cl-35 and Cl-37 abundances
- Teaching weighted-average calculations with Cu isotopes
Frequently asked questions
- What is atomic mass?
- Atomic mass (atomic weight) is the weighted average mass of an element's atoms, expressed in unified atomic mass units (u). It equals molar mass in g/mol for a single element.
- How is average atomic mass from isotopes calculated?
- Average = Σ (isotope mass × fractional abundance). Percent abundances are normalized to sum to 100% before calculating.
- How is this different from molar mass?
- Atomic mass is for a single element. Molar mass sums atomic masses for all atoms in a chemical formula (e.g. H₂O = 18.015 g/mol).
- Which data source is used?
- Standard atomic weights from IUPAC 2021 values for elements 1–118.
