Atomic Mass Calculator

Weighted average atomic mass (IUPAC 2021) from isotope masses and natural abundances. Supports all elements 1–92, atomic weight intervals (boron, lithium...), real‑time pie chart.

Choose any element – loads IUPAC isotopic composition and standard atomic weight.
Isotope composition
Isotope label Mass (amu) Natural abundance (%) Contribution (amu)
11.871600
0.139136
Total abundance does not sum to 100%. Atomic mass uses raw values.
Weighted average atomic mass:
12.010736
amu
IUPAC standard atomic weight:— Radioactive
Abundance sum:100.00 %
Isotope count:2

Isotopic abundance (%)

Privacy-first: all data stays in your browser.

What is Atomic Mass? Weighted Average of Isotopes

The atomic mass (also called relative atomic mass, symbol Ar) of an element is the weighted average mass of its stable isotopes, taking into account their natural abundances. Unlike mass number, atomic mass is not an integer because it reflects isotopic mixtures. This value is fundamental to stoichiometry, mass spectrometry, and understanding periodic trends.

Ar(E) = ∑ (isotope mass × fractional abundance)

Where fractional abundance = (percent abundance) / 100. For example, Chlorine-35 (34.9689 amu, 75.77%) and Chlorine-37 (36.9659 amu, 24.23%) give Ar ≈ 35.45 amu.

Isotope Geochemistry & Mass Spectrometry

Modern atomic weights are determined using high‑precision mass spectrometers (e.g., TIMS, MC‑ICP‑MS). The IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW) evaluates and publishes standard atomic weights every two years. Our calculator uses reference data from NIST and IUPAC Technical Reports. For elements like carbon, the standard atomic weight is 12.011 ± 0.001 due to 13C natural variations.

Why Weighted Average Matters

In chemical reactions, atoms combine in mass ratios determined by atomic masses. Without accurate weighted averages, molar mass calculations for compounds like CO2 or H2O would be inconsistent. The concept also underpins isotope dilution analysis and radiometric dating. The tool’s dynamic table allows you to experiment with hypothetical isotope compositions — essential for teaching isotope fractionation and nuclear chemistry.

Step-by-Step Calculation Guide

  1. Enter each isotope’s exact mass in atomic mass units (amu). You can use precise values from mass spectrometry.
  2. Provide its natural abundance percentage (sum may be less than or greater than 100% — tool shows warning but calculates raw weighted average).
  3. Click preset buttons to load verified data for Carbon, Hydrogen, Oxygen, and Chlorine.
  4. The table automatically updates the contribution per isotope (mass × abundance/100).
  5. The pie chart visualizes abundance proportions, helping to understand dominant isotopes.
  6. Compare computed value with the IUPAC reference atomic weight displayed on the right.

Case Study: Chlorine’s Atomic Mass

Isotopes of Chlorine: 35Cl (34.968852 amu, 75.76%) and 37Cl (36.965903 amu, 24.24%). Weighted average = (34.968852×0.7576) + (36.965903×0.2424) = 35.453 amu. The IUPAC standard atomic weight of chlorine is 35.45 ± 0.01. The slight variation in natural samples (e.g., marine vs. terrestrial) leads to the uncertainty. Our calculator reproduces this with high accuracy, demonstrating the power of isotopic weighting.

Frequently Asked Questions

Because most elements exist as mixtures of isotopes. The atomic mass is the weighted average, which usually includes fractional contributions from each isotope (e.g., carbon: 12.011).

Yes, for any isotope with known mass and abundance (even short‑lived). However, standard atomic weights only consider naturally occurring, long‑lived isotopes.

The calculator shows a warning but still computes the raw weighted average. For accurate standard atomic weight, ensure total is 100% (or use normalization in your analysis).

Reference values are from IUPAC Atomic Weights of the Elements 2021 (CIAAW) and NIST database. They are updated periodically.
Sources: IUPAC Technical Report (2021), NIST Physical Measurement Laboratory, “Atomic Weights of the Elements 2021” – Pure Appl. Chem. (2022). Data for presets verified against standard reference materials.

Scientific rigor & transparency – This tool implements the exact weighted average formula recommended by IUPAC. The interactive isotope editor supports real educational scenarios, from introductory chemistry to advanced mass spectrometry. Reviewed by getzenquery Tech team. Last updated April 2026.