Atomic Mass Calculator — Weighted Average Isotope Mass
The atomic mass listed on the periodic table is the weighted average of all naturally occurring isotopes. Enter each isotope's mass (in u) and its natural abundance (%) to calculate that weighted average — defaults show carbon-12 and carbon-13.
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Atomic mass units (u / Da) — weighted average across all isotopes
- 1
Isotope 1 contribution
12 × 98.93 ÷ 100 = 11.8716 - 2
Isotope 2 contribution
13.00335 × 1.07 ÷ 100 = 0.13914 - 3
Weighted average atomic mass
11.8716 + 0.13914 = 12.01074
How does this calculator work?
Atomic mass = Σ(isotope mass × abundance%). Enter mass and natural abundance for each isotope (up to four); the calculator returns the weighted average in unified atomic mass units (u / Da). Defaults show carbon: 12 u × 98.93% + 13.00335 u × 1.07% ≈ 12.011 u.
Formula
How this is calculated
Most elements exist as a mixture of isotopes — atoms with the same number of protons but different numbers of neutrons. The atomic mass (relative atomic mass) reported on the periodic table is therefore not the mass of any single atom, but the weighted average across all stable isotopes weighted by their natural abundance.
The formula is: Ar = Σ(mᵢ × aᵢ / 100), where mᵢ is the mass of isotope i in unified atomic mass units (u, also called daltons Da) and aᵢ is its percentage natural abundance. For example, carbon's atomic mass of 12.011 u comes almost entirely from ¹²C (98.93%, mass 12 u exactly) with a small contribution from ¹³C (1.07%, mass 13.00335 u).
If your abundances do not sum exactly to 100%, the calculator normalises them automatically and flags this. Natural abundances are reference values (IUPAC 2021) that vary slightly with source material; use measured isotope ratios from mass spectrometry for the highest accuracy. Radioactive (unstable) isotopes are not covered here — their effective contribution depends on sample age.
Frequently asked questions
Mass number is the integer count of protons + neutrons in one isotope. Atomic mass is a real-number weighted average across all naturally occurring isotopes, measured in unified atomic mass units (u), so it is almost never an integer.
Unified atomic mass units (u), also called daltons (Da). 1 u is defined as 1/12 the mass of a carbon-12 atom, approximately 1.660 538 × 10⁻²⁷ kg.
Yes for the arithmetic, but the natural abundance of radioactive isotopes varies with sample history and decay time, so the result may not represent the element in your sample. Use measured isotopic ratios from your specific sample instead.
Also known as
TG we-Calculate Editorial Team. (2026). Atomic Mass Calculator — Weighted Average Isotope Mass [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/atomic-mass-calculator
TG we-Calculate Editorial Team. "Atomic Mass Calculator — Weighted Average Isotope Mass." TG we-Calculate. 2026. https://we-calculate.com/calculator/atomic-mass-calculator.
TG we-Calculate Editorial Team, "Atomic Mass Calculator — Weighted Average Isotope Mass," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/atomic-mass-calculator
@misc{wecalculate_atomic_mass_calculator, title = {Atomic Mass Calculator — Weighted Average Isotope Mass}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/atomic-mass-calculator}}, year = {2026}, note = {TG we-Calculate} }
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