Average Atomic Mass Calculator
Enter up to three isotope masses and their percent abundances to get the abundance-weighted average atomic mass shown on the periodic table.
📊 What is Average Atomic Mass?
Average atomic mass, also called relative atomic mass or standard atomic weight, is the abundance-weighted average of the masses of every naturally occurring isotope of an element. It is the decimal number printed on a periodic table, and this calculator computes it directly from two or three isotope masses and their percent natural abundances, exactly the way it is derived in real reference tables.
This calculation appears throughout chemistry: explaining why periodic table masses are almost never whole numbers (chlorine at 35.45, copper at 63.55), computing molar masses for stoichiometry using the correct average isotope composition, understanding mass spectrometry data, where a sample's isotope peaks and their relative heights directly give the abundances used in this exact calculation, and building the conceptual bridge between individual atoms and bulk laboratory measurements.
A common misconception is that average atomic mass is simply the arithmetic mean of an element's isotope masses. It is not, it is a weighted average, so an isotope that makes up 90% of natural samples pulls the result far closer to its own mass than an isotope making up only 10%, even if the two isotope masses themselves are numerically similar.
This calculator also makes clear how this differs from the Atomic Mass Calculator on this site: that tool finds one single isotope's mass from its particle counts, no abundance or averaging involved, while this one always combines multiple isotopes weighted by how common each one actually is in nature.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Chlorine (two isotopes)
Cl-35: 34.9689 u at 75.77%. Cl-37: 36.9659 u at 24.23%. Isotope 3 unused.
Example 2 — Copper (two isotopes)
Cu-63: 62.9296 u at 69.15%. Cu-65: 64.9278 u at 30.85%. Isotope 3 unused.
Example 3 — Magnesium (three isotopes)
Mg-24: 23.9850 u at 78.99%. Mg-25: 24.9858 u at 10.00%. Mg-26: 25.9826 u at 11.01%.
❓ Frequently Asked Questions
🔗 Related Calculators
What is average atomic mass?
Average atomic mass, also called relative atomic mass or standard atomic weight, is the abundance-weighted average of the masses of all naturally occurring isotopes of an element. It is the decimal number shown on a periodic table, and it reflects the actual mixture of isotopes found in a typical natural sample of that element.
How is average atomic mass calculated?
Multiply each isotope's precise mass by its fractional (decimal) natural abundance, then add those products together across every isotope, average mass = sum of (isotope mass times fractional abundance). Fractional abundance is the percent abundance divided by 100.
Why don't natural isotope abundances always add up to exactly 100%?
In principle they must sum to exactly 100%, since every atom of the element is one isotope or another. In practice, published abundance values are experimentally measured and rounded, so they can be off by a small fraction of a percent, this calculator allows a small tolerance around 100% to account for that rounding.
How is this different from the Atomic Mass Calculator?
The Atomic Mass Calculator computes the estimated mass of a single, specific isotope directly from its proton and neutron counts, a simple arithmetic sum with no abundance involved. This calculator instead combines the precisely measured masses of two or more different isotopes of the same element, weighted by their natural abundance, to reproduce the overall average shown on a periodic table.
Why is chlorine's atomic mass 35.45 instead of a whole number?
Chlorine's periodic table value of 35.45 is a weighted average of chlorine-35 (mass about 34.97 u, about 75.77% abundant) and chlorine-37 (mass about 36.97 u, about 24.23% abundant). No single chlorine atom actually weighs 35.45 u, it is a population-level average across every naturally occurring chlorine atom.
What happens if I only have two isotopes to enter?
Leave the third isotope row's abundance at 0%, this calculator treats any row with 0% abundance as unused and excludes it from the weighted sum entirely, so a two-isotope element calculates correctly using only the first two rows.
Can average atomic mass be lower than every individual isotope's mass?
No. A weighted average always falls somewhere between the smallest and largest values being averaged, so the result is always between the lightest and heaviest isotope's mass, never below the lightest or above the heaviest one included.
Does the calculator work if abundances are given as decimals instead of percentages?
This calculator expects abundance as a percentage (for example, enter 75.77 for 75.77%, not 0.7577). If your source lists fractional abundances instead, multiply each one by 100 before entering it.
Why does isotope mass matter more than mass number for this calculation?
Mass number is always a whole number (protons plus neutrons), but real isotope masses are not exactly whole numbers because of nuclear mass defect and the small contribution of electron mass. Using precise isotope masses like 34.9689 u instead of the whole number 35 gives a far more accurate weighted average, matching published periodic table values closely.
Is average atomic mass the same everywhere on Earth?
For almost every element, yes, isotope abundances are remarkably constant across natural terrestrial sources, which is why a single standard atomic weight works globally. A small number of elements show slight, well-documented natural variation (for example some samples of lithium or boron), which is one reason IUPAC now lists standard atomic weights as intervals for a few elements rather than single fixed numbers.