Quick answer
Average atomic mass = Σ(isotope mass × abundance) ÷ Σ abundance. Chlorine-35 (34.96885 amu, 75.76%) and chlorine-37 (36.965 amu, 24.24%) average to 35.4529 amu.
Enter isotope masses and their percent abundances. The calculator returns the weighted average and the total abundance.
Related tools: Molar Mass Calculator: Molar mass and mass percent composition of a chemical formula. Mass Percent Calculator: Mass percent of a component in a mixture or compound. Mole Calculator: Convert between mass, moles and number of particles.
How to use the Average Atomic Mass Calculator
- Type isotope masses (amu), for example 34.96885, 36.96590.
- Type abundances (%), for example 75.76, 24.24.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Isotope masses (amu) | Text, such as 34.96885, 36.96590 | — |
| Abundances (%) | Text, such as 75.76, 24.24 | — |
Formula and method
Formula
average mass = Σ (massᵢ × abundanceᵢ) ÷ Σ abundanceᵢ
IEEE-754 double precision; shown to the decimals indicated.
Abundances are normalized by their total, so small rounding differences do not matter.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Isotope masses (amu) = 34.96885, 36.9659; Abundances (%) = 75.76, 24.24.
| Result | Value |
|---|---|
| Average atomic mass | 35.4529 amu |
| Total abundance | 100% |
Example 2
Inputs: Isotope masses (amu) = 10.0129, 11.0093; Abundances (%) = 19.9, 80.1.
| Result | Value |
|---|---|
| Average atomic mass | 10.811 amu |
| Total abundance | 100% |
Example 3
Inputs: Isotope masses (amu) = 23.985, 24.986, 25.983; Abundances (%) = 78.99, 10.0, 11.01.
| Result | Value |
|---|---|
| Average atomic mass | 24.3051 amu |
| Total abundance | 100% |
The first line is the headline result (average atomic mass). Below it the average Atomic Mass Calculator lists total abundance, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
The weighted sum
0.7576 × 34.96885 + 0.2424 × 36.965 = 26.492 + 8.960 = 35.453. This matches the periodic table's value for chlorine, about 35.45.
Requirements
The mass and abundance lists must have the same length. Abundances should sum to 100%. Atomic mass units are 1/12 the mass of carbon-12.
Weighted by abundance
Chlorine-35 (34.96885 amu, 75.76%) and chlorine-37 (36.9659 amu, 24.24%) give 35.4529 amu, close to the periodic table value of about 35.45.
The result is not a whole number because it is a weighted average and isotope masses are not whole numbers. If the abundances do not total 100, the formula divides by their sum.
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Abundances (%): entering "75" shows "Enter one abundance per isotope." beside that field and no result.
- Isotope masses (amu): left empty shows "Enter isotope masses (amu)." beside that field and no result.
Choosing between the Average Atomic Mass Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Molar Mass Calculator](/science/molar-mass-calculator/) | chemical formula | Molar mass |
| [Mass Percent Calculator](/science/mass-percent-calculator/) | mass of component and total mass | Mass percent |
| [Mole Calculator](/science/mole/) | start from and molar mass | Amount |
| [Percent Yield Calculator](/science/percent-yield/) | actual yield and theoretical yield | Percent yield |
| [Electron Configuration Calculator](/science/electron-configuration/) | element and charge | Electron configuration of H |
Background: chemistry calculations
Most basic chemistry calculations chain three relations: moles n = mass ÷ molar mass, molarity M = n ÷ volume in litres, and dilution C₁V₁ = C₂V₂. Keep units consistent: grams with g/mol, litres with mol/L.
Molar masses come from standard atomic weights, which differ slightly between tables and with the isotopic make-up of a real sample.
Assumptions and limitations
- Lists must be the same length
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for abundances (%) and isotope masses (amu) to confirm this.
Frequently asked questions
Why is atomic mass not a whole number?
It's a weighted average of isotopes.
What is an isotope?
An atom with the same protons and different neutrons.
Can I use fractions instead of percent?
Yes. Abundances are used as weights, so 0.7576 and 0.2424 give the same average as 75.76% and 24.24%.
What if my abundances do not add to 100?
The calculator divides by the total abundance, so relative counts also work.
What does the Average Atomic Mass Calculator calculate?
Weighted average of isotope masses. It returns Average atomic mass and Total abundance.
What formula does the Average Atomic Mass Calculator use?
average mass = Σ (massᵢ × abundanceᵢ) ÷ Σ abundanceᵢ. Abundances are normalized by their total, so small rounding differences do not matter.
What do I need to enter in the average Atomic Mass Calculator?
You enter Isotope masses (amu) and Abundances (%).
What is an example of a average Atomic Mass result?
With isotope masses (amu) 34.96885, 36.9659 and abundances (%) 75.76, 24.24, the average atomic mass is 35.4529 amu.
Where does the method come from?
The method follows: OpenStax, Chemistry 2e (Rice University): isotopes and the weighted-average atomic mass.
Related calculators
- Molar Mass Calculator — Molar mass and mass percent composition of a chemical formula.
- Mass Percent Calculator — Mass percent of a component in a mixture or compound.
- Mole Calculator — Convert between mass, moles and number of particles.
- Percent Yield Calculator — Percent yield of a reaction.
- Electron Configuration Calculator — Ground-state electron configuration of an element or monatomic ion, with noble-gas shorthand and unpaired electrons.
- Center of Mass Calculator — Center of mass of point masses in one or two dimensions.
- Stoichiometry Calculator — Mass of a product (or reactant) from the mass of another species in a balanced equation.
- Mole Fraction Calculator — Mole fraction of one component in a mixture.
Sources
- OpenStax, Chemistry 2e (Rice University): isotopes and the weighted-average atomic mass
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.