What is the average atomic mass?
Type each isotope's mass and natural abundance, or pick one of seven elements to fill them in from NIST data. The atomic mass calculator gives the abundance-weighted average atomic mass. Switch to isotope abundances to work backwards from a known average to the share of each of two isotopes.
- Average atomic mass (u)
- 35.4527
The average atomic mass is 35.4527 u.
- Abundances add to
- 100%
- Molar mass (g/mol)
- 35.4527
- Working
- 34.969 × 0.7578 + 36.966 × 0.2422
- Answer
- The average atomic mass is 35.4527 u
Average atomic mass (u): 35.4527. The average atomic mass is 35.4527 u.
How to calculate
Finds the average atomic mass of an element from its isotopes’ masses and abundances, Σ(mass × abundance), or the abundance of each of two isotopes from the average.
Example with the default inputs (Find Average atomic mass, Element My own isotopes, Isotopes [Isotope mass (u) 34.969, Abundance 75.78%; Isotope mass (u) 36.966, Abundance 24.22%]): The average atomic mass is 35.4527 u.
Method: Average atomic mass = Σ(mass × abundance) ÷ Σ(abundance). For two isotopes and an average A: abundance 1 = (A − m₂) ÷ (m₁ − m₂) × 100%, abundance 2 = 100% − abundance 1.
- Abundances are percents of atoms (amount fractions), and must add to 100% within 0.5 points; the average divides by their sum.
- Element presets use NIST’s representative isotopic compositions; natural samples vary a little, which is why IUPAC gives some atomic weights as ranges.
- The molar mass in g/mol has the same number as the average atomic mass in u.
Worked examples
Each example is checked against the calculator on every build.
- Find Average atomic mass, Element Chlorine (Cl) gives Average atomic mass (u) 35.452938, Abundances add to 100%.Source: NIST, Atomic Weights and Isotopic Compositions for Chlorine (35Cl 34.968852682 u, 0.7576; 37Cl 36.965902602 u, 0.2424), https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=Cl (retrieved 2026-10-02); OpenStax, Chemistry 2e, §2.3 Atomic Structure and Symbolism (average atomic mass = Σ(fractional abundance × isotopic mass)), https://openstax.org/books/chemistry-2e/pages/2-3-atomic-structure-and-symbolism (retrieved 2026-10-02)
- Find Average atomic mass, Element My own isotopes, Isotopes 34.969 75.78; 36.966 24.22 gives Average atomic mass (u) 35.452673, Answer The average atomic mass is 35.4527 u.Source: OpenStax, Chemistry 2e, §2.3 Atomic Structure and Symbolism (average atomic mass = Σ(fractional abundance × isotopic mass)), https://openstax.org/books/chemistry-2e/pages/2-3-atomic-structure-and-symbolism (retrieved 2026-10-02)
- Find Average atomic mass, Element My own isotopes, Isotopes 10.0129 19.9; 11.0093 80.1 gives Average atomic mass (u) 10.811016.Source: OpenStax, Chemistry 2e, §2.3 Atomic Structure and Symbolism (average atomic mass = Σ(fractional abundance × isotopic mass)), https://openstax.org/books/chemistry-2e/pages/2-3-atomic-structure-and-symbolism (retrieved 2026-10-02): about 10.81 amu
- Find Isotope abundances, Isotope 1 mass 34.96885, Isotope 2 mass 36.9659, Average atomic mass 35.453 gives Isotope 1 abundance 75.756741%, Isotope 2 abundance 24.243259%.Source: OpenStax, Chemistry 2e, §2.3 Atomic Structure and Symbolism (average atomic mass = Σ(fractional abundance × isotopic mass)), https://openstax.org/books/chemistry-2e/pages/2-3-atomic-structure-and-symbolism (retrieved 2026-10-02): 75.76% 35Cl and 24.24% 37Cl
- Find Isotope abundances, Isotope 1 mass 62.9296, Isotope 2 mass 64.9278, Average atomic mass 63.546 gives Isotope 1 abundance 69.152237%, Isotope 2 abundance 30.847763%.Source: NIST, Atomic Weights and Isotopic Compositions for Copper (63Cu 62.92959772 u, 0.6915; 65Cu 64.92778970 u, 0.3085; standard atomic weight 63.546), https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=Cu (retrieved 2026-10-02)
- Find Average atomic mass, Element Magnesium (Mg) gives Average atomic mass (u) 24.305052.Source: NIST, Atomic Weights and Isotopic Compositions for Magnesium (standard atomic weight [24.304, 24.307]), https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=Mg (retrieved 2026-10-02)
How it works
Average atomic mass. For isotopes with masses m₁, m₂, … (in u) and abundances a₁, a₂, … (in percent):
- average atomic mass = (m₁ × a₁ + m₂ × a₂ + …) ÷ (a₁ + a₂ + …)
- when the abundances add to exactly 100%, this is Σ(fractional abundance × isotopic mass), as in OpenStax
- the molar mass in g/mol is the same number
Isotope abundances (two isotopes). With masses m₁ and m₂ and a known average A:
- abundance of isotope 1 = (A − m₂) ÷ (m₁ − m₂) × 100%
- abundance of isotope 2 = 100% − abundance of isotope 1
Element presets. Choosing an element uses these isotope masses (u) and compositions from NIST:
| Element | Isotopes: mass (composition) |
|---|---|
| Boron | 10.01293695 (0.199); 11.00930536 (0.801) |
| Carbon | 12 (0.9893); 13.00335483507 (0.0107) |
| Magnesium | 23.985041697 (0.7899); 24.985836976 (0.1000); 25.982592968 (0.1101) |
| Chlorine | 34.968852682 (0.7576); 36.965902602 (0.2424) |
| Copper | 62.92959772 (0.6915); 64.9277897 (0.3085) |
| Bromine | 78.9183376 (0.5069); 80.9162897 (0.4931) |
| Silver | 106.9050916 (0.51839); 108.9047553 (0.48161) |
Each composition is multiplied by 100 to give the abundance in percent. The maths runs in double precision.
Rules
- My own isotopes: 1 to 10 isotopes, each mass more than 0 and at most 300 u, each abundance 0% to 100%. The abundances must add to 100% within 0.5 percentage points (99.5% to 100.5%); otherwise there is no answer.
- Two isotopes: masses more than 0 and at most 300 u, and different; the average must be from the lighter to the heavier mass (inclusive).
Output format. Average atomic mass and molar mass to 4 decimal places; abundances to 2. The answer sentence writes the average to exactly 4 decimals ("35.4527 u"), and the abundances rounded to 2 decimals with trailing zeros dropped. The working lists each isotope as "mass × fraction", numbers to 10 significant figures.
Worked examples by hand
Chlorine preset. 34.968852682 × 0.7576 + 36.965902602 × 0.2424 = 35.4529 u.
Two typed isotopes. (34.969 × 75.78 + 36.966 × 24.22) ÷ 100 = 35.4527 u.
Boron (OpenStax). 10.0129 × 0.199 + 11.0093 × 0.801 = 10.8110 u.
Magnesium preset. 23.985041697 × 0.7899 + 24.985836976 × 0.1000 + 25.982592968 × 0.1101 = 24.3051 u (IUPAC range 24.304 to 24.307).
Chlorine abundances from 35.453 u. (35.453 − 36.96590) ÷ (34.96885 − 36.96590) = 0.7576: 75.76% ³⁵Cl and 24.24% ³⁷Cl.
Copper abundances from 63.546 u. (63.546 − 64.9278) ÷ (62.9296 − 64.9278) = 69.15% ⁶³Cu and 30.85% ⁶⁵Cu.
Other questions people ask
How do I calculate average atomic mass?
Multiply each isotope's mass by its abundance as a fraction, and add the results. For boron, 19.9% is ¹⁰B (10.0129 u) and 80.1% is ¹¹B (11.0093 u): 10.0129 × 0.199 + 11.0093 × 0.801 = 10.81 u.
Why is the atomic mass on the periodic table not a whole number?
It is an average over the element's isotopes, weighted by how common each one is. Chlorine is about three quarters ³⁵Cl and one quarter ³⁷Cl, so its average is about 35.45, not 35 or 37.
How do I find the abundance of two isotopes from the average?
Call the share of isotope 1 x, so isotope 2 is 1 − x. Then x × m₁ + (1 − x) × m₂ = average, so x = (average − m₂) ÷ (m₁ − m₂). For chlorine, (35.453 − 36.96590) ÷ (34.96885 − 36.96590) = 0.7576, or 75.76% ³⁵Cl.
What if my abundances do not add to 100%?
Rounded abundances often add to 99.99% or 100.01%. The page divides by their sum, so small gaps do not matter, but it asks you to check them when the sum is more than half a point away from 100%.
Is atomic mass the same as molar mass?
They have the same number. An average atomic mass of 35.45 u means one mole of the element's atoms has a mass of 35.45 g, so its molar mass is 35.45 g/mol.
Why do some atomic weights have a range?
The isotope mix of some elements, such as chlorine, boron and carbon, varies a little between natural samples. IUPAC gives their standard atomic weights as intervals, such as [35.446, 35.457] for chlorine. The presets use NIST's representative compositions.