Average Atomic Mass Calculator
⚛ Atomic & Quantum. Use the Average atomic mass formula to find average mass, isotope 1 mass, isotope 1 abundance, isotope 2 mass, isotope 2 abundance, isotope 3 mass, isotope 3 abundance: pick the value you want, type the others, and the answer updates as you go.
Up to three isotopes: mass × fractional abundance. Default: chlorine.
Pick the ○ next to the value you want to find, fill in the others, and it solves as you type.
What each letter means
| Quantity | Unit |
|---|---|
| Average mass | u |
| Isotope 1 mass | u |
| Isotope 1 abundance | fraction |
| Isotope 2 mass | u |
| Isotope 2 abundance | fraction |
| Isotope 3 mass | u |
| Isotope 3 abundance | fraction |
Worked example
Find the average mass when Isotope 1 mass = 34.969 u, Isotope 1 abundance = 0.7576 fraction, Isotope 2 mass = 36.966 u, Isotope 2 abundance = 0.2424 fraction, Isotope 3 mass = 0 u, Isotope 3 abundance = 0 fraction.
Using Mavg = Σ Mi Ai: Average mass = 35.453 u.
Frequently asked questions
What is the formula for Average atomic mass?
Mavg = Σ Mi Ai. Up to three isotopes: mass × fractional abundance. Default: chlorine.
What do the letters stand for, and what units are used?
Average mass in u, Isotope 1 mass in u, Isotope 1 abundance in fraction, Isotope 2 mass in u, Isotope 2 abundance in fraction, Isotope 3 mass in u, Isotope 3 abundance in fraction.
How do you calculate average mass?
Put the values into Mavg = Σ Mi Ai. For example, with Isotope 1 mass = 34.969 u, Isotope 1 abundance = 0.7576 fraction, Isotope 2 mass = 36.966 u, Isotope 2 abundance = 0.2424 fraction, Isotope 3 mass = 0 u, Isotope 3 abundance = 0 fraction, average mass = 35.453 u.
Can I solve for a different value?
Yes. In the calculator above, pick the circle next to any value (average mass, isotope 1 mass, isotope 1 abundance, isotope 2 mass, isotope 2 abundance, isotope 3 mass, isotope 3 abundance) and fill in the others: it rearranges the formula for you.
