Relative Atomic Mass Calculator
Calculate the weighted average mass of an element’s isotopes. Enter the relative isotopic masses and percentage abundances to find the relative atomic mass.
Isotope Details
Enter the mass and abundance of your isotopes to calculate the weighted average
The mass of the first isotope (e.g., 35 for Chlorine-35).
Calculation Results
Weighted average mass and step-by-step breakdown
Enter your isotope details above and click Calculate RAM to reveal the relative atomic mass and calculation breakdown.
Common Elements & Atomic Masses
A quick reference guide for the relative atomic masses of common elements found in chemistry calculations.
| Element | Symbol | Common Isotopes | Relative Atomic Mass |
|---|---|---|---|
| Hydrogen | H | 1, 2 | 1.008 |
| Carbon | C | 12, 13 | 12.011 |
| Chlorine | Cl | 35, 37 | 35.45 |
| Copper | Cu | 63, 65 | 63.55 |
| Neon | Ne | 20, 21, 22 | 20.18 |
Relative Atomic Mass FAQ
Everything you need to know about calculating relative atomic mass, understanding isotopes, and mastering chemistry calculations.
Relative atomic mass (Ar) is the weighted average mass of an atom of an element, taking into account the relative abundances of its isotopes, compared to one-twelfth the mass of a carbon-12 atom. It is a dimensionless quantity.
To calculate relative atomic mass, multiply the relative isotopic mass of each isotope by its percentage abundance, add these values together, and then divide by the total abundance (usually 100). The formula is: Ar = (Mass1 × Abundance1 + Mass2 × Abundance2) / Total Abundance.
The mass number is the total number of protons and neutrons in a specific isotope of an element (always a whole number). Relative atomic mass is the weighted average of the masses of all naturally occurring isotopes of an element, which is why it is often a decimal (e.g., Chlorine is 35.5).
Chlorine has two main stable isotopes: Chlorine-35 (about 75% abundance) and Chlorine-37 (about 25% abundance). The weighted average is calculated as (35 × 0.75) + (37 × 0.25) = 26.25 + 9.25 = 35.5.
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. This means they have the same atomic number but different mass numbers.
No, relative atomic mass is a ratio and is therefore dimensionless. It is relative to 1/12th the mass of a carbon-12 atom. However, when expressed in grams per mole, it is numerically equivalent to the molar mass (g/mol).